Abstract:
A semiconductor device comprises a plurality of field-effect transistors and a common source line. Each of the plurality of memory cell transistors includes a semiconductor region of a first conductivity type formed in a semiconductor substrate, a source region and a drain region of a second conductivity type formed in the semiconductor region, an information storage portion capable of electrically writing and erasing data, and at least one control gate electrode including a conductive layer. The common source line is formed on the semiconductor region of the first conductivity type, and electrically connects the source regions of the memory cell transistors. The common source line includes a conductive layer that has a film thickness substantially equal to a film thickness of the conductive layer included in the control gate electrode and is formed of the same material as that of the conductive layer included in the control gate electrode.

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. 2002-165698, filed Jun. 6, 2002, 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 generally to a semiconductor device with a source line and a fabrication method thereof, and more particularly to a nonvolatile semiconductor memory device having a source line and a method of fabricating the same.  
           [0004]    2. Description of the Related Art  
           [0005]    A description will now be given of a conventional semiconductor device, referring, by way of example, to a NAND-type EEPROM (electrically erasable and programmable read-only memory) that is a kind of nonvolatile semiconductor memory device.  
           [0006]    [0006]FIG. 1 is a circuit diagram of a conventional NAND-type EEPROM, FIG. 2A is a layout of the NAND-type EEPROM, and FIG. 2B is a cross-sectional view taken along line  2 B- 2 B in the layout of FIG. 2A.  
           [0007]    As is shown in FIG. 1, memory cell transistors M 0 , M 1 , . . . , M 15  are connected in series. Select transistors SGD and SGS are connected to both ends of the series-connected memory cell transistors. A bit line BL is connected to the select transistor SGD, and a source line SL is connected to the select transistor SGS. Word lines WL 0 , WL 1 , . . . , WL 15  are connected to the gate electrodes of the memory cell transistors M 0  to M 15 , respectively. Select lines SSL and GSL are connected to the gate electrodes of the select transistors SGD and SGS, respectively.  
           [0008]    As is shown in FIG. 2A, the word lines WL 0  to WL 15  and select lines SSL and GSL are arranged in parallel. The bit lines BL are arranged perpendicular to the word lines WL 0  to WL 15 . Bit line contacts BLC are arranged between the select lines SSL.  
           [0009]    In the NAND-type EEPROM, as shown in FIG. 2B, a plurality ( 16  in FIG. 2B) of memory cell transistors M 0  to M 15  are arranged in series between the bit line contact BLC and source line SL, with the select transistors SGD and SGS interposed.  
           [0010]    As a structure of the source line SL in the NAND-type EEPROM, a prior-art document (Jung-Dai Choi, et al.: IEDM Tech. Dig., pp. 767-770 (2000)) discloses a source line structure (local interconnect structure) wherein impurity-doped polysilicon is buried in an insulating film, as shown in FIG. 2B.  
           [0011]    However, the above-described conventional method has the following problems.  
           [0012]    A first problem is a great height of the source line.  
           [0013]    According to the fabrication method disclosed in the aforementioned document, the source line is formed to have a height that is greater than, at least, the height of the gate of the memory cell. On the other hand, in order that the source line may have a sufficiently low resistivity, it is necessary that the source line be formed to have a sufficient height. If the width of the source line decreases in accordance with device miniaturization, the source line needs to be formed with a greater height for compensation of the decrease in width.  
           [0014]    To ensure insulation between the source line and bit line, the bit line needs to be formed at a higher position than the source line. Accordingly, the height of the bit line contact increases as the height of the source line increases, and the aspect ratio of the bit line contact increases. As a result, in the prior art, it would be difficult to achieve a satisfactory device yield.  
           [0015]    A major factor of the above problem is the use of polysilicon as buried material for forming the source line, the polysilicon having a lower conductivity than a metal or a metal compound.  
           [0016]    A second problem is a decrease in lithography margin, e.g. exposure latitude, occurring in a step of patterning a gate electrode. In general, a regular line-and-space pattern is desirable for ensuring a good lithography margin. In the aforementioned prior-art method, however, a wide space needs to be provided for forming the source line between the select transistors. This leads to a pattern irregularity and a decrease in lithography margin such as an exposure latitude.  
         BRIEF SUMMARY OF THE INVENTION  
         [0017]    According to an aspect of the present invention, there is provided a semiconductor device comprising: a plurality of memory cell transistors, each of the plurality of memory cell transistors including a semiconductor region of a first conductivity type formed in a semiconductor substrate, a source region and a drain region of a second conductivity type formed in the semiconductor region of the first conductivity type, an information storage portion capable of electrically writing and erasing data, and at least one control gate electrode including a conductive layer; and a common source line formed on the semiconductor region of the first conductivity type, the common source line electrically connecting a plurality of the source regions of the plurality of memory cell transistors, and the common source line including a conductive layer that has a film thickness substantially equal to a film thickness of the conductive layer included in the control gate electrode and is formed of the same material as that of the conductive layer included in the control gate electrode.  
           [0018]    According to another aspect of the invention, there is provided a method of fabricating a semiconductor device having a memory cell transistor and a source line formed adjacent to the memory cell transistor and electrically connected to the memory cell transistor, comprising: forming a first gate insulation film and a first polysilicon film in succession on a semiconductor substrate in a memory cell region where the memory cell transistor is to be formed and in a source line region where the source line is to be formed; forming a device isolation insulation film in the semiconductor substrate in the memory cell region and the source line region; removing the first polysilicon film and the first gate insulation film in the source line region; forming a second polysilicon film on the first polysilicon film, the semiconductor substrate and the device isolation insulation film in the memory cell region and the source line region; flattening the second polysilicon film, thereby exposing a surface of the device isolation insulation film; forming a second gate insulation film on the second polysilicon film and the device isolation insulation film in the memory cell region and the source line region; forming a third polysilicon film on the second gate insulation film in the memory cell region and the source line region; removing the third polysilicon film and the second gate insulation film in the source line region; forming a conductive film on the third polysilicon film in the memory cell region, and forming the conductive film on the third polysilicon film and the device isolation insulation film in the source line region; processing the first polysilicon film, the second gate insulation film, the second polysilicon film and the third polysilicon film in the memory cell region and the source line region, thus forming a gate electrode of the memory cell transistor in the memory cell region and a source line in the source line region; and implanting impurities in the semiconductor substrate between the gate electrodes of the memory cell transistors, and in the semiconductor substrate between the gate electrode and the source line, thus forming a source diffusion layer and a drain diffusion layer. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0019]    [0019]FIG. 1 is a circuit diagram showing the structure of a conventional semiconductor device;  
         [0020]    [0020]FIG. 2A is a layout of the semiconductor device;  
         [0021]    [0021]FIG. 2B is a cross-sectional view of the semiconductor device, taken along line  2 B- 2 B in the layout of FIG. 2A;  
         [0022]    [0022]FIG. 3 is a circuit diagram showing the structure of a semiconductor device according to a first embodiment of the present invention;  
         [0023]    [0023]FIG. 4A is a layout of the semiconductor device of the first embodiment;  
         [0024]    [0024]FIG. 4B is a cross-sectional view of the semiconductor device of the first embodiment, taken along line A-A in the layout of FIG. 4A;  
         [0025]    [0025]FIG. 4C is a cross-sectional view of a semiconductor device according to another example of the first embodiment, taken along line A-A in the layout of FIG. 4A;  
         [0026]    [0026]FIGS. 5A to  5 E are cross-sectional views illustrating a first step in a method of fabricating the semiconductor device of the first embodiment;  
         [0027]    [0027]FIGS. 6A to  6 D are cross-sectional views illustrating a second step in the method of fabricating the semiconductor device of the first embodiment;  
         [0028]    [0028]FIGS. 7A to  7 D are cross-sectional views illustrating a third step in the method of fabricating the semiconductor device of the first embodiment;  
         [0029]    [0029]FIGS. 8A to  8 D are cross-sectional views illustrating a fourth step in the method of fabricating the semiconductor device of the first embodiment;  
         [0030]    [0030]FIGS. 9A to  9 D are cross-sectional views illustrating a fifth step in the method of fabricating the semiconductor device of the first embodiment;  
         [0031]    [0031]FIGS. 10A to  10 D are cross-sectional views illustrating a sixth step in the method of fabricating the semiconductor device of the first embodiment;  
         [0032]    [0032]FIGS. 11A to  11 D are cross-sectional views illustrating a seventh step in the method of fabricating the semiconductor device of the first embodiment;  
         [0033]    [0033]FIG. 12 is a cross-sectional view illustrating an eighth step in the method of fabricating the semiconductor device of the first embodiment;  
         [0034]    [0034]FIG. 13 is a cross-sectional view illustrating a ninth step in the method of fabricating the semiconductor device of the first embodiment;  
         [0035]    [0035]FIG. 14 is a cross-sectional view illustrating a tenth step in the method of fabricating the semiconductor device of the first embodiment;  
         [0036]    [0036]FIG. 15 is a cross-sectional view showing the structure of a semiconductor device according to a modification of the first embodiment;  
         [0037]    [0037]FIG. 16 is a circuit diagram showing the structure of a semiconductor device according to a second embodiment of the present invention;  
         [0038]    [0038]FIG. 17A is a layout of the semiconductor device of the second embodiment;  
         [0039]    [0039]FIG. 17B is a cross-sectional view of the semiconductor device of the second embodiment, taken along line A-A in the layout of FIG. 17A;  
         [0040]    [0040]FIG. 17C is a cross-sectional view of a semiconductor device according to another example of the second embodiment, taken along line A-A in the layout of FIG. 17A;  
         [0041]    [0041]FIGS. 18A to  18 E are cross-sectional views illustrating a first step in a method of fabricating the semiconductor device of the second embodiment;  
         [0042]    [0042]FIGS. 19A to  19 C are cross-sectional views illustrating a second step in the method of fabricating the semiconductor device of the second embodiment;  
         [0043]    [0043]FIGS. 20A to  20 C are cross-sectional views illustrating a third step in the method of fabricating the semiconductor device of the second embodiment;  
         [0044]    [0044]FIGS. 21A to  21 C are cross-sectional views illustrating a fourth step in the method of fabricating the semiconductor device of the second embodiment;  
         [0045]    [0045]FIGS. 22A to  22 C are cross-sectional views illustrating a fifth step in the method of fabricating the semiconductor device of the second embodiment;  
         [0046]    [0046]FIG. 23 is a cross-sectional view illustrating a sixth step in the method of fabricating the semiconductor device of the second embodiment;  
         [0047]    [0047]FIG. 24 is a cross-sectional view illustrating a seventh step in the method of fabricating the semiconductor device of the second embodiment;  
         [0048]    [0048]FIG. 25 is a cross-sectional view illustrating an eighth step in the method of fabricating the semiconductor device of the second embodiment;  
         [0049]    [0049]FIGS. 26A and 26B are cross-sectional views showing the structures of semiconductor devices according to modifications of the second embodiment;  
         [0050]    [0050]FIG. 27 is a circuit diagram showing the structure of a semiconductor device according to a third embodiment of the present invention;  
         [0051]    [0051]FIG. 28A is a layout of the semiconductor device of the third embodiment;  
         [0052]    [0052]FIG. 28B is a cross-sectional view of the semiconductor device of the third embodiment, taken along line A-A in the layout of FIG. 28A;  
         [0053]    [0053]FIGS. 29A to  29 C are cross-sectional views illustrating a first step in a method of fabricating the semiconductor device of the third embodiment;  
         [0054]    [0054]FIGS. 30A to  30 C are cross-sectional views illustrating a second step in the method of fabricating the semiconductor device of the third embodiment;  
         [0055]    [0055]FIGS. 31A and 31B are cross-sectional views illustrating a third step in the method of fabricating the semiconductor device of the third embodiment;  
         [0056]    [0056]FIG. 32 is a circuit diagram showing the structure of a semiconductor device according to a fourth embodiment of the present invention;  
         [0057]    [0057]FIG. 33A is a layout of the semiconductor device of the fourth embodiment;  
         [0058]    [0058]FIG. 33B is a cross-sectional view of the semiconductor device of the fourth embodiment, taken along line A-A in the layout of FIG. 33A;  
         [0059]    [0059]FIGS. 34A to  34 C are cross-sectional views illustrating a first step in a method of fabricating the semiconductor device of the fourth embodiment;  
         [0060]    [0060]FIGS. 35A to  35 C are cross-sectional views illustrating a second step in the method of fabricating the semiconductor device of the fourth embodiment;  
         [0061]    [0061]FIGS. 36A and 36B are cross-sectional views illustrating a third step in the method of fabricating the semiconductor device of the fourth embodiment;  
         [0062]    [0062]FIG. 37 is a circuit diagram showing the structure of a semiconductor device according to a modification of the fourth embodiment; and  
         [0063]    [0063]FIGS. 38A to  38 C are circuit diagrams of nonvolatile memories to which the embodiments of the present invention can be applied. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0064]    Semiconductor devices according to embodiments of the present invention will now be described with reference to the accompanying drawings, which depict NAND-type EEPROMs by way of example. In the descriptions below, common parts are denoted by like reference numerals throughout the drawings. The semiconductor devices of the present invention, which are described below, include peripheral transistors of peripheral circuits, in addition to memory cell transistors. However, for the purpose of simplicity, the structure and fabrication method of the memory cell region will mainly be described in this specification.  
         [0065]    [First Embodiment] 
         [0066]    [0066]FIG. 3 is a circuit diagram showing the structure of a NAND-type EEPROM according to a first embodiment of the invention.  
         [0067]    As is shown in FIG. 3, memory cell transistors MO, M 1 , . . . ,M 15 , each having a control gate and a floating gate, are connected in series through their current paths. Select transistors SGD and SGS are connected to both ends of the group of the series-connected memory cell transistors.  
         [0068]    The memory cell transistors M 0 , M 1 , . . . ,M 15 , have the control gates connected to word lines WL 0 , WL 1 , . . . ,WL 15 . The select transistor SGD has a gate connected to a select line SSL. The select transistor SGS has a control gate connected to a select line GSL. In addition, the select transistor SGD is connected to a bit line BL, and the select transistor SGS is connected to a source line SL.  
         [0069]    A layout and a cross-sectional structure of the NAND-type EEPROM according to the first embodiment will now be described.  
         [0070]    The NAND-type EEPROM is characterized in that the source line SL has the same stacked-layer structure as the select transistor SGD, SGS, and the memory cell, M 0  to M 15 .  
         [0071]    [0071]FIG. 4A is a layout of the NAND-type EEPROM according to the first embodiment.  
         [0072]    As is shown in FIG. 4A, the word lines WL 0  to WL 15  are arranged in parallel. A select line SSL is arranged in parallel to the word line WL 0  on the word line WL 0  side of the group of the arranged word lines WL 0  to WL 15 . In addition, another select line SSL is arranged adjacent to the select line SSL with a bit line contact BLC interposed therebetween. A select line GSL is arranged in parallel to the word line WL 15  on the word line WL 15  side of the group of the arranged word lines WL 0  to WL 15 . A source line SL is arranged adjacent to the select line GSL.  
         [0073]    Bit lines BL are arranged perpendicular to the word lines WL 0  to WL 15 , select lines SSL and GSL, and source line SL. A bit line contact BLC for connecting the bit line BL and select transistor SGD is formed at that portion of the bit line BL, which lies between the two select lines SSL.  
         [0074]    [0074]FIG. 4B is a cross-sectional view taken along line A-A in the layout of FIG. 4A.  
         [0075]    As is shown in FIG. 4B, n-type diffusion layers  12 , which are source/drain regions, are arranged such that they are spaced apart in a p-type semiconductor substrate or a p-type well region (hereinafter referred to as “semiconductor substrate”)  11 . A floating gate electrode  14  is formed on that portion of the semiconductor substrate  11 , which lies between the n-type diffusion layers  12 , with a tunnel insulation film  13  interposed. A control gate electrode  16  is formed over the floating gate electrode  14 , with an inter-poly-insulation film  15  interposed.  
         [0076]    The floating gate electrode  14  has a thickness of, e.g. about 10 nm to 300 nm, and is formed of impurity-doped polysilicon. The control gate electrode  16  has, for example, one of the following structures: a stacked structure of tungsten silicide (WSi) and polysilicon; a stacked structure of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, and polysilicon; a stacked structure of a metal, such as W, Cu or Al, and polysilicon; and a single-layer structure of a metal compound of silicon, or a metal. The control gate electrode  16  has a thickness of, e.g. about 10 nm to 300 nm.  
         [0077]    A cap insulation film  17  is formed on the control gate electrode  16 . The cap insulation film  17  is formed of a silicon oxide film or a silicon nitride film with a thickness of about 10 nm to 300 nm.  
         [0078]    Thereby, both ends of the current paths of memory cell transistors M 0  to M 15  are connected to constitute a NAND cell. The control gate electrodes  16  of memory cell transistors M 0  to M 15  correspond to word lines WL 0  to WL 15 .  
         [0079]    A select transistor SGD, comprising the tunnel insulation film  13 , floating gate electrode  14 , interpoly-insulation film  15  and control gate electrode  16 , is formed at one end of the NAND cell. A select transistor SGD of another block is formed at one end of this select transistor SGD, with an n-type diffusion layer  12  and a bit line contact BLC interposed therebetween.  
         [0080]    At the other end of the NAND cell, a select transistor SGS is formed, which comprises the tunnel insulation film  13 , floating gate electrode  14 , interpoly-insulation film  15  and control gate electrode  16 . Further, a source line SL is formed at one end of the select transistor SGS.  
         [0081]    A portion of the inter-poly-insulation film  15  in the select transistor SGD, SGS is removed so as to establish electrical conduction between the floating gate electrode  14  and control gate electrode  16 . Thus, the floating gate electrode  14  and control gate electrode  16  of the select transistor SGD correspond to the select line SSL, and the floating gate electrode  14  and control gate electrode  16  of the select transistor SGS correspond to the select line GSL.  
         [0082]    The source line SL has a cross-sectional structure as described below. In the semiconductor substrate  11 , n-type diffusion layers  12  are formed spaced apart. Alternatively, as shown in FIG. 4C, an n-type diffusion layer  12 ′ may be formed under the source line SL. A tunnel insulation film  13  is formed on the semiconductor substrate  11  and n-type diffusion layer  12 . A portion or the entirety of the tunnel insulation film  13  is removed from a region on the semiconductor substrate  11  (or n-type diffusion layer  12 ′), where the source line is to be formed between the n-type diffusion layers  12 , and from a region on the n-type diffusion layer  12 .  
         [0083]    A conductive film  14 , which is formed of the same conductive material as that of the floating gate electrode  14  in the same layer as the floating gate electrode  14 , is formed on the semiconductor substrate  11  (or n-type diffusion layer  12 ′) and n-type diffusion layer  12 , from where the tunnel insulation film  13  has been removed. Thereby, the conductive film  14  is electrically connected to the semiconductor substrate  11  and n-type diffusion layer  12 . The conductive film  14 , like the floating gate electrode  14 , has a thickness of, e.g. about 10 nm to 300 nm and is formed of impurity-doped polysilicon.  
         [0084]    An inter-poly-insulation film  15  such as an ONO film is formed on the conductive film  14 . A conductive film  16 , which is formed of the same conductive material as that of the control gate electrode  16  in the same layer as the control gate electrode  16 , is formed on the inter-poly-insulation film  15 . The inter-poly-insulation film  15  is formed of the same material as that of the inter-poly-insulation film  15  of the memory cell in the same layer as the inter-poly-insulation film  15  of the memory cell. A portion of the inter-poly-insulation film  15  on the conductive film  14  is removed so as to establish electrical conduction between the conductive film  14  and conductive film electrode  16 . As a result, the conductive film  14  and conductive film  16  are electrically connected to the semiconductor substrate  11  and n-type diffusion layer  12  and function as the source line of the nonvolatile memory cell. The source line is formed of the same layer materials as the select line SSL, GSL and the word line, WL 0  to WL 15 .  
         [0085]    A cap insulation film  17 , which is formed of the same material in the same layer as the cap insulation film  17  of each memory cell and each select transistor, is formed on the conductive film  16 .  
         [0086]    As has been described above, the conductive film  16  is formed in the same layer as the control gate electrode  16 , that is, in the same layer as the control gate electrode (word line)  16  of each memory cell, M 0  to M 15 , and the control gate electrode  16  of the select transistor SGD, SGS. The conductive film  16 , like the control gate electrode  16 , has, for example, one of the following structures: a stacked structure of tungsten silicide (WSi) and polysilicon; a stacked structure of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, and polysilicon; a stacked structure of a metal, such as W, Cu or Al, and polysilicon; and a single-layer structure of a metal compound of silicon, or a metal. The conductive film  16  has a thickness of, e.g. about 10 nm to 300 nm.  
         [0087]    The height of the conductive film  16  of the source line SL, as measured from the surface of the semiconductor substrate  11 , is determined by the height of the control gate electrode  16  of each memory cell and each select transistor, as measured from the surface of the semiconductor substrate  11 . Specifically, the height of the conductive film  16  is set to be equal to the height of the control gate electrode  16 , and preferably less than the height of the control gate electrode  16  by about 10 nm to 100 nm.  
         [0088]    Interlayer insulation films  18  are formed between the gate electrodes (floating gate electrodes  14  and control gate electrodes  16 ) of the memory cell transistors M 0  to M 15  and select transistors SGD and SGS, between the gate electrode and the source line SL, and between the source lines SL. A barrier insulation film  19  is formed on the cap insulation films  17  and interlayer insulation films  18  so as to cover the memory cell transistors, select transistors and source line. The barrier insulation film  19  is formed of, e.g. a silicon nitride film about 5 nm to 50 nm thick. A bit line BL is formed over the barrier insulation film  19 , with an interlayer insulation film  20  interposed.  
         [0089]    A bit line contact BLC is formed on the n-type diffusion layer  12  between the select transistors SGD. The bit line contact BLC electrically connects the n-type diffusion layer  12  and bit line BL.  
         [0090]    The barrier insulation film  19  serves as an etching stopper film that prevents the bit line contact BLC from sinking in the device isolation groove. The bit line contact BLC is formed higher than the source line SL. Thereby, insulation is ensured between the source line SL and bit line BL.  
         [0091]    A fabrication method of the semiconductor device according to the first embodiment will now be described.  
         [0092]    [0092]FIGS. 5A to  5 E, FIGS. 6A to  6 D, FIGS. 7A to  7 D, FIGS. 8A to  8 D, FIGS. 9A to  9 D, FIGS. 10A to  10 D, FIGS. 11A to  11 D and FIGS.  12  to  14  are cross-sectional views illustrating the fabrication method of the semiconductor device according to the first embodiment.  
         [0093]    [0093]FIG. 5A is a cross-sectional view of a NAND-type EEPROM, taken in a direction perpendicular to the device isolation groove. That is, FIG. 5A is a cross-sectional view taken along line B-B in the layout of FIG. 4A, which extends along the source line region, line C-C extending along the select transistor region, and line D-D extending along the memory cell region. The cross-sectional structures along line B-B, line C-C and line D-D are common in the fabrication steps from the formation of the tunnel insulation film  13  to the formation of device isolation insulation films, and therefore FIG. 5A is used to commonly represent these structures. Similarly, FIGS. 5B to  5 E are cross-sectional views taken along lines B-B, C-C and D-D. FIGS. 5A to  5 E show cross-sectional structures of regions indicated by double-headed arrows in FIG. 4A. Similarly, the cross-sectional structures of regions indicated by double-headed arrows in FIG. 4A are illustrated in FIGS. 6A to  6 D, FIGS. 7A to  7 D, FIGS. 8A to  8 D, FIGS. 9A to  9 D, FIGS. 10A to  10 D, FIGS. 11A to  11 D.  
         [0094]    Impurities for forming wells and channels are implanted in the semiconductor substrate  11 . As is shown in FIG. 5A, a tunnel insulation film  13  with a thickness of about 5 nm to 15 nm is formed on the semiconductor substrate  11 . The tunnel insulation film  13  is formed of, e.g. an oxide film or an oxinitride film. A polysilicon film  14  with a thickness of about 10 nm to 200 nm is formed on the tunnel insulation film  13 .  
         [0095]    Subsequently, as shown in FIG. 5B, a stopper insulation film  22  is formed on the polysilicon film  14 . A mask insulation film  23  is formed on the stopper insulation film  22 . The stopper insulation film  22  is formed of, e.g. a silicon nitride film about 20 nm to 200 nm thick. The mask insulation film  23  is formed of, e.g. a silicon oxide film about 50 nm to 200 nm thick. The stopper insulation film  22  serves as a stopper film in a CMP step that is performed later.  
         [0096]    Then, a resist film is patterned by photolithography. Using the patterned resist film and mask insulation film  23  as a mask, anisotropic etching is performed, as illustrated in FIG. 5C, thus forming device isolation grooves  24 A in the semiconductor substrate  11 .  
         [0097]    The device isolation grooves  24 A are filled with insulation films  24  such as silicon oxide films. The insulation films  24  are flattened by CMP, as shown in FIG. 5D. Further, as shown in FIG. 5E, the stopper insulation film  22  is removed by wet etching. FIGS. 6A to  6 D are cross-sectional views in which the device isolation regions have been formed by the above process.  
         [0098]    [0098]FIG. 6A, FIG. 7A, FIG. 8A, FIG. 9A, FIG. 10A, FIG. 11A and FIGS.  12  to  14  are cross-sectional views taken along line A-A perpendicular to the word lines. FIGS. 6A to  11 A show cross-sectional structures of the source line and the select transistor regions arranged on both sides of the source line. FIGS.  12  to  14  are cross-sectional views of a NAND memory cell block including the source line and bit line contact. FIG. 6B, FIG. 7B, FIG. 8B, FIG. 9B, FIG. 10B and FIG. 11B are cross-sectional views taken along line B-B that extends along the source region. FIG. 6C, FIG. 7C, FIG. 8C, FIG. 9C, FIG. 10C and FIG. 11C are cross-sectional views taken along line C-C that extends along the select transistor region. FIG. 6D, FIG. 7D, FIG. 8D, FIG. 9D, FIG. 10D and FIG. 11D are cross-sectional views taken along line D-D that extends along the memory cell region.  
         [0099]    Following the above-described step, a resist film is patterned by photolithography. Using the patterned resist film as a mask, as shown in FIGS. 7A and 7B, anisotropic etching is performed to remove the polysilicon film  14  and tunnel insulation film  13  in the source line region. In this step, there is a case where upper portions of the device isolation insulation films in the source line region may also be etched away. Further, if impurities such as phosphorus or arsenic are implanted in the semiconductor substrate, an n-type diffusion layer may be formed at an interface between the source line and the semiconductor substrate, as shown in FIG. 4C.  
         [0100]    An impurity-heavily-doped polysilicon film  14  with a thickness of about 10 nm to 300 nm is deposited on the polysilicon film  14 , semiconductor substrate  11  and device isolation insulation films  24 , which are shown in FIGS. 7A to  7 D. Then, as shown in FIGS. 8A to  8 D, the deposited polysilicon film  14  is flattened by CMP.  
         [0101]    Subsequently, after the device isolation insulation films  24  are recessed, an inter-poly-insulation film  15  is formed on the polysilicon film  14  and device isolation insulation films  24 , as shown in FIGS. 9A to  9 D. The inter-poly-insulation film  15  comprises, e.g. an ONO film in which an oxide film, a nitride film and an oxide film are stacked in the named order. An impurity-heavily-doped polysilicon film  16  with a thickness of about 10 nm to 100 nm is formed on the inter-poly-insulation film  15 .  
         [0102]    Then, using a photolithographically patterned resist film as a mask, as illustrated in FIGS. 10A to  10 D, anisotropic etching is carried out to remove portions or the entirety of the polysilicon film  16  and inter-poly-insulation film  15  in the source line region and select transistor region.  
         [0103]    Thereafter, as shown in FIGS. 11A to  11 D, a conductive film  16  with a thickness of about 10 nm to 300 nm is formed on the structure shown in FIGS. 10A to  10 D. The conductive film  16  has, for example, one of the following structures: a stacked structure of tungsten silicide (WSi) and polysilicon; a stacked structure of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, and polysilicon; a stacked structure of a metal, such as W, Cu or Al, and polysilicon; and a single-layer structure of a metal compound of silicon, or a metal. A cap insulation film  17  is formed on the conductive film  16 .  
         [0104]    In the source region and select transistor region, as shown in FIGS. 11B and 11C, part or all of the inter-poly-insulation film  15  is removed. Accordingly, the polysilicon film  14  forming the floating gate electrode and the conductive film  16  forming the control gate electrode are electrically connected.  
         [0105]    Following the above step, using a photolithographically patterned resist film and cap insulation film  17  as a mask, anisotropic etching is carried out to process the gate electrodes of the memory cell transistors MO to MIS and select transistors SGD and SGS and the source line SL, as shown in FIG. 12.  
         [0106]    Then, as shown in FIG. 13, impurities such as phosphorus or arsenic are implanted in the semiconductor substrate  11  between the gate electrodes of the memory cell transistors and select transistors and between the gate electrode and source line SL, thereby forming n-type diffusion layers  12  that are source/drain regions. In addition, interlayer insulation films  18  such as silicon oxide films are filled in between the gate electrodes and between the gate electrode and source line.  
         [0107]    Next, a barrier insulation film  19 , which is, e.g. about 5 nm to 50 nm thick, is formed on the gate electrodes of the memory cell transistors and select transistors, the source line SL and the interlayer insulation films  18 .  
         [0108]    Then, as shown in FIG. 14, an interlayer insulation film  20  such as a silicon oxide film is deposited, and the interlayer insulation film  20  is flattened by CMP. Further, a bit line contact BLC connected to the n-type diffusion layer  12  is formed in the interlayer insulation film  20 . A bit line BL is formed on the bit line contact BLC and interlayer insulation film  20 . An insulation film  21  is formed over the bit line BL and interlayer insulation film  20 .  
         [0109]    Subsequently, although not illustrated, upper wiring, a passivation film, etc. are formed to complete fabrication of the NAND-type EEPROM.  
         [0110]    In the semiconductor device with the above structure and the fabrication method, the source line SL is formed of the material having a higher conductivity than polysilicon, as mentioned above. Thus, the source line resistivity can be made lower than in the conventional semiconductor device shown in FIG. 2B. Hence, the height of the source line can be decreased. Thereby, the bit line BL can also be formed lower than in the conventional semiconductor device, and the aspect ratio of the bit line contact BLC can be reduced. As a result, the processing of the semiconductor device can be facilitated, and the device yield is increased.  
         [0111]    In the above-described fabrication method, the source line SL is formed in the same step as the step of forming the gate electrodes of the memory cell transistors and select transistors. This makes it unnecessary to provide a margin for alignment between the select transistor and source line, and reduces the space between the select transistors, compared to the prior art. Hence, the area of the NAND-type EEPROM can be reduced.  
         [0112]    The source line SL is formed of the same material, and in the same layer, as the gate electrode of each of the memory cell transistors and select transistors. Therefore, compared to the prior-art semiconductor device fabrication method wherein the formation of the source line is performed in a separate process, the number of fabrication steps can be reduced and the manufacturing cost of the semiconductor device can be reduced. Furthermore, the design of circuitry is easier since the resistivity of the source line becomes substantially equal to that of the select line SGD, SGS, or the word line, WL 0  to WL 15 .  
         [0113]    In the above-described first embodiment, as shown in FIG. 14, the size of the opening in the tunnel insulation film  13  on the semiconductor substrate  11  is smaller than the bottom surface area of the polysilicon film  14  of the source line SL in the contact region between the source line SL and semiconductor substrate  11 . However, as shown in FIG. 15, the size of the opening in the tunnel insulation film  13  may be greater than the bottom surface area of the polysilicon film  14 .  
         [0114]    The operation of the NAND-type EEPROM according to the first embodiment will now be described.  
         [0115]    In an erase operation, for example, a high voltage (e.g. 10 V to 25 V) is applied to the semiconductor substrate  11  in the state in which the control gate electrode  16  is set at 0 V. Thereby, electrons are released from the floating gate electrode (i.e. charge accumulation layer)  14  to the semiconductor substrate  11 , and the erase operation is effected.  
         [0116]    In a write operation, for example, a high voltage (e.g. 10 V to 25 V) is applied to the control gate electrode  16  in the state in which the semiconductor substrate  11  is set at 0 V. Thereby, electrons are injected from the semiconductor substrate  11  into the floating gate electrode  14 , and the write operation is effected. In another method, hot electrons, which are accelerated in the channel, are produced by positively biasing the drain potential relative to the source potential. In addition, by positively biasing the control gate electrode  16  relative to the source potential, the hot electrons are injected in the floating gate electrode  14 . Thus, the write operation is effected.  
         [0117]    In a read-out operation, the bit line BL is precharged and then set in a floating state. The voltage of the control gate electrode  16  of the memory cell selected for read-out is set at 0 V, and the voltage of the control gate electrodes of the other memory cell transistors is set at a non-selected readout voltage Vread. Further, the voltage of the control gate electrode  16  of the select transistor SGS, SGD is set at a power supply voltage Vcc, and the source line SL is set at 0 V. Whether current flows in the memory cell selected for read-out is detected by the bit line BL. Thus, the read-out operation is effected. Specifically, if the selected memory cell is at a threshold voltage Vth&gt;0 (write state), the memory cell is in the off state and the bit line BL maintains a precharge potential.  
         [0118]    On the other hand, if the selected memory cell is at a threshold voltage Vth&lt;0 (read state), the memory cell is turned on, and the potential of the bit line BL lowers from the precharge potential by ΔV. This potential change is sensed by a sense amplifier, and the data in the memory cell is read out.  
         [0119]    As has been described above, in the first embodiment, it is possible to prevent an increase in height of the source line and to thus prevent an increase in the aspect ratio of the bit line contact. Moreover, it is possible to suppress a decrease in lithography margin in the step of patterning the gate electrode. Besides, since the source line is formed at the same time as the select transistors and memory cell transistors, the number of fabrication steps can be reduced.  
         [0120]    [Second Embodiment] 
         [0121]    A semiconductor device according to a second embodiment of the present invention will now be described.  
         [0122]    The second embodiment differs from the first embodiment in that the gate electrode of each of the memory cell transistors and select transistors has a single-layer structure without an inter-poly-insulation film. Similarly, the source line does not include an inter-poly-insulation film. Further, each of the memory cell transistors and select transistors has a MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) structure. Structural parts common to those of the first embodiment are denoted by like reference numerals, and a description thereof is omitted. Only different parts will be described.  
         [0123]    [0123]FIG. 16 is a circuit diagram showing the structure of a NAND-type EEPROM according to the second embodiment of the invention. FIG. 17A is a layout of the NAND-type EEPROM according to the second embodiment. FIG. 17B is a cross-sectional view taken along line A-A in the layout of FIG. 17A. The layout in FIG. 17A is the same as that of the first embodiment.  
         [0124]    As is shown in FIG. 17B, a gate insulation film of each of the memory cell transistors and select transistors, which is formed on the semiconductor substrate  11 , has a stacked-layer structure comprising a tunnel insulation film  31 , a charge accumulation layer  32  and a block insulation film  33  which are stacked in the named order. Each of the memory cell transistors and select transistors has a so-called MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) structure. The tunnel insulation film  31  is formed of a silicon oxide film, a silicon oxinitride film or a silicon nitride film, with a thickness of, e.g. about 1 nm to 10 nm. The charge accumulation layer  32  is formed of a silicon nitride film with a thickness of, e.g. about 3 nm to 30 nm. The block insulation film  33  is formed of a silicon nitride film, a silicon oxinitride film or a silicon nitride film, with a thickness of, e.g. about 3 nm to 20 nm.  
         [0125]    The gate electrode of each of the memory cell transistors and select transistors comprises a polysilicon film  14  formed on the gate insulation film, and a conductive film  16  formed on the polysilicon film  14 . The polysilicon film  14  has a thickness of about 10 nm to 200 nm. The conductive film  16  has, for example, one of the following structures: a stacked structure of tungsten silicide (WSi) and polysilicon; a stacked structure of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, and polysilicon; a stacked structure of a metal, such as W, Cu or Al, and polysilicon; and a single-layer structure of a metal compound of silicon, or a metal. The conductive film  16  has a thickness of, e.g. about 10 nm to 300 nm.  
         [0126]    In the region of the source line SL, a portion or the entirety of the gate insulation film comprising the tunnel insulation film  31 , charge accumulation layer  32  and block insulation film  33 , which is formed on the semiconductor substrate  11 , is etched away. Thus, a polysilicon film  14  and a conductive film  16 , which are formed of the same material as that of the aforementioned gate electrode, are formed on the n-type diffusion layer  12  and semiconductor substrate  11 . The polysilicon film  14  and conductive film  16  constitute the source line and are electrically connected to the n-type diffusion layer  12 . Alternatively, as shown in FIG. 17C, an n-type diffusion layer  12 ′ may be formed under the source line SL. As a result, the conductive film  14  and conductive film  16  are electrically connected to the semiconductor substrate  11  and n-type diffusion layer  12 ′ and function as the source line of the nonvolatile memory cell. The source line is formed of the material of the same layer as the select line SSL, GSL and word line, WL 0  to WL 15 .  
         [0127]    Further, as shown in FIG. 17B, a cap insulation film  17 , an interlayer insulation film  18 , a barrier insulation film  19 , an interlayer insulation film  20 , a bit line contact BLC, a bit line BL and an insulation film  21  are formed.  
         [0128]    In the NAND-type EEPROM shown in FIGS. 16 and 17B, the select transistors SGD and SGS have the same MONOS structure as the memory cell transistors, but they may have a MOS structure. In the case where the select transistors have the MOS structure, the conductive material of the source line is formed in the same fabrication step, and in the same layer, as the conductive material of the select transistor. In addition, the insulation film between the semiconductor substrate and the conductive material in the source line region is formed in the same fabrication step, and in the same layer, as the insulation film of the select transistor.  
         [0129]    A fabrication method of the semiconductor device according to the second embodiment will now be described.  
         [0130]    [0130]FIGS. 18A to  18 E, FIGS. 19A to  19 C, FIGS. 20A to  20 C, FIGS. 21A to  21 C, FIGS. 22A to  22 C, and FIGS.  23  to  25  are cross-sectional views illustrating the fabrication method of the semiconductor device according to the second embodiment.  
         [0131]    [0131]FIG. 18A is a cross-sectional view of a NAND-type EEPROM, taken in a direction perpendicular to the device isolation groove. That is, FIG. 18A is a cross-sectional view taken along line B-B in the layout of FIG. 17A, which extends along the source line region, line C-C extending along the select transistor region, and line D-D extending along the memory cell region. The cross-sectional structures along line B-B, line C-C and line D-D are common in the fabrication steps from the formation of the tunnel insulation film  31  to the formation of device isolation insulation films, and therefore FIG. 18A is used to commonly represent these structures. Similarly, FIGS. 18B to  18 E are cross-sectional views taken along lines B-B, C-C and D-D.  
         [0132]    Impurities for forming wells and channels are implanted in the semiconductor substrate  11 . As is shown in FIG. 18A, a gate insulation film comprising a tunnel insulation film  31 , a charge accumulation layer  32  and a block insulation film  33 , which are stacked in the named order, is formed on the semiconductor substrate  11 . The tunnel insulation film  31  is formed of a silicon oxide film, a silicon oxinitride film or a silicon nitride film, with a thickness of, e.g. about 1 nm to 10 nm. The charge accumulation layer  32  is formed of a silicon nitride film with a thickness of, e.g. about 3 nm to 30 nm. The block insulation film  33  is formed of a silicon nitride film, a silicon oxinitride film or a silicon nitride film, with a thickness of, e.g. about 3 nm to 20 nm. Then, the semiconductor substrate  11  with the gate insulation film formed is annealed, where necessary, thereby to densify the gate insulation film.  
         [0133]    Subsequently, a polysilicon film  14  with a thickness of about 10 nm to 200 nm is formed on the block insulation film  33 .  
         [0134]    As shown in FIG. 18B, a stopper insulation film  22  is formed on the polysilicon film  14 . A mask insulation film  23  is formed on the stopper insulation film  22 . The stopper insulation film  22  is formed of, e.g. a silicon nitride film about 20 nm to 200 nm thick. The mask insulation film  23  is formed of, e.g. a silicon oxide film about 50 nm to 200 nm thick. The stopper insulation film  22  serves as a stopper film in a CMP step that is performed later.  
         [0135]    Then, a resist film is patterned by photolithography. Using the patterned resist film and mask insulation film  23  as a mask, anisotropic etching is performed, as illustrated in FIG. 18C, thus forming device isolation grooves  24 A in the semiconductor substrate  11 .  
         [0136]    The device isolation grooves  24 A are filled with insulation films  24  such as silicon oxide films. The insulation films  24  are flattened by CMP, as shown in FIG. 18D. Further, as shown in FIG. 18E, the stopper insulation film  22  is removed by wet etching. FIGS. 19A to  19 C are cross-sectional views in which the device isolation regions have been formed by the above process.  
         [0137]    [0137]FIG. 19A, FIG. 20A, FIG. 21A, FIG. 22A, FIGS.  23  to  25 , FIG. 26A and FIG. 26B are cross-sectional views taken along line A-A perpendicular to the word lines. FIGS. 19A to  22 A show cross-sectional structures of the source line and the select transistor regions arranged on both sides of the source line. FIGS.  23  to  25  and FIGS. 26A and 26B are cross-sectional views of a NAND memory cell block including the source line and bit line contact. FIG. 19B, FIG. 20B, FIG. 21B and FIG. 22B are cross-sectional views taken along line B-B that extends along the source region. FIG. 19C, FIG. 20C, FIG. 21C, and  22 C are cross-sectional views taken along line D-D that extends along the memory cell. In the case where the memory cell transistors and select transistors have the MONOS structure, as in the second embodiment, the cross section along line C-C extending along the select transistor region is the same as the cross section along line D-D extending along the memory cell region. Thus, the cross section along line C-C is omitted here.  
         [0138]    Following the above-described step, a resist film is patterned by photolithography. Using the patterned resist film as a mask, as shown in FIGS. 20A to  20 C, anisotropic etching is performed to remove the polysilicon film  14  and the gate insulation film comprising the tunnel insulation film  31 , charge accumulation layer  32  and block insulation film  33  in the source line region. In this step, there is a case where upper portions of the device isolation insulation films in the source line region may also be etched away. Further, if impurities such as phosphorus or arsenic are implanted in the semiconductor substrate, an n-type diffusion layer may be formed at an interface between the source line and the semiconductor substrate, as shown in FIG. 17C.  
         [0139]    An impurity-heavily-doped polysilicon film  14  with a thickness of about 10 nm to 300 nm is deposited on the polysilicon film  14 , semiconductor substrate  11  and device isolation insulation films  24 , which are shown in FIGS. 20A to  20 C. Then, as shown in FIGS. 21A to  21 C, the deposited polysilicon film  14  is flattened by CMP.  
         [0140]    Thereafter, as shown in FIGS. 22A to  22 C, a conductive film  16  with a thickness of about 10 nm to 300 nm is formed on the polysilicon  14  and device isolation insulation films  24  shown in FIGS. 21A to  21 C. The conductive film  16  has, for example, one of the following structures: a stacked structure of tungsten silicide (WSi) and polysilicon; a stacked structure of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, and polysilicon; a stacked structure of a metal, such as W, Cu or Al, and polysilicon; and a single-layer structure of a metal compound of silicon, or a metal. A cap insulation film  17  is formed on the conductive film  16 .  
         [0141]    Following the above step, using a photolithographically patterned resist film and cap insulation film  17  as a mask, anisotropic etching is carried out to process the gate electrodes of the memory cell transistors M 0  to M 15  and select transistors SGD and SGS and the source line SL, as shown in FIG. 23.  
         [0142]    Then, as shown in FIG. 24, impurities such as phosphorus or arsenic are then implanted in the semiconductor substrate  11  between the gate electrodes of the memory cell transistors and select transistors and between the gate electrode and source line SL, thereby forming n-type diffusion layers  12  that are source/drain regions. In addition, interlayer insulation films  18  such as silicon oxide films are filled in between the gate electrodes and between the gate electrode and source line SL.  
         [0143]    Next, a barrier insulation film  19 , which is, e.g. about 5 nm to 50 nm thick, is formed on the gate electrodes of the memory cell transistors and select transistors, the source line SL and the interlayer insulation films  18 .  
         [0144]    Then, as shown in FIG. 25, an interlayer insulation film  20  such as a silicon oxide film is deposited, and the interlayer insulation film  20  is flattened by CMP. Further, a bit line contact BLC connected to the n-type diffusion layer  12  is formed in the interlayer insulation film  20 . A bit line BL is formed on the bit line contact BLC and interlayer insulation film  20 . An insulation film  21  is formed over the bit line BL and interlayer insulation film  20 .  
         [0145]    Subsequently, although not illustrated, upper wiring, a passivation film, etc. are formed to complete fabrication of the NAND-type EEPROM.  
         [0146]    In the second embodiment, too, the conductive material of the source line is formed of the same material, and in the same layer, as the control gate electrode of each memory cell and each select transistor. Therefore, the same advantage as with the first embodiment can be obtained. Furthermore, in the second embodiment, the gate electrode of each memory cell has the single-layer structure without an interpoly-insulation film. This eliminates the necessity to etch the inter-poly-insulation film in the select transistor region and source line region, and the fabrication steps can be simplified.  
         [0147]    In the second embodiment, as shown in FIG. 25, the gate insulation film comprising the tunnel insulation film  31 , charge accumulation layer  32  and block insulation film  33  is left between the gate electrodes. However, as shown in FIGS. 26A and 26B, the gate insulation film between the gate electrodes may be removed. Alternatively, only the block insulation film  33 , or portions of the block insulation film  33  and charge accumulation layer  32 , may be removed.  
         [0148]    The operation of the NAND-type EEPROM according to the second embodiment will now be described.  
         [0149]    In an erase operation, for example, a high voltage (e.g. 5 V to 25 V) is applied to the semiconductor substrate  11  in the state in which the control gate electrode  16  is set at 0 V. Thereby, holes are injected from the semiconductor substrate  11  into the charge accumulation layer  32 . In another method, hot holes are produced by positively biasing the diffusion layer potential relative to the semiconductor substrate  11 . In addition, by negatively biasing the control gate electrode  16  relative to the semiconductor substrate  11 , the hot holes are injected in the charge accumulation layer  32 . Thus, the erase operation is effected.  
         [0150]    In a write operation, for example, a high voltage (e.g. 5 V to 20 V) is applied to the control gate electrode  16  in the state in which the semiconductor substrate  11  is set at 0 V. Thereby, electrons are injected from the semiconductor substrate  11  into the charge accumulation layer  32 , and the write operation is effected. In another method, hot electrons, which are accelerated in the channel, are produced by positively biasing the drain potential relative to the source potential. In addition, by positively biasing the control gate electrode  16  relative to the source potential, the hot electrons are injected in the charge accumulation layer  32 . Thus, the write operation is effected.  
         [0151]    In a read-out operation, the bit line BL is precharged and then set in a floating state. The voltage of the control gate electrode  16  of the memory cell selected for read-out is set at Vref, and the voltage of the control gate electrodes of the other memory cell transistors is set at a non-selected readout voltage Vread. Further, the voltage of the control gate electrode  16  of the select transistor SGS, SGD is set at a power supply voltage Vcc, and the source line SL is set at 0 V. Whether current flows in the memory cell selected for read-out is detected by the bit line BL. Thus, the read-out operation is effected. Specifically, if the selected memory cell is at a threshold voltage Vth&gt;Vref (write state), the memory cell is in the off state and the bit line BL maintains a precharge potential.  
         [0152]    On the other hand, if the selected memory cell is at a threshold voltage Vth&lt;Vref (read state), the memory cell is turned on, and the potential of the bit line BL lowers from the precharge potential by ΔV. This potential change is sensed by a sense amplifier, and the data in the memory cell is read out.  
         [0153]    As has been described above, in the second embodiment, it is possible to prevent an increase in height of the source line SL and to thus prevent an increase in the aspect ratio of the bit line contact. Moreover, it is possible to suppress a decrease in lithography margin in the step of patterning the gate electrode. Besides, since the source line is formed at the same time as the select transistors and memory cell transistors, the number of fabrication steps can be reduced. Furthermore, the gate electrode of each memory cell has the single-layer structure without an interpoly-insulation film. This eliminates the necessity to etch the inter-poly-insulation film in the select transistor region and source line region, and the fabrication steps can be simplified.  
         [0154]    [Third Embodiment] 
         [0155]    A semiconductor device according to a third embodiment of the present invention will now be described.  
         [0156]    The third embodiment differs from the first and second embodiments in that the source line is processed after the gate electrodes of the memory cell transistors and select transistors are processed. Structural parts common to those of the first embodiment are denoted by like reference numerals, and a description thereof is omitted. Only different parts will be described.  
         [0157]    [0157]FIG. 27 is a circuit diagram showing the structure of a NAND-type EEPROM according to the third embodiment of the invention. FIG. 28A is a layout of the NAND-type EEPROM according to the third embodiment. FIG. 28B is a cross-sectional view taken along line A-A in the layout of FIG. 28A. The circuit diagram of FIG. 27 and the layout of FIG. 28A are the same as those of the first embodiment.  
         [0158]    As is shown in FIG. 28B, n-type diffusion layers  12 , which are source/drain regions, are arranged such that they are spaced apart in a p-type semiconductor substrate or a p-type well region (hereinafter referred to as “semiconductor substrate”)  11 . A floating gate electrode  14  is formed on that portion of the semiconductor substrate  11 , which lies between the n-type diffusion layers  12 , with a tunnel insulation film  13  interposed. A control gate electrode  16  is formed over the floating gate electrode  14 , with an inter-poly-insulation film  15  interposed. In addition, a silicide layer  36  is formed on the control gate electrode  16 . Thereby, both ends of the current paths of memory cell transistors M 0 , M 1 , . . . ,M 15  are connected to constitute a NAND cell. The control gate electrode  16  is an impurity-heavily-doped polysilicon film about 10 nm to 300 nm thick. The silicide layer  36  is formed of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, with a thickness of about 10 nm to 300 nm.  
         [0159]    The control gate electrodes  16  and silicide layers  36  of the memory cell transistors M 0  to M 15  correspond to word lines WL 0  to WL 15 .  
         [0160]    A select transistor SGD, comprising the aforementioned tunnel insulation film  13 , floating gate electrode  14 , inter-poly-insulation film  15 , control gate electrode  16  and silicide layer  36 , is formed at one end of the NAND cell. A select transistor SGD of another block is formed at one end of this select transistor SGD, with an n-type diffusion layer  12  and a bit line contact BLC interposed therebetween.  
         [0161]    At the other end of the NAND cell, a select transistor SGS is formed, which comprises the aforementioned tunnel insulation film  13 , floating gate electrode  14 , inter-poly-insulation film  15 , control gate electrode  16  and silicide layer  36 .  
         [0162]    A portion of the inter-poly-insulation film  15  in the select transistor SGD, SGS is removed so as to establish electrical conduction between the floating gate electrode  14  and control gate electrode  16 . Thus, the floating gate electrode  14 , control gate electrode  16  and silicide layer  36  of the select transistor SGD correspond to the select line SSL, and the floating gate electrode  14 , control gate electrode  16  and silicide layer  36  of the select transistor SGS correspond to the select line GSL. Further, a source line SL is formed at one end of the select transistor SGS.  
         [0163]    The source line SL has a cross-sectional structure as described below. In the semiconductor substrate  11 , an n-type diffusion layer  12  is formed. Interlayer insulation films  18  and  20  and a barrier insulation layer  19  are formed on the n-type diffusion layer  12 . A groove reaching the n-type diffusion layer  12  is formed in the interlayer insulation films  18  and  20  and barrier insulation layer  19  provided over the n-type diffusion layer  12 . A polysilicon film  35  is buried in the groove. A silicide layer  36  is formed on the polysilicon film  35 .  
         [0164]    The polysilicon film  35  and silicide layer  36  constitute a source line SL. The source line SL and n-type diffusion layer  12  are electrically connected. The height of the source line SL from the surface of the semiconductor substrate  11  is substantially equal to the height of the gate electrode of each of the memory cell transistors and select transistors from the surface of the semiconductor substrate  11 . The silicide layer  36  of the source line SL is formed in the same layer as the silicide layer  36  of each select transistor SGS, SGD and the silicide layer  36  of each memory cell, M 0  to M 15 .  
         [0165]    Further, as is shown in FIG. 28B, an interlayer insulation film  37 , a bit line contact BLC, a bit line BL and an insulation film  21  are formed.  
         [0166]    A fabrication method of the semiconductor device according to the third embodiment will now be described.  
         [0167]    [0167]FIGS. 29A to  29 C, FIGS. 30A to  30 C, FIG. 31A and FIG. 31B are cross-sectional views illustrating the steps of the fabrication method of the semiconductor device according to the third embodiment. These Figures are cross-sectional views taken along line A-A in the layout of FIG. 28A.  
         [0168]    To start with, gate electrodes of the memory cell transistors M 0  to M 15  and select transistors SGD and SGS are processed. The memory cell has a stacked structure wherein a tunnel insulation film  13 , a polysilicon film  14 , an inter-poly-insulation film  15  and a polysilicon  16  are stacked in the named order on the semiconductor substrate  11 . The select transistor also has a stacked structure wherein a tunnel insulation film  13 , a polysilicon film  14 , an interpoly-insulation film  15  and a polysilicon  16  are stacked in the named order on the semiconductor substrate  11 . In the select transistor, a part of the inter-poly-insulation film  15  is removed to establish electrical conduction between the polysilicon  14  and polysilicon  16 . The polysilicon films  14  and  16  are doped with impurities. The polysilicon film  14  has a thickness of about 10 nm to 200 nm, and the polysilicon film  16  has a thickness of about 10 nm to 300 nm.  
         [0169]    Then, as is shown in FIG. 29A, impurities such as phosphorus or arsenic are implanted in the semiconductor substrate  11  between the gate electrodes of the memory cell transistors and select transistors, thereby forming n-type diffusion layers  12  that are source/drain regions. In addition, interlayer insulation films  18  such as silicon oxide films are filled in between the gate electrodes of the memory cell transistors and select transistors.  
         [0170]    Next, a barrier insulation film  19  such as a silicon nitride film, which is, e.g. about 5 nm to 50 nm thick, is formed on the gate electrodes of the memory cell transistors and select transistors. Cap insulation films  17  such as silicon oxide films or silicon nitride films, which are about 10 nm to 100 nm thick, are formed between the gate electrodes of the memory cell transistors and select transistors and the barrier insulation film  19 .  
         [0171]    An interlayer insulation film  20  such as a silicon oxide film is then deposited, as shown in FIG. 29B, and the interlayer insulation film  20  is flattened by CMP. Further, as shown in FIG. 29C, the interlayer insulation film  20  and barrier insulation film  19  in the source region are etched away, thus forming a groove  34  for burying the source line. In FIG. 29C, the source line groove  34  is formed perpendicular to the semiconductor substrate  11 . Alternatively, the source line groove  34  may be formed in a tapered shape, which can advantageously achieve improved burying of conductive material.  
         [0172]    As is shown in FIG. 30A, an impurity-doped polysilicon film  35  is deposited on the structure illustrated in FIG. 29C. The polysilicon film  35  is then etched back by means of CMP, etc., as shown in FIG. 30B. Thereby, the polysilicon film  35  constituting the source line, which is buried in the groove  34  in the interlayer insulation film  20 , is formed. In this case, the height of the polysilicon film  35  that constitutes the source line is determined by the height of the gate electrode of each memory cell and each select transistor.  
         [0173]    Subsequently, the barrier insulation film  19  and cap insulation films  17  covering the gate electrodes of the memory cell transistors and select transistors are removed by wet etching or anisotropic etching. Thereby, as shown in FIG. 30C, the polysilicon films  35  forming the gate electrodes  16  and source line are exposed. In this case, the height of the source line  35  from the semiconductor substrate is substantially equal to the height of each select transistor and each memory cell. However, in order to decrease the resistivity of the source line, it is preferable that the height of the source line  35  be higher than each select transistor or memory cell by about 10 nm to 100 nm. This is realized by the technique wherein the source line  35  is formed higher than the gate electrodes of the select transistors and memory cell transistors by a degree corresponding to the thickness of the cap insulation film  17  and barrier insulation film  19 , as shown in FIG. 30B.  
         [0174]    Then, a metal film of nickel (Ni), molybdenum (Mo), titanium (Ti) or cobalt (Co) is deposited on the structure shown in FIG. 30C, and the resultant structure is annealed. Thereby, as shown in FIG. 31A, upper portions of the polysilicon films  16  and  36  constituting the gate electrodes and source line are silicided into silicon-metal compounds  36  such as NiSi, MoSi, TiSi or CoSi.  
         [0175]    On the structure shown in FIG. 31A, an interlayer insulation film  37  such as a silicon oxide film is deposited. The interlayer insulation film  37  is flattened by CMP. Further, a bit line contact BLC connected to the n-type diffusion layer  12  is formed in the interlayer insulation films  18 ,  20  and  37 . A bit line BL is formed on the bit line contact BLC and interlayer insulation film  37 . An insulation film  21  is formed on the bit line BL and interlayer insulation film  37 .  
         [0176]    Subsequently, although not illustrated, upper wiring, a passivation film, etc. are formed to complete fabrication of the NAND-type EEPROM.  
         [0177]    As regards the semiconductor device with the above-described structure and the fabrication method thereof, compared to the first and second embodiments, the number of fabrication steps can be reduced since there is no need to remove the tunnel insulation film (gate insulation film)  13  between the polysilicon film  35  constituting the source line SL and the semiconductor substrate  11 . In addition, since it is not necessary to provide a margin for alignment between the pattern after removal of the tunnel insulation film  13  and the polysilicon film  35  of the source line SL, the area of the NAND-type EEPROM can be reduced.  
         [0178]    This embodiment differs from the prior art shown in FIG. 2B in that the buried conductive material of the source line SL has a stacked structure. The upper conductive material of the stacked structure of the source line is a silicon-metal compound (silicide) such as NiSi, MoSi, TiSi or CoSi. Thus, compared to the prior-art source line formed of single-layer polysilicon, the resistivity of the source line is low and the height of the source line can be reduced. Preferably, the height of the source line should be made equal to the height of the gate electrode of each select transistor and each memory cell. If the fabrication method of this embodiment is used, the height of the source line is determined by the gates of the select transistors and memory cell transistors and therefore the height of the source line can stably be set. Since a variance in source line resistivity is decreased, a variance in characteristics of the nonvolatile memory, in particular, in the data read-out mode, can be reduced.  
         [0179]    Furthermore, the silicide, which is the upper conductive material of the stacked structure of the source line SL, is the same as the upper conductive material of the gate electrode of each memory cell and each select transistor, and the silicidation is performed in the same fabrication step. Therefore, the fabrication process can be simplified. In addition, since the resistivity of the source line SL is equal to that of each select line SSL, GSL and each word line, WL 0  to WL 15 , the circuitry design becomes easier.  
         [0180]    In the third embodiment, like the first embodiment, the memory cell is of the floating gate type. Alternatively, it may be of the MONOS type, like the second embodiment.  
         [0181]    The barrier insulation film  19 , which is formed of, e.g. a silicon nitride film about 5 nm to 50 nm thick, is provided between the select transistors. The barrier insulation film  19  serves as an etching stopper film that prevents the bit line contact BLC from sinking in the device isolation groove.  
         [0182]    In the semiconductor device of the third embodiment, the source line SL is formed of the silicide layer having a higher conductivity than polysilicon, as mentioned above. Thus, the source line resistivity can be made lower than in the conventional semiconductor device shown in FIG. 2B. Hence, the height of the source line can be decreased. Thereby, the bit line BL can also be formed lower than in the conventional semiconductor device, and the aspect ratio of the bit line contact BLC can be reduced. As a result, the processing of the semiconductor device can be facilitated, and the device yield is increased.  
         [0183]    The write, erase and read-out operations of the NAND-type EEPROM of the third embodiment are the same as those in the first or second embodiment, so a description thereof is omitted here.  
         [0184]    As has been described above, according to the third embodiment, the conductive material that forms the source line has the stacked structure, and the source line resistivity is decreased. Thereby, an increase in height of the source line SL is prevented, and also an increase in the aspect ratio of the bit line contact is prevented. Thus, the processing is facilitated. Besides, a part of the conductive material that forms the source line is provided in the same layer as the control gates of the select transistors and memory cell transistors (i.e. select lines and word lines). Hence, the fabrication steps can be simplified, the source line resistivity is made equal to the resistivity of the select lines and word lines, and the circuitry design is facilitated.  
         [0185]    [Fourth Embodiment] 
         [0186]    A semiconductor device according to a fourth embodiment of the present invention will now be described.  
         [0187]    In the fourth embodiment, like the third embodiment, the source line SL is processed after the gate electrodes of the memory cell transistors and select transistors are processed. In addition, the source line comprises stacked conductive films, and the upper conductive film is formed of a conductive material having a lower resistivity than impurity-doped polysilicon. The fourth embodiment differs from the third embodiment in that the upper conductive film is formed in the same fabrication step using damascene processing, and in the same layer, as the conductive films constituting the control gate electrodes of the memory cell transistors and select transistors.  
         [0188]    [0188]FIG. 32 is a circuit diagram showing the structure of a NAND-type EEPROM according to the fourth embodiment of the invention. FIG. 33A is a layout of the NAND-type EEPROM according to the fourth embodiment. FIG. 33B is a cross-sectional view taken along line A-A in the layout of FIG. 33A.  
         [0189]    An upper conductive film  41  of the source line SL is formed of a silicon-metal compound such as tungsten silicide (WSi) or a metal such as Cu, Al or W. The conductive film  41  is formed not by silicidation of polysilicon, as in the third embodiment, but by damascene processing in which a conductive film is buried after the cap insulation film  17  of the gate electrode is removed. The damascene processing, compared to the silicidation, can reduce the manufacturing cost and enhance controllability.  
         [0190]    A fabrication method of the semiconductor device according to the fourth embodiment will now be described.  
         [0191]    [0191]FIGS. 34A to  34 C, FIGS. 35A to  35 C, FIG. 36A and FIG. 36B are cross-sectional views illustrating the steps of the fabrication method of the semiconductor device according to the fourth embodiment. These Figures are cross-sectional views taken along line A-A in the layout of FIG. 33A.  
         [0192]    To start with, gate electrodes of the memory cell transistors M 0  to M 15  and select transistors SGD and SGS are processed. The gate electrode of the memory cell has a stacked structure wherein a tunnel insulation film  13 , a polysilicon film  14 , an interpoly-insulation film  15  and a polysilicon  16  are stacked in the named order on the semiconductor substrate  11 . The select transistor also has a stacked structure wherein a tunnel insulation film  13 , a polysilicon film  14 , an inter-poly-insulation film  15  and a polysilicon  16  are stacked in the named order on the semiconductor substrate  11 . In the select transistor, a part of the inter-poly-insulation film  15  is removed to establish electrical conduction between the polysilicon  14  and polysilicon  16 . The polysilicon films  14  and  16  are doped with impurities. The polysilicon film  14  has a thickness of about 10 nm to 200 nm, and the polysilicon film  16  has a thickness of about 10 nm to 300 nm.  
         [0193]    Then, as is shown in FIG. 34A, impurities such as phosphorus or arsenic are implanted in the semiconductor substrate  11  between the gate electrodes of the memory cell transistors and select transistors, thereby forming n-type diffusion layers  12  that are source/drain regions. In addition, interlayer insulation films  18  such as silicon oxide films are filled in between the gate electrodes of the memory cell transistors and select transistors.  
         [0194]    Next, a barrier insulation film  19  such as a silicon nitride film, which is, e.g. about 5 nm to 50 nm thick, is formed on the gate electrodes of the memory cell transistors and select transistors. Cap insulation films  17  such as silicon oxide films or silicon nitride films, which are about 10 nm to 100 nm thick, are formed between the gate electrodes of the memory cell transistors and select transistors and the barrier insulation film  19 .  
         [0195]    An interlayer insulation film  20  such as a silicon oxide film is then deposited, as shown in FIG. 34B, and the interlayer insulation film  20  is flattened by CMP. Further, as shown in FIG. 34C, the interlayer insulation film  20  and barrier insulation film  19  in the source region are etched away, thus forming a groove  34  for burying the source line. In FIG. 34C, the source line groove  34  is formed perpendicular to the semiconductor substrate  11 . Alternatively, the source line groove  34  may be formed in a tapered shape, which can advantageously achieve improved burying of conductive material.  
         [0196]    An impurity-doped polysilicon film is deposited on the structure illustrated in FIG. 34C. The polysilicon film is then etched back by means of CMP, etc., as shown in FIG. 35A. Thereby, the polysilicon film  35  constituting the source line, which is buried in the groove  34  in the interlayer insulation film  20 , is formed. In this case, it is desirable that the amount of etch-back of the polysilicon film  35  be 30 nm or more. As the amount of etch-back increases, a greater amount of conductive film with low resistivity can be buried in a later fabrication step, and the source line resistivity can be reduced. On the other hand, in order to reduce a variance in resistivity, it is desirable that the amount of etch-back be small. Taking both factors into account, an optimal amount of etch-back is determined.  
         [0197]    Subsequently, as shown in FIG. 35B, the barrier insulation film  19  and cap insulation films  17  covering the gate electrodes of the memory cell transistors and select transistors are removed by wet etching or anisotropic etching. Thereby, the polysilicon films  16  and  35  forming the gate electrodes and source line are exposed.  
         [0198]    On the structure shown in FIG. 35B, a silicon-metal compound  41  such as tungsten silicide (WSi) or a metal  41  such as Cu, Al or W is deposited, as shown in FIG. 35C. Then, the deposited silicon-metal compound  41  or metal  41  is etched back by CMP, etc., as shown in FIG. 36A.  
         [0199]    On the structure shown in FIG. 36A, an interlayer insulation film  37  such as a silicon oxide film is deposited. The interlayer insulation film  37  is flattened by CMP. Further, a bit line contact BLC connected to the n-type diffusion layer  12  is formed in the interlayer insulation films  18 ,  20  and  37 . A bit line BL is formed on the bit line contact BLC and interlayer insulation film  37 . An insulation film  21  is formed on the bit line BL and interlayer insulation film  37 .  
         [0200]    Subsequently, although not illustrated, upper wiring, a passivation film, etc. are formed to complete fabrication of the NAND-type EEPROM.  
         [0201]    In the fourth embodiment, like the first embodiment, the memory cell is of the floating gate type. Alternatively, it may be of the MONOS type, like the second embodiment.  
         [0202]    The barrier insulation film  19 , which is formed of, e.g. a silicon nitride film about 5 nm to 50 nm thick, is provided between the select transistors. The barrier insulation film  19  serves as an etching stopper film that prevents the bit line contact BLC from sinking in the device isolation groove.  
         [0203]    In the fourth embodiment, too, the source line SL comprises stacked conductive films, and the upper conductive film is formed of the conductive material having a higher conductivity than impurity-doped polysilicon. Therefore, the same advantage as with the third embodiment can be obtained. Furthermore, in the fourth embodiment, the upper conductive film of the source line is buried by damascene processing, as mentioned above. Thus, compared to the use of silicidation, the manufacturing cost can be reduced and the controllability enhanced. If the fabrication method of this embodiment is used, the height of the source line is determined by the gates of the select transistors and memory cell transistors and therefore the height of the source line can stably be set. Since a variance in source line resistivity is decreased, a variance in characteristics of the nonvolatile memory, in particular, in the data read-out mode, can be reduced.  
         [0204]    The write, erase and read-out operations of the NAND-type EEPROM of the fourth embodiment are the same as those in the first or second embodiment, so a description thereof is omitted here.  
         [0205]    As has been described above, according to the fourth embodiment, the conductive material that forms the source line has the stacked structure, and the source line resistivity is decreased. Thereby, an increase in height of the source line SL is prevented, and also an increase in the aspect ratio of the bit line contact is prevented. Thus, the processing is facilitated. Besides, a part of the conductive material that forms the source line is provided in the same layer as the control gates of the select transistors and memory cell transistors (i.e. select lines and word lines). Hence, the fabrication steps can be simplified, the source line resistivity is made equal to the resistivity of the select lines and word lines, and the circuitry design is facilitated.  
         [0206]    [Modification of the Fourth Embodiment] 
         [0207]    [0207]FIG. 37 shows a modification of the fourth embodiment. Like the fourth embodiment, the source line has a stacked structure. The lower layer  35  (in contact with the n-type diffusion layer  12 ) of the stacked structure is formed of, e.g. impurity-doped polysilicon, and the upper layer  41  is formed of a material having a lower resistivity than polysilicon. Specifically, the upper layer  41  of the source line is formed of a metal compound of silicon, such as NiSi, MoSi, TiSi or CoSi, having a thickness of, e.g. about 10 nm to 300 nm. Alternatively, the upper layer  41  is formed of a silicon metal compound such as tungsten silicide (WSi), or a metal such as Cu, Al or W.  
         [0208]    This modification differs from the fourth embodiment in that the control gate electrode  16  of each memory cell and each select transistor is not formed in the same layer as the upper layer  41  of the source line.  
         [0209]    In this modification, the upper layer of the source line is formed in a layer different from the layer of the control gate  16  of each select transistor or each memory cell. Thus, the step of removing the barrier insulation film  19  and cap insulation  17  covering the select transistors and memory cell portions is not necessary, and the fabrication steps can be simplified.  
         [0210]    Like the fourth embodiment, the conductive material that forms the source line has the stacked structure, and the source line resistivity is decreased. Thereby, an increase in height of the source line SL is prevented, and also an increase in the aspect ratio of the bit line contact is prevented. Thus, the processing is facilitated. Further, the height of the source line is determined by the gates of the select transistors and memory cell transistors and therefore the height of the source line can stably be set. Since a variance in source line resistivity is decreased, a variance in characteristics of the nonvolatile memory, in particular, in the data read-out mode, can be reduced. Note that this modification is applicable not only to the floating-gate-type memory cell transistors but also to MONOS type memory cell transistors.  
         [0211]    In the first to fourth embodiments and modifications, the common source line SL of the NAND-type EEPROM as shown in FIG. 38A is described by way of example. However, a common source line of a NOR-type EEPROM as shown in FIG. 38B or a common source line of an AND-type EEPROM as shown in FIG. 38C may be formed to have the stacked structure as shown in the first to fourth embodiments and modifications. The first and second embodiments use an SA-STI (Self Align-Shallow Trench Isolation) process for forming device isolation regions in a self-alignment manner with respect to the floating gate or control gate electrode. Alternatively, the gate insulation film and gate electrode may be formed after the device isolation regions are formed.  
         [0212]    The memory cell transistor may have a stacked gate structure such as a floating gate type, or a single-layer gate structure such as a MONOS type. Further, the select transistor and memory cell transistor may have the same structure or different structures. The select line and word line may have the same structure or different structures. For example, the gate electrode  14  and gate electrode  16  of the select transistor in the first embodiment may not be connected, and the gate electrode  14  formed in the same layer as the floating gate electrode may be used as a select line. In these modifications, too, the common source line may have the stacked structure, and the source line resistivity is made lower than in the conventional single-layer structure of polysilicon. Thereby, an increase in height of the source line is prevented, and also an increase in the aspect ratio of the bit line contact is prevented. Thus, the processing is facilitated. Preferably, the height of the source line may be determined by the gates of the select transistors and memory cell transistors. Thereby, since a variance in source line resistivity is decreased, a variance in characteristics of the device can be reduced. More preferably, a part or the entirety of the source line may be provided in the same layer as the select line (the control gate electrode of the select transistor) and the word line (the control gate line of the memory cell transistor). Hence, the number of fabrication steps can be reduced and the processing of the semiconductor device can be facilitated. Therefore, the manufacturing cost is reduced and the device yield is increased.  
         [0213]    As has been described above, according to the semiconductor devices and fabrication methods of the embodiments of the present invention, the conductive material that forms the source line has the stacked structure, and the source line resistivity is decreased. Thereby, an increase in height of the source line SL is prevented, and also an increase in the aspect ratio of the bit line contact is prevented. Thus, the processing is facilitated. Besides, a part or the entirety of the conductive material that forms the source line is provided in the same layer as the control gates of the select transistors and memory cell transistors (i.e. select lines and word lines). Hence, the fabrication steps can be simplified, the source line resistivity is made equal to the resistivity of the select lines and word lines, and the circuitry design is facilitated. Furthermore, a decrease in lithography margin such as an exposure latitude in the step of patterning the gate electrodes can be suppressed.  
         [0214]    The above-described embodiments can be practiced singly or in combination, as desired. Furthermore, each embodiment includes inventions in various stages, and various inventions can be derived from desired combinations of structural elements disclosed herein.  
         [0215]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.