Abstract:
There is provided an EEPROM semiconductor device including (a) a plurality of field insulating films each extending perpendicularly to word lines, (b) a plurality of memory cells arranged in a matrix, each memory cell having a floating gate, a control gate formed on the floating gate and doubling as a word line, and source and drain regions located at either sides of the control gate, (c) a common source line extending in parallel with the word lines and connecting source regions of the memory cells with each other, and (d) a first bit line extending perpendicularly to the word lines and connecting drain regions of the memory cells with each other. The above-mentioned EEPROM semiconductor device makes it possible to form CMOS logic circuit together with a non-volatile memory on a common semiconductor substrate without increasing fabrication steps, and also makes it possible for the non-volatile memory to write data thereinto and read data therefrom at a higher rate without an increase in a cell size.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a semiconductor device, and more particularly to a semiconductor device including an electrically erasable programmable read only memory having a two-gate structure of a floating gate and a control gate deposited on the floating gate.  
           [0003]    2. Description of the Related Art  
           [0004]    An electrically erasable programmable read only memory (hereinafter, referred to simply as “EEPROM”) generally includes, as a memory cell, MISFET memory transistor having a two-gate structure of a floating gate and a control gate formed on the floating gate. Data is written into or eliminated from the two-gate type EEPROM by introducing electric charges into or discharging electric charges from a floating gate.  
           [0005]    For instance, data is written into the two-gate type EEPROM by introducing channel hot electrons, generated in drain regions, into a floating gate, whereas data is eliminated from EEPROM by introducing electrons into a source, for instance, by virtue of Fowler-Nordheim tunneling.  
           [0006]    A conventional method of fabricating a two-gate type memory cell array is explained hereinbelow with reference to FIGS. 1, 2 and  3 A to  3 D, wherein FIG. 1 is a plan view of a conventional two-gate type memory cell array, FIG. 2 is a plan view illustrating the memory cell array being fabricated, and FIGS. 3A to  3 D are cross-sectional views of the memory cell array taken along the line III-III in FIG. 3A, showing respective steps of a method of fabricating the memory cell array.  
           [0007]    As illustrated in FIG. 3A, a p-type well  2  is formed in a p-type semiconductor substrate  1  in a region where a memory cell array is to be formed. Then, a plurality of field insulating films  3  is formed in the form of islands by selective oxidation. The field insulating films  3  are not illustrated only in FIG. 3A, but illustrated in FIG. 2.  
           [0008]    Then, a first gate insulating film  4  is formed all over the p-type well  2 , and a first polysilicon layer  5   a  is formed all over the first gate insulating film  4  for forming a floating gate. Then, impurities such as phosphorus (P) are doped into the first polysilicon layer  5   a  by thermal diffusion or ion-implantation to thereby lower a resistance of the first polysilicon layer  5   a . Then, as illustrated in FIG. 2, the first polysilicon layer  5   a  is patterned into a plurality of layers  5   a  in parallel with each other so that the layers  5   a  extend perpendicularly to word lines which will be formed later, in order to define a width thereof in a direction of a channel width of a floating gate.  
           [0009]    Then, a second gate insulating film  6  is formed all over the product, and a second polysilicon layer  7   a  is formed over the second gate insulating film  6 . Then, as illustrated in FIG. 3A, a patterned photoresist film  18   a  is formed on the second polysilicon layer  7   a  by photolithography and dry etching. The photoresist film  18   a  has a pattern for forming word lines.  
           [0010]    Then, as illustrated in FIG. 3B, the second and first polysilicon layers  7   a  and  5   a  are patterned with the patterned photoresist film  18   a  being used as a mask, to thereby form control gates  7  and floating gates  5 . After removal of the photoresist film  18   a , impurities such as arsenic (As) are ion-implanted into the product with the deposited gates  5  and  7  and the field insulating films  3  being used as a mask, to thereby form drain regions  8   a  and source regions  8   b.    
           [0011]    Then, as illustrated in FIG. 3C, sidewall spacers  9  are formed around a sidewall of the deposited gates  5  and  7  of each of memory cells in order to cause CMOS transistors located outside memory cell array regions to have a LDD-structure. Thereafter, a first interlayer insulating film  10  is deposited all over the product. The first interlayer insulating film  10  is composed of boron phospho silicate glass (BPSG), and has a thickness in the range of 6000 to 8000 angstroms.  
           [0012]    Then, there is formed a photoresist film  18   e  having a hole above the drain region  8   a . Then, the first interlayer insulating film  10  is etched with the photoresist film  18   e  being used as a mask, to thereby form a contact hole  11  leading to the drain region  8   a.    
           [0013]    After removal of the photoresist film  18   e , aluminum alloy is deposited by sputtering by a thickness in the range of 4000 to 6000 angstroms. Then, the aluminum alloy is patterned by photolithography and dry etching to thereby form bit lines  12  extending perpendicularly to the word lines. Then, the product is entirely covered with a passivation film  16  composed of PSG and having a thickness of about 5000 angstroms. Thus, there is completed a memory cell array.  
           [0014]    While the above-mentioned method is being carried out a region  3   a  (a hatched region in FIG. 2) which is sandwiched between the field insulating films  3  and will become a source region is exposed to etching twice, namely, when the first polysilicon layer  5   a  is patterned and when the second polysilicon layer  7   a  is patterned. When the first polysilicon layer  5   a  is patterned, the region  3   a  is covered merely with the thin first gate insulating film  4  after the first polysilicon layer  5   a  has been etched. Hence, the first gate insulating film  4  is first removed, and then, the p-type semiconductor substrate  1  is undesirably etched. In addition, when the second polysilicon layer  7   a  is patterned, the region  3   a  is covered merely with the thin second gate insulating film  6  after the second polysilicon layer  7   a  has been etched. Hence, the p-type semiconductor substrate  1  is undesirably further etched.  
           [0015]    As a result, as illustrated in FIG. 4 which is a cross-sectional view taken along the line IV-IV in FIG. 1, there is formed an undesirable recess  19  at a surface of the semiconductor substrate  1 . The undesirable recess  19  causes junction leakage therein, which pauses a problem that data-writing and data-eliminating properties are deteriorated.  
           [0016]    If a diffusion layer had a depth shallower than a depth of the recess  19 , there is formed a breakage in a source region at the recess  19 , since impurities are not ion-implanted into an inner sidewall of the recess  19 . This causes a reduction in a fabrication yield.  
           [0017]    The above-mentioned problem can be solved by a semiconductor device structure as suggested in Japanese Unexamined Patent Publications Nos. 3-52267 and 3-126266, for instance. Hereinafter is explained the suggested structure with reference to FIGS. 5, 6,  7  and  8 A to  8 D, wherein FIG. 5 is a plan view of the suggested memory cell array, FIG. 6 is a cross-sectional view taken along the line VI-VI in FIG. 5, FIG. 7 is a cross-sectional view taken along the line VII-VII in FIG. 5, and FIGS. 8A to  8 D are cross-sectional views taken along the line VI-VI in FIG. 5, showing respective steps of a method of fabricating the suggested memory cell array.  
           [0018]    The suggested memory cell array is characterized by that a plurality of the field insulating films  3  extend perpendicularly to the word lines  7 , and that the common source line  17   a  connecting the source regions  8   b  to each other in a direction in which the word lines  7  extend is formed to extend perpendicularly to the field insulating films  3 . Hereinafter is explained a method of fabricating the suggested memory cell array, with reference to FIGS. 8A to  8 D.  
           [0019]    As illustrated in FIG. 8A, a p-type well  2  is formed in a p-type semiconductor substrate  1  by introducing p-type impurities into the semiconductor substrate  1  and thermally diffusing the p-type impurities in the semiconductor substrate  1 . Then, a plurality of field insulating films  3  are formed on a principal surface of the p-type well  2  by selective oxidation so that the field insulating films  3  extend in parallel with one another, but perpendicularly to word lines which will be formed later. The field insulating films  3  are not illustrated only in FIG. 8A, but illustrated in FIG. 5.  
           [0020]    Then, a first gate insulating film  4  and then a first polysilicon layer  5   a  are formed all over the product. Then, impurities such as phosphorus (P) are ion-implanted into the first polysilicon layer  5   a  to thereby lower a resistance of the first polysilicon layer  5   a . Then, as illustrated in FIG. 2, the first polysilicon layer  5   a  is patterned into a plurality of layers  5   a  in parallel with each other in order to define a width thereof in a direction of a channel width of a floating gate. When the first polysilicon layer  5   a  is patterned, the thick field insulating films  3  exist below a region where the first polysilicon layer  5   a  is etched, which ensures that the substrate  1  is not etched, and hence a recess such as the recess  19  illustrated in FIG. 4 is not formed.  
           [0021]    Then, a second gate insulating film  6  is formed all over the product, and a second polysilicon layer  7   a  is formed over the second gate insulating film  6 . Then, impurities such as phosphorus (P) are ion-implanted into the second polysilicon layer  7   a  to thereby lower a resistance thereof. Then, as illustrated in FIG. 8A, a patterned photoresist film  18   a  is formed on the second polysilicon layer  7   a  by photolithography and dry etching. The photoresist film  18   a  has a pattern for forming word lines.  
           [0022]    Then, as illustrated in FIG. 8B, the second and first polysilicon layers  7   a  and  5   a  are patterned by etching with the patterned photoresist film  18   a  being used as a mask, to thereby form control gates  7  and floating gates  5 . After removal of the photoresist film  18   a , n-type impurities are ion-implanted into the product with the deposited gates  5  and  7  and the field insulating films  3  being used as a mask, to thereby form drain regions  8   a  and source regions  8   b.    
           [0023]    Then, as illustrated in FIG. 8C, sidewall spacers  9  are formed around a sidewall of the deposited gates  5  and  7  of each of memory cells. Thereafter, a first interlayer insulating film  10  is deposited all over the product by chemical vapor deposition (CVD). The first interlayer insulating film  10  is composed of silicon dioxide. Then, the first interlayer insulating film  10  is etched in selected regions to thereby form contact holes C 1  reaching a surface of the source regions  8   b  and contact holes C 2  reaching a surface of the drain regions  8   a.    
           [0024]    Then, as illustrated in FIG. 8D, an electrically conductive layer composed of polysilicon is formed all over the product, and then patterned to thereby form a common source line  17   a  and an extended bit line  17   b . The common source line  17   a  connects the source regions  8   b  in a direction in which the word lines extend. The extended bit line  17   b  makes electrical contact with the drain region  8   a  through the contact hole C 2 , and covers a portion of the first interlayer insulating film  10  around the contact hole C 2  therewith. The electrically conductive layer from which the common source line  17   a  and the extended bit line  17   b  are formed may be composed of refractory metal, silicide thereof, or polycide thereof, as well as polysilicon.  
           [0025]    Then, a second interlayer insulating film  13  composed of BPSG is deposited all over the product. Thereafter, a photoresist film  18   c  is formed, and then, patterned by photolithography and dry etching so as to have an opening above the extended bit line  17   b . Then, the second interlayer insulating film  13  is etched with the patterned photoresist film  18   c  being used as a mask, to thereby form through-holes  14  reaching the extended bit line  17   b.    
           [0026]    After removal of the photoresist film  18   c , aluminum alloy is deposited by sputtering. Then, the aluminum alloy is patterned by photolithography and dry etching to thereby form bit lines  12  (see FIGS. 5, 6 and  7 ) extending perpendicularly to the word lines. Then, the product is entirely covered with a passivation film  16  (see FIGS. 4, 6 and  7 ) composed of PSG. Thus, there is completed a non-volatile semiconductor memory device as illustrated in FIGS.  4  to  7 .  
           [0027]    In accordance with the above-mentioned method, when the first polysilicon layer  5   a  is etched, the thick field insulating films  3  exist below a region to be etched. When the second and first polysilicon layers  7   a  and  5   a  are patterned to thereby form the control gate  7  and the floating gate  5 , a region where only a single polysilicon layer is etched is a region located above the field insulating regions  3 . Hence, the above-mentioned undesirable recess  19  caused by etching a polysilicon layer is not formed. Accordingly, there is solved a problem that junction leakage junction leakage occurs due to the recess, and resultingly data-writing and data-eliminating properties are deteriorated, and that a fabrication yield due to the breakage in a source region is reduced.  
           [0028]    A semiconductor device including a high-rate CMOS logic circuit is generally designed to have two or more wiring layers. When a non-volatile memory is formed on a common semiconductor substrate on which a high-rate CMOS logic circuit is also formed, it is required that an increase in the number of additional fabrication steps is avoided and that the non-volatile memory is small in size, in order to reduce fabrication costs and integrate the device in a higher density.  
           [0029]    In the conventional method having been explained with reference to FIGS. 5, 6,  7  and  8 A to  8 D, the common source line is formed of the electrically conductive layer composed of electrically conductive material such as polysilicon, after the contact hole has been formed. Hence, the above-mentioned conventional method has a problem that the number of additional fabrication steps is increased relative to the number of steps for fabricating CMOS logic circuit having two or more wiring layers, and hence, fabrication costs are also increased.  
           [0030]    In addition, since the common source line is formed of an electrically conductive layer composed of polysilicon, the common source line unavoidably has high resistivity, which causes problems that data-writing and data-eliminating properties of a non-volatile memory are deteriorated, and that a speed at which a memory cell reads out data is reduced.  
           [0031]    The common source line may be designed to have a smaller resistance by increasing an area of the electrically conductive layer and/or forming a backing wiring layer composed of aluminum. However, this makes it difficult to reduce a size of a memory cell, and reduce fabrication costs per a chip.  
         SUMMARY OF THE INVENTION  
         [0032]    It is an object of the present invention to provide an electrically erasable programmable read only memory which is capable of being formed commonly on a semiconductor substrate on which a high-rate CMOS semiconductor device is also formed, without an increase in the number of additional fabrication steps, and also capable of writing data thereinto and reading data therefrom at a high rate without an increase in a cell size.  
           [0033]    The above-mentioned object can be accomplished by presenting a non-volatile memory including memory cells having a floating gate and a control gate doubling as a word line, field insulating films each extending perpendicularly to word lines to thereby electrically insulate the memory cells from one another, a common source line extending in parallel to the word lines to thereby connect source regions of the memory cells to one another, and a bit line extending perpendicularly to the word lines to thereby connect drain regions of the memory cells to one another. The common source line may be formed of a first metal wiring layer, and the bit line may be formed of a second metal wiring layer.  
           [0034]    Specifically, in one aspect of the present invention, there is provided an EEPROM semiconductor device including (a) a plurality of field insulating films each extending perpendicularly to word lines, (b) a plurality of memory cells arranged in a matrix, each memory cell including a floating gate, a control gate formed on the floating gate and doubling as a word line, and source and drain regions located at either sides of the control gate, (c) a common source line extending in parallel with the word lines and connecting source regions of the memory cells with each other, and (d) a first bit line extending perpendicularly to the word lines and connecting drain regions of the memory cells with each other.  
           [0035]    The common source line may be constituted of a first metal wiring layer, which is preferably composed of aluminum. The bit line may be constituted of a second metal wiring layer, which is preferably composed of aluminum.  
           [0036]    The EEPROM semiconductor device may further include a plurality of second bit lines formed above the drain regions of the memory cells, in which case, it is preferable that the first bit line connects the second bit lines with one another.  
           [0037]    It is preferable that both the second bit lines and the common source line are constituted of a first metal wiring layer, which is preferably composed of aluminum.  
           [0038]    The EEPROM semiconductor device may further include CMOS logic circuit including both the common source line and the first bit line, and formed on a common semiconductor substrate.  
           [0039]    There is further provided an EEPROM semiconductor device including (a) a plurality of field insulating films each extending perpendicularly to word lines, (b) a plurality of memory cells arranged in a matrix, each memory cell including a floating gate, a control gate formed on the floating gate and doubling as a word line; and source and drain regions located at either sides of the control gate, (c) a first bit line extending perpendicularly to the word lines and connecting drain regions of the memory cells with each other, and (d) a first common source line extending in parallel with the word lines and connecting source regions of the memory cells with each other.  
           [0040]    The EEPROM semiconductor device may further include a plurality of second common source lines formed above the source regions of the memory cells, in which case, the first common source line preferably connects the second common source lines with one another.  
           [0041]    It is preferable that both the second common source lines and the bit line are constituted of a first metal wiring layer, which is preferably composed of aluminum.  
           [0042]    The EEPROM semiconductor device may further include backing wiring layers each of which is connected to the word lines at every certain number of bits, in which case, it is preferable that both the backing wiring layers and the first common source lines are constituted of a second metal wiring layer.  
           [0043]    In another aspect, there is provided a method of fabricating an EEPROM semiconductor device, including the steps of (a) forming a plurality of field insulating films in parallel on a semiconductor substrate, (b) forming a first gate insulating film in each of active regions, (c) forming a plurality of first polysilicon layers in parallel with one another perpendicularly to word lines, (d) forming a second gate insulating film and a second polysilicon layer all over the product resulting from the step (c), (e) patterning the second polysilicon layer, the second gate insulating film, and the first polysilicon layer to thereby form a control gate and a floating gate, (f) forming drain and source regions, (g) forming a first interlayer insulating layer all over the product resulting from the step (f), (h) forming a first metal wiring layer which is patterned so as to form both a common source line extending in parallel with the word lines and connecting source regions to one another, and an extended bit line connecting the drain region to a bit line, (i) forming a second interlayer insulating layer all over the product resulting from the step (h), and (j) forming a second metal wiring layer which is patterned so as to form a bit line connecting the drain regions to one another.  
           [0044]    The second gate insulating film may have a three-layered structure of oxide/nitride/oxide films.  
           [0045]    There is further provided a method of fabricating an EEPROM semiconductor device, including the steps of (a) forming a plurality of field insulating films in parallel on a semiconductor substrate, (b) forming a first gate insulating film in each of active regions, (c) forming a plurality of first polysilicon layers in parallel with one another perpendicularly to word lines, (d) forming a second gate insulating film and a second polysilicon layer all over the product resulting from the step (c), (e) patterning the second polysilicon layer, the second gate insulating film, and the first polysilicon layer to thereby form a control gate and a floating gate, (f) forming drain and source regions, (g) forming a first interlayer insulating layer all over the product resulting from the step (f), (h) forming a first metal wiring layer which is patterned so as to form both a bit line extending almost in parallel with the field insulating films and connecting drain regions to one another, and an extended common source line connecting the source region to a later mentioned common source line, (i) forming a second interlayer insulating layer all over the product resulting from the step (h), and (j) forming a second metal wiring layer which is patterned so as to form a common source line connecting the source regions to one another.  
           [0046]    The method may further include the step of forming backing wiring layers connecting to the control gate at a certain interval, in which case, the backing wiring layers are preferably constituted of the second metal wiring layer.  
           [0047]    The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]    [0048]FIG. 1 is a plan view of a conventional memory cell array.  
         [0049]    [0049]FIG. 2 is a plan view of the conventional memory cell array illustrated in FIG. 1, being fabricated.  
         [0050]    [0050]FIGS. 3A to  3 D are cross-sectional views taken along the line III-III in FIG. 1, illustrating respective steps of a method of fabricating the memory cell array illustrated in FIG. 1.  
         [0051]    [0051]FIG. 4 is a cross-sectional view taken along the line IV-IV in FIG. 1.  
         [0052]    [0052]FIG. 5 is a plan view of another conventional memory cell array.  
         [0053]    [0053]FIG. 6 is a cross-sectional view taken along the line VI-VI in FIG. 5.  
         [0054]    [0054]FIG. 7 is a cross-sectional view taken along the line VII-VII in FIG. 5.  
         [0055]    [0055]FIGS. 8A to  8 D are cross-sectional views taken along the line VI-VI in FIG. 5, illustrating respective steps of a method of fabricating the memory cell array illustrated in FIG. 5.  
         [0056]    [0056]FIG. 9 is a plan view of a memory cell array in accordance with the first embodiment of the present invention.  
         [0057]    [0057]FIG. 10 is a plan view of the memory cell array illustrated in FIG. 9, being fabricated.  
         [0058]    [0058]FIGS. 11A to  11 E are cross-sectional views taken along the line  11 E- 11 E in FIG. 9, illustrating respective steps of a method of fabricating the memory cell array illustrated in FIG. 9.  
         [0059]    [0059]FIG. 12 is a plan view of a memory cell array in accordance with the second embodiment of the present invention.  
         [0060]    [0060]FIG. 13 is a cross-sectional view taken along the line XIII-XIII in FIG. 12.  
         [0061]    [0061]FIG. 14 is a cross-sectional view taken along the line XIV-XIV in FIG. 12.  
         [0062]    [0062]FIG. 15 is a cross-sectional view taken along the line XV-XV in FIG. 12.  
         [0063]    [0063]FIGS. 16A and 16B are cross-sectional views taken along the line XIII-XIII in FIG. 12, illustrating respective steps of a method of fabricating the memory cell array illustrated in FIG. 12.  
         [0064]    [0064]FIG. 17 is a plan view of a memory cell array in accordance with the third embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0065]    [0065]FIG. 9 illustrates a memory cell array in accordance with the first embodiment. As illustrated in FIG. 9, a plurality of field insulating films  3  is formed in parallel perpendicularly to word lines. Control gates  7  doubling as word lines extend perpendicularly to the field insulating films  3 . Floating gates  5  are formed on channel regions located below the control gates  7 . That is, the control gates  7  are deposited on the floating gates  5 . Drain regions  8   a  and source regions  8   b  are formed in a semiconductor substrate at either sides of the deposited gates  7  and  5 .  
         [0066]    The source regions  8   b  are connected to each other via contact holes  11  through a common source line  12   a  extending in parallel with the word lines and composed of a first aluminum wiring layer. The drain regions  8   a  are connected to extended bit lines  12   b  composed of the first aluminum wiring layer via the contact hole  11 , and are connected to one another through a bit line  15   a  composed of a second aluminum wiring layer in a direction perpendicular to the word lines.  
         [0067]    A method of fabricating the memory cell array in accordance with the first embodiment is explained hereinbelow with reference to FIGS. 11A to  11 E.  
         [0068]    As illustrated in FIG. 11A, a p-type semiconductor substrate  1  is ion-implanted at about 100 KeV with doses of about 1×10 13  atoms/cm 2  with p-type impurities such as boron (B), followed by annealing at about 1000° C. Thus, there is formed a p-type well  2  in the p-type semiconductor substrate  1  in a region where a memory cell array is to be formed.  
         [0069]    Then, a plurality of field insulating films  3  composed of silicon dioxide are formed in parallel by selective oxidation. The field insulating films  3  extend perpendicularly to word lines which will be formed later, and have a thickness in the range of 4000 to 8000 angstroms. The field insulating films  3  are not illustrated only in FIG. 11A, but illustrated in FIG. 10.  
         [0070]    Then, a substrate surface of active regions are thermally oxidized at a temperature in the range of 700 to 850 degrees centigrade to thereby form a first gate insulating film  4  which will make a gate oxide film of memory cells. The thus formed first gate insulating film  4  has a thickness of about 100 angstroms.  
         [0071]    Then, a first polysilicon layer  5   a  is formed all over the first gate insulating film  4  by a thickness in the range of about 1500 to about 2500 angstroms by reduced pressure CVD. The first polysilicon layer  5   a  will make a floating gate. Then, n-type impurities such as phosphorus (P) are doped into the first polysilicon layer  5   a  by thermal diffusion or ion-implantation to thereby lower a resistance of the first polysilicon layer  5   a.    
         [0072]    Then, as illustrated in FIG. 10, the first polysilicon layer  5   a  is patterned by photolithography and dry etching into a plurality of layers  5   a  in parallel with each other so that the layers  5   a  extend perpendicularly to word lines which will be formed later, in order to define a width thereof in a direction of a channel width of a floating gate.  
         [0073]    When the first polysilicon layer  5   a  is patterned, the thick field insulating films  3  exist below a region where the first polysilicon layer  5   a  is etched, which ensures that the substrate  1  is not etched in an etching step for forming a gate electrode, and hence a recess such as the recess  19  illustrated in FIG. 4 is not formed.  
         [0074]    Then, a second gate insulating film  6  having a thickness in the range of about 200 to about 300 angstroms is formed all over the product by thermal oxidation or CVD. The second gate insulating film  6  may be designed to have a three-layered structure of oxide/nitride/oxide films, which called ONO film.  
         [0075]    The second gate insulating film  6  formed outside a region where memory cell array is to be formed is removed by wet or dry etching, using acid such as hydrofluoric acid. Thereafter, a second polysilicon layer  7   a  is formed all over the second gate insulating film  6  by reduced pressure CVD. The second polysilicon layer  7   a  will make a control gate and a gate electrode of peripheral transistors. Then, n-type impurities such as phosphorus (P) are introduced into the second polysilicon layer  7   a  by thermal diffusion or ion-implantation to thereby lower a resistance thereof. On the second polysilicon layer  7   a  may be formed a film composed of silicide of refractory metal such as W, Ti and Mo to thereby form a polycide structure film.  
         [0076]    Then, as illustrated in FIG. 11A, a patterned photoresist film  18   a  is formed on the second polysilicon layer  7   a  by photolithography and dry etching. The photoresist film  18   a  has a pattern for forming control gates.  
         [0077]    Then, as illustrated in FIG. 11B, the second polysilicon layer  7   a , the second gate insulating film  6  and the first polysilicon layer  5   a  are patterned by reactive ion etching (RIE) with the patterned photoresist film  18   a  being used as a mask, to thereby form control gates  7  and floating gates  5  in self-align fashion.  
         [0078]    After removal of the photoresist film  18   a , the product is ion-implanted at about 50 to 70 KeV with doses of about 1×10 15  atoms/cm 2  with n-type impurities such as arsenic (As) with the deposited gates  5  and  7  and the field insulating films  3  being used as a mask, to thereby form drain regions  8   a  and source regions  8   b.    
         [0079]    Then, as illustrated in FIG. 11C, sidewall spacers  9  are formed around a sidewall of the deposited gates  5  and  7  of each of memory cells in order to cause CMOS transistors located outside memory cell array regions to have a LDD-structure. Thereafter, a first interlayer insulating film  10  is deposited all over the product by chemical vapor deposition (CVD). The first interlayer insulating film  10  is composed of BPSG and has a thickness in the range of 6000 to 8000 angstroms. Then, there is formed a photoresist film  18   b  by photolithography and dry etching. The photoresist film  18   b  has openings above the source regions  8   b  and the drain regions  8   a . Then, the first interlayer insulating film  10  is etched by RIE in selected regions with the photoresist film  18   b  being used as a mask, to thereby form contact holes  11  reaching all the source and drain regions  8   b  and  8   a  of the memory cells.  
         [0080]    Then, as illustrated in FIG. 11D, aluminum alloy is deposited over the product by sputtering by a thickness in the range of about 4000 to about 6000 angstroms. The thus deposited aluminum alloy is patterned to thereby form a common source line  12   a  and an extended bit line  12   b  both as a first aluminum wiring layer. The common source line  12   a  extends in parallel with the word lines, and connects the source regions  8   b  located in a direction in which the word lines extend, to one another. The extended bit line  12   b  is a junction through which the drain regions  8   a  make electrical contact with a bit line.  
         [0081]    Then, a second interlayer insulating film  13  composed of BPSG is deposited all over the product by CVD. The second interlayer insulating film  13  has a thickness in the range of about 4000 to about 5000 angstroms. Thereafter, a photoresist film  18   c  is formed, and then, patterned by photolithography and dry etching so as to have an opening above the drain regions  8   a . Then, the second interlayer insulating film  13  is etched by RIE with the patterned photoresist film  18   c  being used as a mask, to thereby form through-holes  14  reaching the extended bit line  12   b.    
         [0082]    After removal of the photoresist film  18   c , as illustrated in FIG. 11E, an aluminum alloy film having a thickness in the range of about 4000 to about 6000 angstroms, as a second aluminum wiring layer, is deposited by sputtering over the product. Then, the aluminum alloy film is patterned by photolithography and dry etching to thereby form bit lines  15   a  in parallel with the field insulating film  3 . The bit lines  15   a  connect the drain regions  8   a  located adjacent to the field insulating film  3 , to one another.  
         [0083]    In CMOS logic products where a memory cell is formed on a common substrate, wirings are also made in CMOS logic circuit in first and second metal wiring layers in a memory cell array region. If a contact hole or a through-hole were filled with metal such as tungsten (W) in CMOS logic circuit, wirings can be made in the same manner also in a memory cell array.  
         [0084]    Then, the product is entirely covered with a passivation film  16  composed of PSG. Thus, there is completed the memory cell array in accordance with the first embodiment.  
       Second Embodiment  
       [0085]    FIGS.  12  to  15  illustrate a memory cell array in accordance with the second embodiment.  
         [0086]    The second embodiment is different from the first embodiment in that the bit line  12   c  is constituted of the first aluminum wiring layer, and the common source line  15   b  is constituted of the second aluminum wiring layer. In the second embodiment, the drain regions  8   a  arranged in parallel with the field insulating films  3  are connected to one another through the bit line  12   c  constituted of the first aluminum wiring layer and extending almost in parallel with the field insulating films  3 , and a common extended source line  12   d  constituted of the first aluminum wiring layer is formed on the source regions  8   b . The source regions  8   b  arranged in parallel with the word lines are connected commonly to the common source line  15   b  via the common extended source line  12   d . The common source line  15   b  is constituted of the second aluminum wiring layer, and extends in parallel with the word lines.  
         [0087]    A method of fabricating the memory cell array in accordance with the second embodiment is explained hereinbelow with reference to FIGS. 16A and 16B.  
         [0088]    The method of fabricating the memory cell array in accordance with the second embodiment has the same fabrication steps from the first step to the step illustrated in FIG. 11C as those in the method of fabricating the memory cell array in accordance with the first embodiment.  
         [0089]    As illustrated in FIG. 11C or FIG. 16A, there are formed the contact holes  11  reaching all the drain regions  8   a  and source regions  8   b  formed in the memory cell array.  
         [0090]    Then, as illustrated in FIG. 16B, aluminum alloy is deposited over the product by sputtering by a thickness in the range of about 4000 to about 6000 angstroms. The thus deposited aluminum alloy is patterned to thereby form a bit line  12   c  and a common extended source line  12   d  both as a first aluminum wiring layer. The bit line  12   c  extends almost in parallel with the field insulating films  3 , and connects the drain regions  8   a  located in parallel with the field insulating films  3 , to one another. The common extended source line  12   d  is a junction through which the source regions  8   b  make electrical contact with the common source line  15   b.    
         [0091]    Then, a second interlayer insulating film  13  composed of BPSG is deposited all over the product by CVD. The second interlayer insulating film  13  has a thickness in the range of about 4000 to about 5000 angstroms. Thereafter, a photoresist film  18   d  is formed, and then, patterned by photolithography and dry etching so as to have an opening above the source regions  8   b . Then, the second interlayer insulating film  13  is etched by RIE with the patterned photoresist film  18   d  being used as a mask, to thereby form through-holes  14  reaching the common extended source line  12   d.    
         [0092]    After removal of the photoresist film  18   d , as illustrated in FIG. 16B, an aluminum alloy film having a thickness in the range of about 4000 to about 6000 angstroms, as a second aluminum wiring layer, is deposited by sputtering over the product. Then, the aluminum alloy film is patterned by photolithography and dry etching to thereby form a common source line  15   b  in parallel with the word lines. The common source line  15   b  connects the source regions  8   b  located in parallel with the word lines, to one another.  
         [0093]    Then, the product is entirely covered with a passivation film  16  composed of PSG. Thus, there is completed the memory cell array in accordance with the second embodiment as illustrated in FIG. 13.  
         [0094]    In the above-mentioned second embodiment, a wiring layer constituted of the second aluminum wiring layer is only the common source line  15   b . Hence, it is possible for the common source line  15   b  to have a greater width than a width of a common source line in the first embodiment, which ensures a further reduction in a resistance of the common source line  15   b , resulting in that the memory cell could operate at a higher rate.  
       Third Embodiment  
       [0095]    [0095]FIG. 17 is a plan view illustrating a memory cell array in accordance with the third embodiment. Parts or elements corresponding to those of the memory cell array in accordance with the second embodiment illustrated in FIG. 12 have been provided with the same reference numerals, and are not explained in detail.  
         [0096]    The third embodiment is different from the second embodiment in that backing wiring layers  15   c  constituted of the second aluminum wiring layer are formed above the control gates  7 , and connect to the control gates  7  at a certain interval.  
         [0097]    In the second embodiment, since the second aluminum wiring layer is formed only into the common source line  15   b , the common source line  15   b  was designed to have a greater width for lowering a resistance thereof. In the third embodiment, it is possible to operate a memory cell array at a higher rate by lowering a resistance of word lines.  
         [0098]    The backing wiring layers  15   c  illustrated in FIG. 17 are designed to be connected to the word lines or control gates  7  via contact holes at every 32 bits, for instance.  
         [0099]    The word lines are generally composed of polysilicon or polycide. However, these materials have greater resistivity than other metals. In addition, since the word lines are so long, a great degree of RC is generated in the word lines, and may cause a memory cell array to operate at a lower rate. To the contrary, in accordance with the present embodiment, the backing wiring layers  15   c  lower a resistance of the word lines, and hence, data-reading can be accomplished at a higher rate.  
         [0100]    While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.  
         [0101]    The entire disclosure of Japanese Patent Application No. 9-205592 filed on Jul. 31, 1997 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.