Abstract:
A non-volatile semiconductor memory device comprising a device isolation insulation layer, a floating gate, and control gate, and a booster electrode. The device isolation insulation layer is formed on a semiconductor substrate, and is for defining a device region. The floating gate is formed above the device region and has a pair of first side faces opposed to a side face of the device isolation insulation layer which is located on the device region side. The control gate is formed above the floating gate. The booster electrode has faces opposed to a pair of second surfaces of the floating gate which are substantially perpendicular to the pair of first side faces. A distance between the pair of first side faces of the floating gate is equal or not more than a width of the device region defined by the device isolation insulation layer. Dimensions of the floating gate are determined based on a coupling ratio between the floating gate and the booster electrode.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electrically programmable non-volatile semiconductor memory device having an electrode called a booster plate. 
     An EEPROM having an electrode called a booster plate is described, for example, in 1996 Symposium on VLSI Technology Digest of Technical Papers, pp. 238-239 (I. D. Choi, D. J. Kim, D. S. Jang, J. Kim, H. S. Kim, W. C. Shin, S. T. Ahn, and O. H. Kwon, Samsung Electronics Co., LTD.). 
     In this specification, the electrode called “booster plate” is referred to as “booster electrode.” An EEPROM cell having the booster electrode will now be generally described. 
     FIG. 1A is a plan view of a conventional memory cell, FIG. 1B is a cross-sectional view taken along line B—B in FIG. 1A, and FIG. 1C is a cross-sectional view taken along line C—C in FIG.  1 A. For simple description, bit lines and an underlying interlayer insulating film are omitted in FIG.  1 A. 
     As is shown in FIGS. 1A to  1 C, device isolation insulation films  102  are formed in a surface portion of a P-type silicon substrate  101 . Device regions  103  are defined on a surface of the substrate  101  by the device isolation insulation films  102 . 
     A tunnel insulation film  104 , a floating gate  105 , an insulation film  106  and a word line  107  are successively formed on the device region  103 . A structure wherein the floating gate  105  and word line  107  are stacked is called a stacked-gate structure. 
     Reference numeral  108  denotes a gate of a select transistor. N-type diffusion layers  109 ,  110  and  111  are formed in the device region  103 . The diffusion layer  109  is connected to a source line (not shown), and the diffusion layer  110  is to a bit line  112 . The number of diffusion layers  111  is two or more and these layers  111  function as source/drain regions of memory cell transistors, respectively. 
     A booster electrode insulating film  114  is formed over the periphery of the stacked-gate structure and the diffusion layers  111 . A booster electrode  115  is formed on the insulating film  114 . Reference numeral  144  denotes an interlayer insulation film. 
     FIG. 2A shows an equivalent circuit of the conventional EEPROM. For the purpose of simple description, FIG. 2A shows the case where two word lines (WL 1 , WL 2 ) and two bit lines (BL 1 , BL 2 ) are provided. 
     As is shown in FIG. 2A, a select transistor ST 11 , cell transistors MC 11  and MC 21  and a select transistor ST 21  are connected in series between a bit line BL 1  and a source line SL. 
     Similarly, a select transistor ST 12 , cell transistors MC 12  and MC 22  and a select transistor ST 22  are connected in series between a bit line BL 2  and the source line SL. 
     A word line WL 1  is commonly connected to the gates of the cell transistors MC 11  and MC 12 , and a word line WL 2  is commonly connected to the gates of the cell transistors MC 21  and MC 22 . 
     A drain-side select gate line SG 1  is commonly connected to the gates of the select transistors ST 11  and ST 12 , and a source-side select gate line SG 2  is commonly connected to the gates of the select transistors ST 21  and ST 22 . A back-gate (BULK) of each transistor is common. 
     In the NAND type EEPROM, the potential of the back-gate BULK is varied in accordance with the operation mode. A booster electrode BP is capacitively coupled to the mutual connection nodes and floating gates FG 11 , FG 12 , FG 21  and FG 22  of the respective transistors. 
     The write operation will now be described on the basis of the disclosure in the above-mentioned document. In the following description, a write operation for injecting electrons into the floating gate is called “0” write, and a write operation for injecting no electrons into the floating gate is called “1” write. FIG. 2B shows potentials of respective nodes in the write mode. 
     In the NAND type flash EEPROM disclosed in the above-mentioned document, the potential of the selected word line WL 1  is set at 13 V, the potential of the booster electrode BP is at 13 V, the potential of the bit line BL 1  designated for “0” write is at 0 V, the potential of the drain-side select gate line SG 1  is at 3.3 V, the potential of the source-side select gate line SG 2  is at 0 V, and the potential of the non-selected word line WL 2  is at 3.3 V. 
     At this time, the potentials of both the write-selected word line WL 1  and booster electrode BP are 13 V. A potential corresponding to about a coupling ratio (γ pgm) “0.78” between the floating gate FG 11  and word line WL 1  can be produced at the floating gate FG 11  by a potential of the booster electrode BP, and a potential of about 10 V is applied to the tunnel insulation film. 
     Accordingly, even if the write potential is 13 V, electrons are injected into the floating gate FG 11  through the tunnel oxide film having about 10 nm thick. Thus, “0” write is effected in the cell MC 11 . 
     On the other hand, the gate potential of the cell MC 21  belonging to the same bit line BL 1  and having the gate connected to the non-selected word line WL 2  is 3.3 V, and the potential of the booster electrode BP is 13 V. At this time, the voltage of 3.3 V applied to the word line WL 2  acts to lower the potential of the floating gate FG 21 . Thus, no electrons are injected in the floating gate FG 21 . 
     On the other hand, the potential of the bit line BL 2  designated for “1” write is 3.3 V. Since the potential of the drain-side select gate line SG 1  is 3.3 V at this time, the select transistor ST 12  is cut off when the potential of “3.3 V-VthST” has been transferred to the N-type diffusion layer. As a result, the region  116  of the diffusion layer  111  shown in FIG.  1 B and channel  113  of the memory cell (hereinafter referred to as “NAND cell channel  116 ” or simply “cell channel  116 ”) is set in the floating state. 
     In this case, “VthST” is a threshold voltage of the select transistor ST 12 . At this time the potential of the cell channel  116  is raised by the potential of booster electrode BP. 
     The potential, 13 V, of the selected word line WL 1  contributes to raising the potential of cell channel  116  through the floating gate FG 12 . In this manner the potential of cell channel  116  is raised up to about 8 V. 
     In the cell MC 12  having the gate connected to the selected word line WL 1 , a potential difference between the channel thereof and the word line WL 1  decreases to “13 V−8 V=5 V” and no electrons are injected in the floating gate FG 12 . 
     Thus, data “1” is written in the cell MC 12 . As described above, in the EEPROM having the booster electrode BP, the potential of the cell channel  116  is greatly raised up to about 8 V in the write-selected cell MC 12  connected to the bit line BL 2  designated for “1” write. 
     In addition, in the cell MC 22  having the gate connected to the non-selected word line WL 2 , a potential difference between the channel thereof and the word line WL 2  is “3.3 V−8 V=−4.7 V” and no electrons are injected in the floating gate FG 22 . 
     As has been described above, the main function of the booster electrode BP is to increase the effective coupling ratio γ pgm so that the potential of the floating gate is sufficiently raised at the time of “0” write, thereby lowering the potential (write potential VPP) of the selected word line from 17 V to 13 V. 
     Furthermore, the channel potential of the cell for “1” write is raised from “3.3-VthST”, as in the prior art, to about 8 V, thereby making it difficult for electrons to be injected in the floating gate. Thereby, occurrence of “erroneous write”, such as erroneous write of “0”, can be prevented. 
     However, in the conventional EEPROM having the booster electrode, the coupling ratio γ pgm in write mode varies due to “processing error” at the time of forming the device isolation region  102  and “processing error” at the time of forming the floating gate  105 , as will be described below in detail. 
     FIG. 3 is a bird&#39;s eye view showing dimensions of the floating gate. 
     Suppose, as shown in FIG. 3, that the dimension of the floating gate  105  along the bit line is “a”, the dimension of floating gate  105  along the word line is “b”, the height of floating gate  105  is “c”, and the width of the device region  103  is “d”. 
     In addition, suppose that the thickness of the tunnel insulation film  104  between the substrate  101  and floating gate  105 , as shown in FIGS. 1A to  1 C, is “tox1”, the thickness of the insulation film  106  between the floating gate  105  and word line  107  is “tox2”, and the thickness of the booster electrode insulating film  114  between the floating gate  105  and booster electrode  115  is “tox3.” 
     At this time, the capacitance C1 between the substrate  101  and floating gate  105  is given by 
     
       
           C 1=∈0·∈ r ( a·d )/tox1. 
       
     
     The capacitance C2 between the floating gate  105  and word line  107  is given by 
     
       
           C 2=⊂0·∈ r ( b+ 2 c ) a /tox2. 
       
     
     The capacitance C3 between the floating gate  105  and booster electrode  115  is given by 
     
       
           C 3=∈0·∈ r (2 b·c )/tox3. 
       
     
     When the potential of word line  107  is write potential VPP, the potential VFG of the floating gate  105  is given by the following, if the charge in the floating gate  105  is ignored: 
     
       
         ( VPP−VFG )·( C 2+ C 3)= VFG·C 1 
       
     
     Accordingly, 
     
       
           VFG= ( C 2+ C 3)· VPP /( C 1 +C 2+ C 3)=γ  pgm·Vpp.   
       
     
     As the capacitance C2, C3 increases, the potential VFG becomes closer to the potential VPP and increases. At this time, the width “b” of floating gate  105  along the word line  107  is not included in the capacitance C1 but is included in the capacitance C2, C3. 
     Accordingly, as the width “b” increases, the capacitance C2, C3 increases and the value of potential VFG also increases. In other words, if the width “b” varies, the value of potential VFG varies. 
     The variance in potential VFG results in a variance in write charge (quality of electrons injected in the floating gate), and the variance in threshold voltage of the cell in which data “0” has been written increases. 
     In particular, in these years, data to be stored in the EEPROM has gradually changed from general two-value data to multi-value data. Thus, there is a demand that the threshold voltage of the cell be distributed in a very narrow range. 
     In order to meet the demand, the quantity of electrons injected in the floating gate needs to be controlled with higher precision. However, the variance in potential VFG makes the control difficult. 
     In addition, if the value of potential VFG varies, a possibility increases that electrons may be injected in the floating gate of the non-selected cell in which a gate is the word line or the cell for “1” write at the time of data write. 
     Although the width “d” of the device region  103  is not included in the capacitance C2, C3, it is included in the capacitance C1. The effective coupling ratio γ pgm is expressed by 
     
       
         γ  pgm= ( C 2+ C 3)/( C 1+ C 2+ C 3)=[{( b+ 2 c ) a /tox2}+{(2 b·c )/tox3}]/[{( a·d )/tox1}+{( b+ 2 c ) a / tox2}+{(2 b·c )/tox3}] 
       
     
     Accordingly, if the width “d” of device region  103  varies, the coupling ratio γ pgm varies at the time of data write. 
     FIG. 4 is a graph showing the dependency of the coupling ratio γ pgm upon the width “d” of the device region  103 . In FIG. 4, the variation of the coupling ratio γ pgm is plotted when the width “d” of the device region  103  has varied in the cell having substantially the following values: a=0.25 μm, b=0.45 μm, c=0.1 μm, d=0.25 μm, tox1=10 nm, tox2=14 nm, and tox3=30 nm. 
     As is shown in FIG. 4, the coupling ratio γ pgm decreases as the width “d” of device region  103  increases. 
     If the coupling ratio γ pgm varies, the variation in distribution of threshold voltage of the cell increases. In order to decrease the variance of distribution of threshold voltage, it is possible, for example, to divide the write pulse into small components and inject electrons into the floating gate little by little. In this case, however, the write time increases. 
     Furthermore, if there is a cell wherein electrons may be easily injected due to variance in coupling ratio γ pgm, defects such as erroneous write or read disturb (weak write occurring when a voltage is produced between the word line and substrate) may easily occur. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above circumstances, and the present invention provides a non-volatile semiconductor memory device and a method of manufacturing the same, wherein a variation in potential VFG due to a variation in coupling ratio γ pgm  can be suppressed, and defects such as erroneous write, in which electrons are erroneously injected in a floating gate of a cell non-selected for write or a cell designated for “1” write, or read disturb can be prevented. 
     According to a first aspect of the invention, there is provided a non-volatile semiconductor memory device comprising: 
     a device isolation insulation layer, formed on a semiconductor substrate, for defining a device region; 
     a floating gate formed above the device region and having a pair of first side faces opposed to a side face of the device isolation insulation layer which is located on the device region side; 
     a control gate formed above the floating gate; and 
     a booster electrode having faces opposed to a pair of second surfaces of the floating gate which are substantially perpendicular to the pair of first side faces; 
     wherein a distance between the pair of first side faces of the floating gate is equal or not more than a width of the device region defined by the device isolation insulation layer, and dimensions of the floating gate are determined based on a coupling ratio between the floating gate and the booster electrode. 
     According to a second aspect of the invention, there is provided the device of the first aspect, further comprising: 
     a first insulation film formed between the floating gate and the substrate; 
     a second insulation film formed between the floating gate and the control gate; and 
     a third insulation film formed between the floating gate and the booster electrode. 
     According to a third aspect of the invention, there is provided the device of the first aspect, wherein a plurality of stacked gates each having the floating gate and the control gate are formed on the semiconductor substrate, and 
     the booster electrode is formed between adjacent two of the stacked gates. 
     According to a fourth aspect of the invention, there is provided the device of the first aspect, further comprising a plug for contact with a bit line, the plug being formed of the same conductive material as the booster electrode. 
     According to a fifth aspect of the invention, there is provided the device of the first aspect, further comprising a wiring formed of the same conductive material as the booster electrode. 
     According to a sixth aspect of the invention, there is provided a non-volatile semiconductor memory device comprising: 
     a device isolation insulation layer, formed on a semiconductor substrate, for defining a device region; 
     a floating gate formed above the device region and having a pair of first side faces opposed to a side face of the device isolation insulation layer which is located on the device region side; 
     a control gate formed above the floating gate; and 
     a booster electrode having faces opposed to a pair of second surfaces of the floating gate which are substantially perpendicular to the pair of first side faces, 
     wherein a distance between a pair of first side faces of the floating gate is equal or not more than a width of the device region defined by the device isolation insulation layer, and the control gate comprises: 
     a first conductive film formed above the floating gate; and 
     a second conductive film formed on the first conductive film and the device isolation insulation film. 
     According to a seventh aspect of the invention, there is provided the device of the first aspect, further comprising an insulation layer formed on the control gate. 
     According to an eighth aspect of the invention, there is provided the device of the first aspect, wherein the distance between the pair of first side faces is substantially equal to the width of the device region. 
     According to a ninth aspect of the invention, there is provided the device of the first aspect, wherein the device isolation insulation layer is formed of an insulation material buried in a trench formed on the semiconductor substrate, the trench being self-aligned with the pair of first side faces of the floating gate. 
     According to a tenth aspect of the invention, there is provided a non-volatile semiconductor memory device comprising: 
     a floating gate formed above a semiconductor substrate via a first insulation film; 
     a control gate opposed to a first face of the floating gate via a second insulation film; and 
     a booster electrode opposed to a second face of the floating gate via a third insulation film, 
     wherein a width of the floating gate opposed to the semiconductor substrate via the first insulation film, a width of the floating gate opposed to the control gate via the second insulation film, and a width of the floating gate opposed to the booster electrode via the third insulation film are substantially equal to one another, and dimensions of the floating gate are determined based on a coupling ratio between the floating gate and the booster electrode. 
     According to an eleventh aspect of the invention, there is provided a non-volatile semiconductor memory device comprising: 
     a floating gate formed above a semiconductor substrate via a first insulation film; 
     a control gate opposed to a first face of the floating gate via s second insulation film; and 
     a booster electrode opposed to a second face of the floating gate via a third insulation film, 
     wherein a width of the floating gate opposed to the semiconductor substrate via the first insulation film, a width of the floating gate opposed to the control gate via the second insulation film and a width of the floating gate opposed to the booster electrode via the third insulation film are substantially equal to one another; and 
     a cell array portion, where a plurality of stacked gates in which the floating gate and the control gate are stacked on each other are provided, and the booster electrode is buried between the stacked gates adjacent to each other. 
     According to a twelfth aspect of the invention, there is provided a non-volatile semiconductor memory device comprising: 
     a floating gate formed above a semiconductor substrate via a first insulation film; 
     a control gate opposed to a first face of the floating gate via a second insulation film; and 
     a booster electrode opposed to a second face of the floating gate via a third insulation film, 
     wherein a width of the floating gate opposed to the semiconductor substrate via the first insulation film, a width of the floating gate opposed to the control gate via the second insulation film and a width of the floating gate opposed to the booster electrode via the third insulation film are substantially equal to one another, and 
     the control gate comprises a first portion capacitively coupling with the floating gate via the second insulation film and a second portion for connecting the first portion to another first portion adjacent to the first portion. 
     According to a thirteenth aspect of the invention, there is provided the device of the sixth aspect, further comprising an insulation layer formed on the control gate. 
     According to a fourteenth aspect of the invention, there is provided the device of the sixth aspect, wherein the distance between the pair of first side faces is substantially equal to the width of the device region. 
     According to a fifteenth aspect of the invention, there is provided the device of the tenth aspect, wherein a plurality of stacked gates each having the floating gate and the control gate are formed on the semiconductor substrate, and the booster electrode is formed between adjacent two of the stacked gates. 
     According to a sixteenth aspect of the invention, there is provided the device of the tenth aspect, further comprising a plug for contact with a bit line, the plug being formed of the same conductive material as the booster electrode. 
     According to a seventeenth aspect of the invention, there is provided the device of the tenth aspect, further comprising a wiring formed of the same conductive material as the booster electrode. 
     According to an eighteenth aspect of the invention, there is provided the device of the tenth aspect, further comprising an insulation layer formed on the control gate. 
     According to a nineteenth aspect of the invention, there is provided the device of the tenth aspect, further comprising a device isolation insulation layer, formed on the semiconductor substrate, for defining a device region, wherein the distance between the first face and the second face of the floating gate is substantially equal to the width of the device region. 
     According to a twentieth aspect of the invention, there is provided the device of the nineteenth aspect, wherein the device isolation insulation layer is formed of an insulation material buried in a trench formed on the semiconductor substrate, the trench being self-aligned with the first face and the second face of the floating gate. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinbefore. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments give below, serve to explain the principles of the invention. 
     FIG. 1A is a plan view of a conventional memory cell; 
     FIG. 1B is a cross-sectional view taken along line B—B in FIG. 1A; 
     FIG. 1C is a cross-sectional view taken along line C—C in FIG. 1A; 
     FIG. 2A is an equivalent circuit diagram of a conventional EEPROM; 
     FIG. 2B shows a relationship between node potentials in the write mode; 
     FIG. 3 is a bird&#39;s eye view of a conventional floating gate; 
     FIG. 4 is a graph showing the dependency of a coupling ratio upon the width of a device region; 
     FIG. 5A is a plan view of a memory cell according to a first embodiment of the invention; 
     FIG. 5B is a cross-sectional view taken along line  5 B— 5 B in FIG. 5A; 
     FIG. 5C is a cross-sectional view taken along line  5 C— 5 C in FIG. 5A; 
     FIG. 6 is a bird&#39;s eye view of a floating gate of the memory cell according to the present invention; 
     FIG. 7A is an equivalent circuit diagram of an EEPROM having the memory cell according to the first embodiment of the invention; 
     FIG. 7B shows a relationship between node potentials in the write mode; 
     FIG. 7C shows a relationship between node potentials in the read mode; 
     FIG. 7D shows a relationship between node potentials in the erase mode; 
     FIG. 8A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 8B is a cross-sectional view taken along line  8 B— 8 B in FIG. 8A; 
     FIG. 8C is a cross-sectional view taken along line  8 C— 8 C in FIG. 8A; 
     FIG. 9A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 9B is a cross-sectional view taken along line  9 B— 9 B in FIG. 9A; 
     FIG. 9C is a cross-sectional view taken along line  9 C— 9 C in FIG. 9A; 
     FIG. 10A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 10B is a cross-sectional view taken along line  10 B— 10 B in FIG. 10A; 
     FIG. 10C is a cross-sectional view taken along line  10 C— 10 C in FIG. 10A; 
     FIG. 11A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 11B is a cross-sectional view taken along line  11 B— 11 B in FIG. 11A; 
     FIG. 11C is a cross-sectional view taken along line  11 C— 11 C in FIG. 11A; 
     FIG. 12A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 12B is a cross-sectional view taken along line  12 B— 12 B in FIG. 12A; 
     FIG. 12C is a cross-sectional view taken along line  12 C— 12 C in FIG. 12A; 
     FIG. 13A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 13B is a cross-sectional view taken along line  13 B— 13 B in FIG. 13A; 
     FIG. 13C is a cross-sectional view taken along line  13 C— 13 C in FIG. 13A; 
     FIG. 14A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 14B is a cross-sectional view taken along line  14 B— 14 B in FIG. 14A; 
     FIG. 14C is a cross-sectional view taken along line  14 C— 14 C in FIG. 14A; 
     FIG. 15A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 15B is a cross-sectional view taken along line  15 B— 15 B in FIG. 15A; 
     FIG. 15C is a cross-sectional view taken along line  15 C— 15 C in FIG. 15A; 
     FIG. 16A is a plan view illustrating a manufacturing step of the memory cell according to the first embodiment of the invention; 
     FIG. 16B is a cross-sectional view taken along line  16 B— 16 B in FIG. 16A; 
     FIG. 16C is a cross-sectional view taken along line  16 C— 16 C in FIG. 16A; 
     FIG. 17A is a plan view of a memory cell according to a second embodiment of the invention; 
     FIG. 17B is a cross-sectional view taken along line  17 B— 17 B in FIG. 17A; 
     FIG. 17C is a cross-sectional view taken along line  17 C— 17 C in FIG. 17A; 
     FIG. 18A is an equivalent circuit diagram of an EEPROM having the memory cell according to the second embodiment of the invention; 
     FIG. 18B shows a relationship between node potentials in the write mode; 
     FIG. 18C shows a relationship between node potentials in the read mode; 
     FIG. 18D shows a relationship between node potentials in the erase mode; 
     FIG. 19A is a cross-sectional view of the memory cell according to the first embodiment of the invention; 
     FIG. 19B is a cross-sectional view of the memory cell according to the second embodiment of the invention; 
     FIG. 20A is a plan view illustrating a manufacturing step of the memory cell according to the second embodiment of the invention; 
     FIG. 20B is a cross-sectional view taken along line  20 B— 20 B in FIG. 20A; 
     FIG. 20C is a cross-sectional view taken along line  20 C— 20 C in FIG. 20A; 
     FIG. 21A is a plan view illustrating a manufacturing step of the memory cell according to the second embodiment of the invention; 
     FIG. 21B is a cross-sectional view taken along line  21 B— 21 B in FIG. 21A; 
     FIG. 21C is a cross-sectional view taken along line  21 C— 21 C in FIG. 21A; 
     FIG. 22A is a plan view illustrating a manufacturing step of the memory cell according to the second embodiment of the invention; 
     FIG. 22B is a cross-sectional view taken along line  22 B— 22 B in FIG. 22A; 
     FIG. 22C is a cross-sectional view taken along line  22 C— 22 C in FIG. 22A; 
     FIG. 23A is a plan view of a memory cell according to a third embodiment of the invention; 
     FIG. 23B is a cross-sectional view taken along line  23 B— 23 B in FIG. 23A; 
     FIG. 23C is a cross-sectional view taken along line  23 C— 23 C in FIG. 23A; 
     FIG. 24A is a cross-sectional view of the memory cell according to the second embodiment of the invention; 
     FIG. 24B is a cross-sectional view of the memory cell according to the third embodiment of the invention; 
     FIG. 25A is a plan view illustrating a manufacturing step of the memory cell according to the third embodiment of the invention; 
     FIG. 25B is a cross-sectional view taken along line  25 B— 25 B in FIG. 25A; 
     FIG. 25C is a cross-sectional view taken along line  25 C— 25 C in FIG. 25A; 
     FIG. 26A is a plan view illustrating a manufacturing step of the memory cell according to the third embodiment of the invention; 
     FIG. 26B is a cross-sectional view taken along line  26 B— 26 B in FIG. 26A; 
     FIG. 26C is a cross-sectional view taken along line  26 C— 26 C in FIG. 26A; 
     FIG. 27A is a plan view illustrating a manufacturing step of the memory cell according to the third embodiment of the invention; 
     FIG. 27B is a cross-sectional view taken along line  27 B— 27 B in FIG. 27A; 
     FIG. 27C is a cross-sectional view taken along line  27 C— 27 C in FIG. 27A; 
     FIG. 28A is a plan view illustrating a manufacturing step of the memory cell according to the third embodiment of the invention; 
     FIG. 28B is a cross-sectional view taken along line  28 B— 28 B in FIG. 28A; 
     FIG. 28C is a cross-sectional view taken along line  28 C— 28 C in FIG. 28A; 
     FIG. 29A is a plan view illustrating a manufacturing step of the memory cell according to the third embodiment of the invention; 
     FIG. 29B is a cross-sectional view taken along line  29 B— 29 B in FIG. 29A; 
     FIG. 29C is a cross-sectional view taken along line  29 C— 29 C in FIG. 29A; 
     FIG. 30A is a plan view illustrating a manufacturing step of the memory cell according to the third embodiment of the invention; 
     FIG. 30B is a cross-sectional view taken along line  30 B— 30 B in FIG. 30A; 
     FIG. 30C is a cross-sectional view taken along line  30 C— 30 C in FIG. 30A; 
     FIG. 31A is a plan view of a memory cell according to a fourth embodiment of the invention; 
     FIG. 31B is a cross-sectional view taken along line  31 B— 31 B in FIG. 31A; 
     FIG. 31C is a cross-sectional view taken along line  31 C— 31 C in FIG. 31A; 
     FIG. 32A is a plan view illustrating a manufacturing step of the memory cell according to the fourth embodiment of the invention; 
     FIG. 32B is a cross-sectional view taken along line  32 B— 32 B in FIG. 32A; 
     FIG. 32C is a cross-sectional view taken along line  32 C— 32 C in FIG. 32A; 
     FIG. 33A is a plan view of a memory cell according to a fifth embodiment of the invention; 
     FIG. 33B is a cross-sectional view taken along line  33 B— 33 B in FIG. 33A; 
     FIG. 33C is a cross-sectional view taken along line  33 C— 33 C in FIG. 33A; 
     FIG. 34A is a plan view illustrating a manufacturing step of the memory cell according to the fifth embodiment of the invention; 
     FIG. 34B is a cross-sectional view taken along line  34 B— 34 B in FIG. 34A; and 
     FIG. 34C is a cross-sectional view taken along line  34 C— 34 C in FIG.  34 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described with reference to the accompanying drawings, referring to a NAND type EEPROM as an example. In the drawings, common parts are denoted by like reference numerals, and an overlapping description will be omitted. 
     FIG. 5A is a plan view of an EEPROM cell according to a first embodiment of the invention, FIG. 5B is a cross-sectional view taken along line  5 B— 5 B in FIG. 5A, and FIG. 5C is a cross-sectional view taken along line  5 C— 5 C in FIG.  5 A. For the purpose of simple description, FIG. 5A does not show the bit line and the underlying interlayer insulation film. 
     As is shown in FIGS. 5A to  5 C, device isolation insulation films  2  are formed in a surface portion of a P-type silicon substrate (BULK)  1 . Device regions  3  are defined on a surface of the substrate  1  by the device isolation insulation films  2 . 
     A tunnel insulation film  4 , a floating gate (FG)  5 , an insulation film  6  and a control gate (word line WL)  7  are successively formed in a stacked-gate structure. Reference numeral  8  denotes a gate of a select transistor. 
     N-type diffusion layers  9 ,  10  and  11  are formed in the device region  3 . The diffusion layer  9  is connected to a source line (SL) (not shown), and the diffusion layer  10  is to a bit line (BL)  12 . 
     The number of diffusion layers  11  is two or more and these layers  11  function as channels  13  of memory cell transistors (MC), respectively. The control gate  7  crosses over the channels  13  and are capacitively coupled to the channels  13  through the floating gates  5 . 
     A booster electrode insulating film  14  is formed over the periphery of the stacked-gate structure and is formed on the diffusion layers  11 , respectively. A booster electrode  15  is formed on the insulating film  14 . An interlayer insulation film  44  is formed on the booster electrode  15 . 
     FIG. 6 is a bird&#39;s eye view of the floating gate shown in FIGS. 5A to  5 C. 
     The cell of this invention is characterized in that, as shown in FIG. 6, the width of a plane  21  of the floating gate FG opposed to the channel  13  with tunnel insulating film  4  interposed, the width of a plane  22  opposed to the word line WL with insulation film  6  interposed and the width of a plane  23  opposed to the booster electrode  15  with booster electrode insulating film  14  interposed are equal to one another. 
     The three widths are the width “b” of the floating gate  5  along the word line. The width “b” is equal to the width “d” of device region  3  between the device isolation regions  2 . Therefore, the effective coupling ratio γ pgm in the write mode is different from the ratio in the prior art, i.e. 
     
       
         γ  pgm= ( C 2+ C 3)/( C 1 +C 2 +C 3)=[{( b+ 2 c ) a /tox2}+{(2 b·c )/tox3}]/[{( a·d )/tox1}+{( b+ 2 c ) a/ tox2}+{(2 b·c )/tox3}] 
       
     
     and is expressed by 
     
       
         γ  pgm= ( C 2 +C 3)/( C 1 +C 2 +C 3)=[{( d·a )/tox2}+{(2 d·c )/tox3}]/[{( a·d )/tox1}+{( a·d )/tox2}+{(2 d·c )/tox3}] 
       
     
     Accordingly, the coupling ratio does not depend on the width “d” (“b”). 
     In the above equation, suppose that the dimension of the floating gate  5  along the bit line is “a”, the dimension of floating gate  5  along the word line is “b”, the height of floating gate  5  is “c”, and the width of the device region is “d”. In addition, suppose that the thickness of the tunnel insulation film  4  between the substrate  1  and floating gate  5  is “tox1”, the thickness of the insulation film  6  between the floating gate  5  and control gate  7  is “tox2”, and the thickness of the booster electrode insulating film  14  between the floating gate  5  and booster electrode  15  is “tox3.” 
     Similar with the above-described prior art, the capacitances C1, C2 and C3 are the capacitance between the substrate  1  and floating gate  5 , the capacitance between the floating gate  5  and control gate  7  and the capacitance between the floating gate  5  and booster electrode  15 , respectively. 
     Since the coupling ratio γ pgm does not depend on the width “d”, the variance in coupling ratio γ pgm is not greatly influenced even by the width “d” of device region  3  varies due to a processing variance. 
     In the present invention, as regards the factors of the variance in coupling ratio γ pgm, in particular, the variance in width “d” of device region  3 , which is one of the factors, can be eliminated. In this invention, the variance in coupling ratio γ pgm can be reduced accordingly, compared to the prior art. 
     Still more, even if the width “d” along a word line of the floating gates is not equal to the width “d” of the device region  3 , if a side surface along a column direction of the floating gates  5  opposes to the device isolation regions  2  but does not oppose to control gate  7 , as shown in FIG. 6, the condition which is not depending on width “d” of the device region  3  is satisfied as described above with regard to the coupling ratio γ pgm. In other word, in the case where a relation between the width “b” along a word line of the floating gates  5  and width “d” of the device region  3  is set to be “b≦d”, it should suffice if a surface along a column direction of the floating gates  5  is opposed to the device isolation region  2 . 
     Since the variance in coupling ratio γ pgm is reduced, the possibility of occurrence of a cell in which electrons are easily injected decreases, compared to the prior art. Defects such as erroneous write or read disturb can be more prevented than in the prior art. 
     An operation method of the NAND type EEPROM according to the first embodiment of the invention will now be described. FIG. 7A is an equivalent circuit diagram of the EEPROM, FIG. 7B shows a relationship between node potentials in a write mode, FIG. 7C shows a relationship between node potentials in a read mode, and FIG. 7D shows a relationship between node potentials in a erase mode. For the purpose of simple description, FIG. 7A shows the case where two word lines (WL 1 , WL 2 ) and two bit lines (BL 1 , BL 2 ) are provided. 
     At first the write operation will be described. 
     The potential of the selected word line WL 1  is set at 13 V, the potential of the booster electrode BP is at 13 V, the potential of the bit line BL 1  designated for “0” write is at 0 V, the potential of the drain-side select gate line SG 1  is at 3.3 V, the potential of the source-side select gate line SG 2  is at 0 V, and the potential of the non-selected word line WL 2  is at 3.3 V. 
     At this time, the potentials of both the write-selected word line WL 1  and booster electrode BP are 13 V. Although the gate potential of the cell MC 11  having the gate connected to the word line WL 1  is 13 V, the effective coupling ratio γ pgm in the write mode is increased to “0.78” by the booster electrode BP, and a potential of about 10 V is applied to the tunnel insulation film. 
     Accordingly, even if the write potential is 13 V, electrons are injected into the floating gate FG 11  through the tunnel insulation film about 10 nm thick. Thus, “0” write is effected in the cell MC 11 . 
     On the other hand, the gate potential of the cell MC 21  belonging to the same bit line BL 1  and having the gate connected to the non-selected word line WL 2  is 3.3 V, and the potential of the booster electrode BP is 13 V. At this time, the voltage of 3.3 V applied to the word line WL 2  acts to lower the potential of the floating gate FG 21 . Thus, no electrons are injected in the floating gate FG 21 . 
     On the other hand, the potential of the bit line BL 2  designated for “1” write is 3.3 V. Since the potential of the drain-side select gate line SG 1  is 3.3 V at this time, the select transistor ST 12  is cut off when the potential of “3.3 V-VthST” has been transferred to the N-type diffusion layer  7 . As a result, the cell channel  16  including the diffusion layer  11  shown in FIG.  5 B and channel  13  is set in the floating state. 
     In this case, “VthST” is a threshold voltage of the select transistor ST 12 . At this time the potential of the cell channel  16  is raised by the potential of booster electrode BP. 
     The potential, 13 V, of the selected word line WL 1  contributes to raising the potential of cell channel  16  through the floating gate FG 12 . In this manner the potential of cell channel  16  is raised up to about 8 V. 
     In the cell MC 12  having the gate connected to the selected word line WL 1 , a potential difference between the channel thereof and the word line WL 1  decreases to “13 V−8 V=5 V” and no electrons are injected in the floating gate FG 12 . 
     Thus, data “1” is written in the cell MC 12 . As described above, in the EEPROM having the booster electrode BP, the potential of the cell channel  16  is greatly raised up to about 8 V in the write-selected cell MC 12  connected to the bit line BL 2  designated for “1” write. 
     In addition, in the cell MC 22  having the gate connected to the non-selected word line WL 2 , a potential difference between the channel thereof and the word line WL 2  is “3.3 V−8 V=−4.7 V” and no electrons are injected in the floating gate FG 22 . 
     Next, the read mode will be described. 
     The potential of the word line WL 1  selected for data read (read-selected word line WL 1 ) is set at 0 V, and the potentials of the booster electrode BP, drain-side select gate line SG 1  and source-side select gate line SG 2  are set at 3.3 V, respectively. 
     The non-selected word line WL 2  is set at a potential at which it is turned on independently of the state of the threshold voltage of the cell MC  21 , MC 22 . In this embodiment, this potential is 3.3 V. 
     Since the cell MC 11  is “0”-written (electrons being injected), its threshold voltage is 0 V or above. Since the cell MC 12  is “1”-written (no electrons being injected), its threshold voltage is 0 V or less. 
     Since the potential of the read-selected word line WL 1  is 0 V, the cell MC 11  is turned off and the cell MC 12  is turned on. Thereby, the potentials of the bit lines BL 1  and BL 2 , which are pre-charged prior to data read, are at “H” level (non-discharged) and at “L” level (discharged), respectively. 
     These potentials are amplified by sense amplifiers (not shown), and thus data “0” is read out from the cell MC 11  and data “1” is read out from the cell MC 12 . 
     The erase operation will now be described. 
     The potential of the word line WL 1  selected for data erase (erase-selected word line WL 1 ) and the potential of the booster electrode BP) are set at 0 V. The bit lines BL 1  and BL 2 , source-side select gate line SG 1 , drain-side select gate line SG 2 , source line SL and non-selected word line WL 2  are set in the floating state. 
     The potential of the substrate BULK is set at 13 V. Thus, a positive voltage relative to the floating gates FG 11  and FG 21  is applied to the substrate BULK, and electrons injected in the floating gate FG 11  is released to the substrate BULK. Accordingly, the data in the cells MC 11  and MC 21  is erased. 
     As regards the cells MC 12  and MC 22 , since the word line WL 2  is in the floating state, the potential of the word line WL 2  is coupled to the substrate BULK and increased. 
     As a result, the electrons injected in the floating gates FG 12  and FG 22  are not released. Of course, if the potential of the word line WL 2  is set at 0 V, the data in the cells MC 11 , MC 21 , MC 12  and MC 22  can be erased at a time. 
     A method of fabricating the EEPROM according to the first embodiment will now be described. 
     FIGS. 8 to  16  illustrate principal manufacturing steps of the EEPROM according to the first embodiment of the invention. In FIGS. 8 to  16 , each FIG. A is a plan view, each FIG. B is a cross-sectional view taken along line B—B in FIG. A, and each FIG. C is a cross-sectional view taken along line C—C in FIG. A. 
     As is shown in FIGS. 8A to  8 C, a first stacked-film structure  34  is formed on a P-type silicon substrate  1 . The first stacked-film structure  34  comprises a silicon dioxide film  31  which will become a tunnel insulation film, a conductive polysilicon layer  32  which will become a floating gate, and a silicon nitride film  33  which will become a mask in forming a device isolation trench. 
     That portion of the silicon nitride film  33 , which corresponds to the trench, is removed, and the silicon nitride film  33  is patterned in accordance with the device region. Then, using the silicon nitride mask  33  as a mask, the substrate  1  is etched. The device region  3  is formed in a self-alignment manner at the left portion of the first stacked-film structure  34 , and the device isolation trench  35  is formed in the substrate  1 . 
     As is shown in FIGS. 9A to  9 C, silicon dioxide is deposited on the structure shown in FIGS. 8A to  8 C, and a silicon dioxide film to be buried in the trench  35  is formed. 
     The silicon dioxide film is subjected to chemical mechanical polishing (CMP), and the silicon nitride film is buried in the trench  35  and the device isolation region  2  is formed. Then, the silicon nitride film  33 , if it is left, is removed. 
     Subsequently, as shown in FIGS. 10A to  10 C, an ONO film  37  which will become an insulation film is formed by successively depositing silicon dioxide, silicon nitride, and silicon dioxide on the structure shown in FIGS. 9A to  9 C. 
     That portion of the ONO film  37 , which will become the gate of the select gate transistor, is removed and a conductive polysilicon film  38  which will become the word line (control gate) is deposited. 
     Then, as shown in FIGS. 11A to  11 C, the film structure including the silicon dioxide film  31 , conductive polysilicon film  32 , ONO film  37  and conductive polysilicon film  38  is patterned to have a word line pattern, and a stacked-gate structure  40  including the tunnel insulation film  4 , floating gate  5 , film  6  and word line  7  is formed. 
     At this time, the floating gate  5  is formed on the device region  3  in a self-alignment manner. In addition, in the region of the select gate transistor, a gate structure  41  wherein the insulation film  6  is not provided and the floating gate  5  and word line  7  are electrically connected is formed. 
     As is shown in FIGS. 12A to  12 C, using the stacked-gate structure  40 , gate structure  41  and device isolation region  2  as a mask, N-type impurities are ion-implanted in the device regions  3  and then diffused to form N-type diffusion layers  9 ,  10  and  11 . 
     As is shown in FIGS. 13A to  13 C, silicon dioxide is deposited on the structure shown in FIGS. 12A to  12 C and the booster electrode insulation film  14  is formed. 
     As is shown in FIGS. 14A to  14 C, conductive polysilicon is deposited on the booster electrode insulation film  14  and a conductive film  42  serving as a booster electrode is formed. 
     As is shown in FIGS. 15A to  15 C, the conductive film  42  is patterned in a booster electrode pattern and a booster electrode  15  is formed. In FIGS. 15A to  15 C, reference numeral  43  denotes a mask layer formed of a photoresist in accordance with the booster electrode pattern. 
     Subsequently, as shown in FIGS. 16A to  16 C, silicon dioxide is deposited on the structure shown in FIGS. 15A to  15 C and a first interlayer insulation film  44  is formed. 
     Then, a source line contact hole (not shown) communicating with the diffusion layer  9 , a bit line contact hole  45  communicating with the diffusion layer  10 , and a booster electrode control line contact hole (not shown) communicating with the booster electrode  15  are formed in the interlayer insulation film  44 . Following this, a source line (not shown) and a booster electrode control line (not shown) are formed at the bit line. 
     At last, a second interlayer insulation film  46  is formed, and the fabrication of the EEPROM cell according to the first embodiment is completed. 
     An EEPROM cell according to a second embodiment of the invention will now be described. 
     FIG. 17A is a plan view of the EEPROM cell according to the second embodiment, FIG. 17B is a cross-sectional view taken along line  17 B— 17 B in FIG.  17 A and FIG. 17C is a cross-sectional view taken along line  17 C— 17 C in FIG.  17 A. For the purpose of simple description, FIG. 17A does not show the bit line and the underlying interlayer insulation film. 
     In the second embodiment, as shown in FIGS. 17A to  17 C, booster electrodes  15  are buried between stacked-gate structures  40  and between the stacked-gate structure  40  and gate structure  41 , and the booster electrodes  15  are formed in a wiring shape in the cell array. 
     In FIGS. 17A to  17 C, the booster electrodes  15  with the wiring shape are denoted by numerals  15 - 1  to  15 - 3 . Hereinafter, these electrodes  15  are referred to as wiring-type boosters. 
     FIG. 18A is an equivalent circuit diagram of the EEPROM having the memory cell according to the second embodiment, FIG. 18B shows a relationship between node potentials in the write mode, FIG. 18C shows a relationship between node potentials in the read mode, and FIG. 18D shows a relationship between node potentials in the erase mode. 
     In the equivalent circuit shown in FIG. 18A, there are provided a first wiring-type booster electrode BP 1  formed between select transistors ST 11 , ST 12  and cells MC 11 , MC 12 , a second wiring-type booster electrode BP 2  formed between cells MC 11 , MC 12  and cells MC 21 , MC 22 , and a third wiring-type booster electrode BP 3  formed between select transistors ST 21 , ST 22  and cells MC 21 , MC 22 . 
     However, if the first to third wiring-type booster electrodes BP 1  to BP 3  are controlled simultaneously as one booster electrode BP, the same operations as in the first embodiment can be performed, as shown in FIGS. 18B to  18 D. 
     In order to simultaneously control the first to third wiring-type booster electrodes BP 1  to BP 3  as single booster electrode BP, it is possible, for example, to interconnect the first to third booster electrodes BP 1  to BP 3  at an end portion of the cell array by means of patterning, or to interconnect them by using other wiring elements. 
     In the second embodiment, like the first embodiment, a variance in coupling ratio γ pgm decreases. In addition, compared to, e.g. the cell of the first embodiment shown in FIG. 19A, the depth “f” of contact hole  45  can be decreased since the booster electrode is not present between the word line and bit line, as shown in FIG.  19 B. 
     Since the aspect ratio “f/e” (“e” indicating the dimension of opening of contact hole) of the bit line contact hole  45  can be reduced, the cell can be effectively miniaturized. 
     In the cell of the first embodiment, as shown in FIG. 19A, the word line  7  has three surfaces opposed to the booster electrode  15 , i.e. side surfaces “g” and “h” and upper surface “i” of the word line  7 . 
     By contrast, in the cell of the second embodiment, as shown in FIG. 19B, only the side surfaces “g” and “h” of the word line  7  are opposed to the booster electrode. Thus, compared to the cell shown in FIG. 19A, a parasitic capacitance around the word line  7  can be reduced. 
     Since the parasitic capacitance of word line  7  is reduced, the rise time of word line  7  (i.e. time needed to charge the word line from 0 V to a predetermined potential) and the fall time of word line  7  (i.e. time needed to discharge the word line from a predetermined potential to 0 V) can be shortened. 
     Since these times can be shortened, the cell of the second embodiment can perform write, read and erase operations at higher speed. 
     According to the structure of the second embodiment, the first to third wiring-type booster electrodes BP 1  to BP 3  are independently formed. This structure can thus be modified so that the first to third wiring-type booster electrodes BP 1  to BP 3  may be independently controlled. 
     The method of fabricating the EEPROM of the second embodiment will now be described. 
     FIGS. 20 to  22  illustrate principal steps of fabricating the EEPROM according to the second embodiment. In FIGS. 20 to  22 , each FIG. A is a plan view, each FIG. B is a cross-sectional view taken along line B—B in FIG. A, and each FIG. C is a cross-sectional view taken along line C—C in FIG. A. 
     According to the steps shown in FIGS. 8 to  14 , the conductive polysilicon is deposited on the booster electrode insulation film  14  and the conductive film  42  which becomes booster electrodes is formed. 
     Subsequently, as shown in FIGS. 20A to  20 C, the surface of the conductive film  42  is etched back by chemical mechanical polishing (CMP) or RIE. Thus, the conductive film  42  is buried only in trenches between the stacked-gate structure  40  and gate structure  41 . 
     Then, as shown in FIGS. 21A to  21 C, that portion of the buried conductive film  42 , which lies on the diffusion layers  9 ,  10 , are removed. Reference numeral  43  denotes a mask layer of a photoresist. Thereby, wiring-type booster electrodes  15 - 1  to  15 - 3  are formed over the diffusion layers  11  with the booster electrode insulating film  14  interposed. 
     As is shown in FIGS. 22A to  22 C, silicon dioxide is deposited on the structure shown in FIGS. 21A to  21 C and a first interlayer insulation film  44  is formed. Then, a source line contact hole (not shown) communicating with the diffusion layer  9 , a bit line contact hole  45  communicating with the diffusion layer  10 , and a booster electrode control line contact hole (not shown) communicating with the booster electrode  15  are formed in the interlayer insulation film  44 . Following this, a bit line, a source line (not shown) and a booster electrode control line (not shown) are formed. 
     At last, a second interlayer insulation film  46  is formed, and the fabrication of the EEPROM cell according to the second embodiment is completed. 
     An EEPROM cell according to a third embodiment of the invention will now be described. 
     FIG. 23A is a plan view of the EEPROM cell according to the third embodiment, FIG. 23B is a cross-sectional view taken along line B—B in FIG.  23 A and FIG. 23C is a cross-sectional view taken along line C—C in FIG.  23 A. For the purpose of simple description, FIG. 23A does not show the bit line and the underlying interlayer insulation film. 
     As is shown in FIGS. 23A to  23 C, in the third embodiment, like the second embodiment, booster electrodes  15  are buried between the stacked-gate structures  40  and between the stacked-gate structure  40  and gate structure  41 . Thus, booster electrodes  15 - 1  to  15 - 3  having a wiring shape are formed in the cell array. 
     In addition, the conductive film forming the booster electrodes  15 - 1  to  15 - 3  is left on the source diffusion layer  9  and drain diffusion layer  10 , and a source wiring  51  and a bit line contact plug  52  formed of the same conductor as the booster electrodes  15 - 1  to  15 - 3  are formed. 
     The source wiring  51  is formed in a wiring shape similarly with the booster electrodes  15 - 1  to  15 - 3  and is connected to the diffusion layer  9 . In this case, the diffusion layer  9  may be formed in a line shape along the intervening region between the gate structures  41  or may be separated for each NAND cell. 
     The plug  52  is formed in an island shape and connected to the diffusion layer  10 . In this case, the diffusion layer  9  is separated for each NAND cell connected to one bit line. 
     According to the third embodiment, like the second embodiment, the variance in coupling ratio γ pgm can be reduced and a parasitic capacitance in the word line  7  can be decreased. 
     As is shown in FIG. 24B, the plug  52  is provided at a contact portion between the diffusion layer  10  and bit line  12 . Accordingly, compared to the cell of the second embodiment shown in FIG. 24A, for example, the depth “f” of contact hole  45  can be further reduced. Therefore, the aspect ratio “f/e” of the bit line contact hole  45  can be further reduced and the cell can be effectively miniaturized. 
     A method of fabricating the EEPROM of the third embodiment will now be described. 
     FIGS. 25 to  30  illustrate in succession the principal steps of fabricating the EEPROM according to the third embodiment. In FIGS. 25 to  30 , each FIG. A is a plan view, each FIG. B is a cross-sectional view taken along line B—B in FIG. A, and each FIG. C is a cross-sectional view taken along line C—C in FIG. A. 
     According to the steps shown in FIGS. 8 to  12 , the stacked-gate structures  40  and gate structures  41  are formed and the N-type diffusion layers  9 ,  10  and  11  are formed. 
     Then, as shown in FIGS. 25A to  25 C, a booster electrode insulating film  14  is formed and a first conductive film  53  is thinly deposited on the booster electrode insulating film  14 . The first conductive film is formed of, e.g. conductive polysilicon. 
     In this manufacturing method, the pitch “i” between the gate structures  41 , at which the N-type diffusion layer (source)  9  is formed, and the pitch “j” between the gate structures  41 , at which the N-type diffusion layer (drain)  10  is formed, are made substantially equal to the pitch “g” between the gate structure  41  and stacked-gate structure  40 , at which the N-type diffusion layer (source/drain of the cell)  11  is formed, and the pitch “h” between the stacked-gate structures  40 , respectively. 
     The reason for this is that if the pitch “i” of the region for formation of the source wiring, the pitch “j” of the region for formation of the plug, and the pitches “g” and “h” of the regions for formation of booster electrodes are equalized, all trenches formed between the stacked-gate structures  40  and gate structures  41  can be easily filled with a conductor. 
     Since contact holes for contact with the substrate  1  are not formed in the regions with pitches “g” and “h”, these pitches can be set at a minimum value. If the pitches “i” and “j” of the regions conventionally having contact holes are made to agree with the pitches “g” and “h”, the degree of density of stacked-gate structures  40  and gate structures  41  is increased in the cell array section. 
     In addition, since the stacked-gate structures  40  and gate structures  41  are patterned to alternately appear at regular intervals, the pitches thus determined contributes to finer processing. Although it is desirable that the pitches “g”, “h”, “i” and “j” are equalized, the pitches “i” and “j” of the regions for formation of the source wiring and plug may be greater than the pitches “g” and “h” of the regions for formation of the booster electrodes, as in the first and second embodiments. 
     As is shown in FIG. 25A by reference symbol “k”, the N-type diffusion layer  9 , like the N-type diffusion layer  10 , is isolated for each NAND cell connected to one bit line, i.e. for each column. 
     The reason for this is that in the third embodiment, even if the N-type diffusion layers  9  are isolated, these may be interconnected later by means of source wiring. If this patterning is adopted, the conventional mesh-like pattern of device regions  3  may be changed to a simple line-and-space pattern, and finer processing can be performed. 
     Although it is desirable that the N-type diffusion layer  9 , like the N-type diffusion layer  10 , be isolated for each column, the N-type diffusion layer  9  may be formed in one region along the intervening regions among the gate structures  40 , as in the first and second embodiments. 
     Following the above steps, a mask layer  54  of a photoresist is formed on the conductive film  53 , as shown in FIGS. 26A to  26 C. Then, linear windows  55  and  56  corresponding to the intervening regions of the gate structures  41  are formed in the mask layer  54 . Using the mask layer  54  as an etching mask, the booster electrode insulating film  14  is removed and the surfaces of the N-type diffusion layers  9  and  10  are exposed. 
     As is shown in FIGS. 27A to  27 C, after the mask layer  54  is removed, a second conductive film  57  is deposited and filled in recesses between the stacked-gate structures  40  and gate structures  41 . The second conductive film  57  is formed of, e.g. tungsten. 
     The first conductive film  53  and second conductive film  57  constitute a so-called “poly-metal structure film”  58 . In this case, the second conductive film  57  is put in electrical contact with the N-type diffusion layers  9  and  10 . 
     Subsequently, as shown in FIGS. 28A to  28 C, the surface of the poly-metal structure film  58  is etched back by chemical mechanical polishing (CMP) or RIE. Thus, the poly-metal structure film  58  is buried only in trenches between the stacked-gate structure  40  and gate structure  41 . 
     As is shown in FIGS. 29A to  29 C, a mask layer  59  of a photoresist is formed on the structure shown in FIGS. 28A to  28 C. Then, windows  60  corresponding to slit portions for isolating the poly-metal structure films  58  for respective N-type diffusion layers  10  are formed in the mask layer  59 . 
     Using the mask layer  59  as an etching mask, the poly-metal structure film  58  is removed and isolated for each N-type diffusion layer  10 . Thus, the poly-metal structure film  58  is formed into the source wiring  51 , plug  52  and wiring-type booster electrodes  15 - 1  to  15 - 3 . 
     As is shown in FIGS. 30A to  30 C, after the mask layer  59  is removed, a first interlayer insulation film  44  is formed, and a bit line contact hole  45  communicating with the plug  52 , a source line contact hole (not shown) communicating with the source wiring  51  and a booster electrode control line contact hole (not shown) communicating with the wiring-type booster electrodes  15 - 1  to  15 - 3  are formed in the interlayer insulation film  44 . Following this, a bit line BL, a source line (not shown) and a booster electrode control line (not shown) are formed. 
     At last, a second interlayer insulation film  46  is formed, and the fabrication of the EEPROM cell according to the third embodiment is completed. 
     An EEPROM cell according to a fourth embodiment of the invention will now be described. 
     FIG. 31A is a plan view of the EEPROM cell according to the fourth embodiment, FIG. 31B is a cross-sectional view taken along line  31 B— 31 B in FIG.  31 A and FIG. 31C is a cross-sectional view taken along line  31 C— 31 C in FIG.  31 A. For the purpose of simple description, FIG. 31A does not show the bit line and the underlying interlayer insulation film. 
     As is shown in FIGS. 31A to  31 C, in the fourth embodiment, the word line  7  has a stacked-structure comprising a first conductive film  61  and a second conductive film  62  formed on the first conductive film  61 , and the first conductive film  61  is not provided on the device isolation insulation layer  2 . 
     FIGS. 31A to  31 C show the fourth embodiment as having the structure including wiring-type booster electrodes  15 - 1  to  15 - 3 , like the second embodiment. However, needless to say, the structure of the fourth embodiment can be applied to the cell of the first embodiment with the booster electrode  15  covering the stacked-gate structure  41  or to the cell of the third embodiment with the source wiring  51  and plug  52  formed of the same conductor as the wiring-type booster electrodes  15 - 1  to  15 - 3 . 
     A method of fabricating the EEPROM cell of the fourth embodiment will now be described. 
     FIGS. 32A to  32 C illustrate in succession the principal steps of fabricating the EEPROM according to the fourth embodiment. FIG. 32A is a plan view, FIG. 32B is a cross-sectional view taken along line  32 B— 32 B in FIG. 32A, and FIG. 32C is a cross-sectional view taken along line  32 C— 32 C in FIG.  32 A. 
     According to the method illustrated in FIGS. 8A to  8 C, the silicon dioxide film  31  which becomes the tunnel insulation film and the conductive polysilicon layer  32  which becomes the floating gate are formed on the P-type silicon substrate  1 . 
     The ONO film  37  which becomes the insulation film is formed on the conductive polysilicon layer  32 . That portion of the ONO film  37 , which corresponds to the region of the select transistor, is removed. 
     As is shown in FIGS. 32A to  32 C, the first conductive film  61  is formed, thereby forming a first stacked-film structure comprising the silicon dioxide film  31 , conductive polysilicon layer  32 , ONO film  37  and first conductive film  61 . 
     The first conductive film is formed of a conductive polysilicon. Then, a silicon nitride film (not shown), which serves as an etching mask in forming the device isolation trench, is formed and the silicon nitride film (not shown) is patterned in accordance with the device region. 
     Subsequently, using the silicon nitride film (not shown) as a mask, the substrate  1  is etched and the device region  3  and device isolation trench  35  which are self-aligned with the remaining portion of the first stacked-film structure are formed on the substrate  1 . 
     The trench  35  is then filled with the silicon dioxide film. The silicon dioxide film is subjected to chemical mechanical polishing (CMP), and the silicon nitride film is buried in the trench  35  and the device isolation region  2  is formed. Then, the silicon nitride film  33 , if it is left, is removed. 
     Although not shown in particular, a second conductive film  62  is formed on the structure shown in FIGS. 32A to  32 C, and a stacked structure of the first conductive film  61  and second conductive film  62  is obtained. The second conductive film is formed of tungsten. 
     Subsequently, for example, according to the manufacturing method illustrated in FIGS. 10 to  14 , the stacked-gate structure  40  and gate structure  41  are formed, the N-type diffusion layers  9 ,  10  and  11  are formed and the booster electrode insulation film  14  is formed. 
     The conductor which becomes the booster electrode is then formed. Following this, according to the manufacturing method described with reference to FIG. 15, FIGS. 20 and 21, or FIGS. 25 to  29 , the booster electrode  15  or wiring-type booster electrodes  15 - 1  to  15 - 3 , and the source wiring  51  and plug  52  are formed. 
     As has been described with reference to FIGS. 16,  22  or  30 , the first interlayer insulation film is then formed. The bit line contact hole, etc. are formed in the first interlayer insulation film, and the bit line, etc. are formed on the first interlayer insulation film. Thereafter, the second interlayer insulation film is formed, and the fabrication of the cell according to the fourth embodiment is completed. 
     According to the fourth embodiment of the invention, like the first embodiment, the variance in the coupling ratio γ pgm can be reduced. In addition, the word line  7  has the stacked structure comprising the first conductive film  61  and second conductive film  62 , and the resistance thereof is decreased. Therefore, the cell of the fourth embodiment can perform write, read and erase operations at higher speed. 
     Although tungsten is used as material of the second conductive film  62 , other high-melting point metals or silicides thereof may be used. 
     An EEPROM cell according to a fifth embodiment of the invention will now be described. 
     FIG. 33A is a plan view of the EEPROM cell according to the fifth embodiment, FIG. 33B is a cross-sectional view taken along line  33 B— 33 B in FIG.  33 A and FIG. 33C is a cross-sectional view taken along line  33 C— 33 C in FIG.  33 A. For the purpose of simple description, FIG. 33A does not show the bit line and the underlying interlayer insulation film. 
     As is shown in FIGS. 33A to  33 C, in the fifth embodiment, cap layers  71  of insulating material are provided on the stacked-gate structure  40  and gate structure  41 . 
     FIGS. 33A to  33 C show the fifth embodiment as having the structure including wiring-type booster electrodes  15 - 1  to  15 - 3 , like the second embodiment. However, needless to say, the structure of the fifth embodiment can be applied to the cell of the first embodiment with the booster electrode  15  covering the stacked-gate structure  41  or to the cell of the third embodiment with the source wiring  51  and plug  52  formed of the same conductor as the wiring-type booster electrodes  15 - 1  to  15 - 3 . 
     The technique of the fifth embodiment can also be applied to the fourth embodiment wherein the word line  7  has the stacked structure. 
     A method of fabricating the EEPROM cell of the fifth embodiment will now be described. 
     FIGS. 34A to  34 C illustrate the principal steps of fabricating the EEPROM according to the fifth embodiment. FIG. 34A is a plan view, FIG. 34B is a cross-sectional view taken along line  34 B— 34 B in FIG. 34A, and FIG. 34C is a cross-sectional view taken along line  34 C— 34 C in FIG.  34 A. 
     According to the method illustrated in FIGS. 8 to  10 , the first stacked-film structure including the silicon dioxide film which becomes the tunnel insulation film, the conductive polysilicon layer which becomes the floating gate, and the silicon nitride film is formed on the P-type silicon substrate  1 . 
     Subsequently, the first stacked-film structure and substrate are etched, and the device region and device isolation trench which are self-aligned with the remaining portion of the first stacked-film structure are formed on the substrate, and the device isolation region  2  is formed. 
     Cap layers  71  of insulating material are formed on the structure shown in FIGS. 10A to  10 C. The cap layers  71  are formed of, e.g. silicon nitride. 
     As is shown in FIGS. 34A and 34B, the stacked-gate structure  40  and gate structure  41  are formed according to the method described with reference to FIGS. 11A to  11 C. The upper surfaces of the stacked-gate structure  40  and gate structure  41  are covered with the cap layers  71 . 
     Although not shown in particular, according to the manufacturing method illustrated in FIGS. 12 to  14 , the N-type diffusion layers  9 ,  10  and  11  are formed and the booster electrode insulation film  14  is formed. A conductor material which becomes the booster electrode is deposited. 
     Following this, according to the manufacturing method described with reference to FIG. 15, FIGS. 20 and 21, or FIGS. 25 to  29 , the booster electrode  15  or wiring-type booster electrodes  15 - 1  to  15 - 3 , and the source wiring  51  and plug  52  are formed. 
     In particular, the cap layers  71  function as stoppers for polishing/etching-back in the method illustrated in FIGS. 20-21 or  25 - 29  wherein the material of the booster electrode is subjected to chemical mechanical polishing or etched back and buried between the stacked-gate structures  40 , between the stacked-gate structure  40  and gate structure  41  and between the gate structures  41 . Therefore, a decrease in film thickness of the word line  7  can be prevented. 
     As has been described with reference to FIGS. 16,  22  or  30 , the first interlayer insulation film is then formed. The bit line contact hole, etc. are formed in the first interlayer insulation film, and the bit line, etc. are formed on the first interlayer insulation film. Thereafter, the second interlayer insulation film is formed, and the fabrication of the cell according to the fifth embodiment is completed. 
     According to the fifth embodiment of the invention, like the first to fourth embodiments, the variance in the coupling ratio γ pgm can be reduced. 
     In the above embodiments, the P-type semiconductor substrate is used as BULK. However, needless to say, the P-type well in an N-type semiconductor substrate may be used as BULK and the cell may be formed on the BULK. Other modifications may be made without departing from the spirit of the invention. 
     As has been described above, the present invention can provide a non-volatile semiconductor memory device and a method of manufacturing the same, wherein a variation in potential VFG due to a variation in coupling ratio γ pgm can be suppressed, and defects such as erroneous write, in which electrons are erroneously injected in a floating gate of a non selected cell in which a gate is to be the word line at the time of the write or a cell designated for “1” write, or read disturb can be prevented. 
     Additional advantages and modifications will readily occurs 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.