Abstract:
A method of manufacturing a nonvolatile semiconductor memory comprising: forming a gate insulating film formed on a surface of a semiconductor substrate; forming a source region and a drain region in the semiconductor substrate; forming a floating gate electrode on the gate insulating film; forming a inter-gate insulating film on the floating gate electrode; forming a control gate electrode on the inter-gate insulating film; forming a source contact region wherein the source contact region is electrically contact to said source region, and top part of said source contact region is lower than bottom part of said control gate electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior-Japanese Patent Application No. 2007-42408, filed Feb. 22, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a non-volatile semiconductor memory, and particularly to a memory cell transistor having a stacked gate structure. 
         [0004]    2. Description of the Related Art 
         [0005]    For example, NAND or NOR type flash memories are used as non-volatile semiconductor memories in electronic equipment. In the NOR type flash memory, a memory cell transistor is a MOS (Metal Oxide Semiconductor) transistor having a stacked gate structure comprising a control gate electrode and a floating gate electrode, for example. The stacked gate structure is formed in the line &amp; space style in the memory cell array portion, and thus the channel region and the source/drain diffusion layer of the memory cell transistor can be easily scaled down (miniaturized). 
         [0006]    On the other hand, the miniaturization of a contact portion provided to connect bit lines and source lines to the source and drain diffusion layer is not easy. The contact portion is formed after the stacked gate electrode and the source/drain diffusion layer are formed. Therefore, a contact hole in which the contact portion is embedded is formed in an aspect ratio based on the height of the stacked gate electrode. Accordingly, the source/drain diffusion layer is increased in size because of the embedding property of a contact member into the contact hole, securement of short margin between the contact portion and the gate electrode, etc. 
         [0007]    This problem is particularly severe in the memory cell transistor having the stacked gate structure, and the miniaturization of the contact portion has not yet followed the miniaturization of the whole of the memory cell array portion. Therefore, the fraction of the total memory cell array area occupied by contacts tends to increase as the device density increases. 
         [0008]    Furthermore, in order to solve the above problem a method known as SAS (Self-Aligned Source) has been frequently used for providing no source-line contact portion, but forming a source diffusion layer so as to be common in the extending direction of the word line and substituting the diffusion layer concerned for the contact portion. However, SAS has a larger resistance than a contact plug made of tungsten (W) or a similar material. Therefore, shunt areas are provided at fixed intervals in the memory cell array portion. A source line having lower electrical resistivity than SAS is formed on the upper portion of SAS within the shunt area, and both are connected to each other through the contact portion. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    A method of manufacturing a nonvolatile semiconductor memory comprising: forming a gate insulating film formed on a surface of a semiconductor substrate; forming a source region and a drain region in the semiconductor substrate; forming a floating gate electrode on the gate insulating film; forming a inter-gate insulating film on the floating gate electrode; forming a control gate electrode on the inter-gate insulating film; forming a source contact region wherein the source contact region is electrically contact to said source region, and top part of said source contact region is lower than bottom part of said control gate electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1  is a plan view showing a memory cell array portion according to a first embodiment; 
           [0011]      FIG. 2  is a cross-sectional view taken along the II-II line of  FIG. 1 ; 
           [0012]      FIG. 3  is a cross-sectional view taken along the III-III line of  FIG. 1 ; 
           [0013]      FIG. 4  is a cross-sectional view taken along the IV-IV line of  FIG. 1 ; 
           [0014]      FIG. 5  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0015]      FIG. 6  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0016]      FIG. 7  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0017]      FIG. 8  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0018]      FIG. 8A  is a plan view showing a manufacturing process of the embodiment; 
           [0019]      FIG. 8B  is a cross-sectional view taken along the VIIIB-VIIIB line of  FIG. 8A ; 
           [0020]      FIG. 9  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0021]      FIG. 10  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0022]      FIG. 11A  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0023]      FIG. 11B  is a cross-sectional view showing a manufacturing process of the embodiment; 
           [0024]      FIG. 12  is a plan view showing the structure of an application example; 
           [0025]      FIG. 13  is a cross-sectional view taken along the XIII-XIII line of  FIG. 12 ; 
           [0026]      FIG. 14  is a cross-sectional view showing a step of the manufacturing process of the application example; 
           [0027]      FIG. 15  is a cross-sectional view showing a modification; 
           [0028]      FIG. 16  is a cross-sectional view showing the structure of a second embodiment; 
           [0029]      FIG. 17  is a cross-sectional view showing a step of the manufacturing process of the second embodiment; 
           [0030]      FIG. 18  is a cross-sectional view showing the structure of a third embodiment; 
           [0031]      FIG. 19  is a cross-sectional view showing a step of the manufacturing process of the third embodiment; and 
           [0032]      FIG. 20  is a cross-sectional view showing a step of the manufacturing process of the third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Summary 
       [0033]    A non-volatile semiconductor memory according to an embodiment of the present invention relates to a memory cell transistor having a stacked gate structure. 
         [0034]    For example, in the NOR type flash memory, a contact portion is provided to connect a diffusion layer of the memory cell transistor to a source line or a bit line. The contact portion comprises one or two or more contact layers. 
         [0035]    The embodiment of this invention has a feature that the upper end of a source-line contact portion in contact with a source diffusion layer is located at a lower position than the lower end of a control gate electrode when the contact portion is constructed by two or more contact layers. Here, a lower end direction is defined as a substrate direction, and an upper end direction is defined as the opposite direction to the substrate direction. Here, “located at a lower position” is defined as “located at a position in the substrate direction (at the substrate side) as compared with a comparison target member/site”. 
         [0036]    A contact hole in which the contact portion in contact with the source diffusion layer is embedded is formed in an aspect ratio based on the height of a floating gate electrode. Therefore, as compared with the aspect ratio when the contact hole is formed in the height of the stacked gate electrode, the aspect ratio can be reduced, and the size of the diffusion layer can be reduced. Thus, according to the embodiment of the present invention, the memory cell transistor can be scaled down further, and the size of the memory cell array portion can be reduced. 
         [0037]    Furthermore, in the structure of the embodiment of the present invention, the contact portion which comes into contact with the source diffusion layer is formed at a step previous to a step of forming a control gate electrode. That is, the embodiment of the present invention adopts a manufacturing method of forming two gate electrodes to be stacked in different steps. Therefore, in the embodiment of this invention, a method of manufacturing the above structure will be described. The source-line and bit-line contact portions described with reference to this embodiment are defined as contact portions located below the source line and the bit line. 
       2. Embodiments 
       [0038]    Next, some possible embodiments will be described. 
         [0039]    The embodiments of the present invention will be described in detail by using a NOR type flash memory as an example. 
         [0040]    The definition of the contact portion is as described in summary. 
       (1) First Embodiment 
     (a) Example 
     (i) Structure 
       [0041]    The structure of this embodiment will be described with reference to  FIGS. 1 to 4 . 
         [0042]      FIG. 1  is a plan view showing a memory cell array portion of an NOR type flash memory,  FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 ,  FIG. 3  is a cross-sectional view taken along line III-III of  FIG. 1 , and  FIG. 4  is a cross-sectional view taken along line IV-IV of  FIG. 1 . 
         [0043]    As shown in  FIG. 1 , the memory cell array portion has active areas AA and element separating areas STI for electrically separating the active areas AA from one another. Memory cell transistors are arranged in a matrix form on the active areas AA surrounded by the element separating areas STI. 
         [0044]    In the NOR type flash memory, a bit-line contact portion BC is connected to the drain diffusion layer  6  of one memory cell transistor, and a first source-line contact portion SC 1  is connected to the source diffusion layer  5  of the memory cell transistor. The source-line contact portion SC 1  extends in a direction traversing the active area AA (channel width direction). 
         [0045]    The bit line BL extends in the channel length direction in the upper layer of the active area AA. The source line SL is provided to the upper layer of the shunt area SA which is provided in the memory cell array portion, and extends in the channel length direction as in the case of the bit line BL. The source line SL is connected to the first source-line contact portion SC 1  through a second source-line contact portion SC 2  which is provided in the shunt area SA. The shunt area SA is provided every 64 active areas AA, for example. Furthermore, a control gate electrode  11  which also serves as a word line extends in a direction perpendicular to the extending direction of the bit line BL. 
         [0046]    As shown in  FIGS. 2 to 4 , the memory cell transistor has a stacked gate structure comprising the floating gate electrode  3  and the control gate electrode  11 . The height of the stacked gate is set to about 300 nm, for example. 
         [0047]    The floating gate electrodes  3  are disposed on gate insulating film  2  formed on the surface of the semiconductor substrate  1 . The floating gate electrodes  3  adjacent in the channel width direction are electrically separated from each other by an element separating insulating layer  4  having an STI (Sallow Trench Isolation) structure. The floating gate electrodes  3  are formed of polysilicon, for example. 
         [0048]    Furthermore, for example, a spacer insulating layer  7  is formed on the side wall in the channel length direction of the floating gate electrode  3 . 
         [0049]    The control gate electrode  11  is disposed on the floating gate electrode  3  through inter-electrode insulating film  10 . The control gate electrode  11  functions as a word line, and thus it extends in the channel width direction and is jointly owned by the memory cell transistors adjacent in the channel width direction. The control gate electrode  11  is formed of polysilicon, for example. In this embodiment, the control gate electrode has a one-layer structure, however, it may be a control gate electrode having a double-layer structure comprising polysilicon film and silicide film formed on the polysilicon film. 
         [0050]    Furthermore, as shown in  FIG. 3 , the control gate electrode  11  may be formed so as to cover the side portion in the channel width direction of the floating gate electrode  3  through the inter-electrode insulating film  10 . The coupling ratio between the floating gate electrode  3  and the control gate electrode  11  can be enhanced by the above structure. 
         [0051]    The source diffusion layer  5  and the drain diffusion layer  6  are formed as the source/drain area of the memory cell transistor on the surface of the semiconductor substrate  1 . The source and drain diffusion layers  5 ,  6  are jointly owned by the memory cell transistors adjacent in the channel length direction. 
         [0052]    The drain diffusion layer  6  is electrically connected to the bit line BL through the bit-line contact portion BC. The bit-line contact portion is embedded in a tapered contact hole. The bit line BL is jointly owned by the memory cell transistors adjacent in the channel length direction. 
         [0053]    The source diffusion layer  5  is electrically connected to the source line SL through the two contact layers of the first and second source-line contact portions SC 1 , SC 2 . The source-line contact portions SC 1 , SC 2  are formed of tungsten (W), for example. The source line SL is formed of a metal material having low resistivity such as aluminum (Al), copper (Cu) or the like. Intermediate metal film and a contact portion for connecting the intermediate metal film to the source line SL and the bit line may be provided between the source line SL, the bit line BL and the contact portions BC, SC 1 , SC 2 . 
         [0054]    The source line and bit line contact portions described in this embodiment are defined as contact portions located at lower layers (substrate direction) than the source line and the bit line. 
         [0055]    The first source-line contact portion SC 1  is embedded in a slit-shaped contact hole formed in the first insulating layer  9 , and comes in contact with the source diffusion layer  5 . As shown in  FIG. 4 , the first source-line contact portion SC 1  is commonly connected between the source diffusion layers  5  of the memory cell transistors adjacent in the channel width direction. That is, the first source-line contact portion SC 1  extends in the channel width direction on the element separating area STI and on the active area AA. 
         [0056]    The second source-line contact portion SC 2  is provided in the shunt area SA. The first source-line contact portion SC 1  is connected to the source line SL in the shunt area SA by the second source-line contact portion SC 2 . 
         [0057]    As described above, the first source-line contact portion SC 1  is jointly owned by the memory cell transistors adjacent in the channel width direction. Therefore, the first source-line contact portion SC 1  may be also used as a source line extending in the extension direction of the word line, and a source voltage may be supplied from the source-line contact portion SC 1  to the memory cell transistor. In this case, it is unnecessary to provide the source line SL and the second contact portion SC 2  as shown in  FIGS. 1 to 4 . 
         [0058]    Furthermore, in this embodiment, the source line SL is provided in the same layer as the bit line BL. However, the bit line BL and the source line SL may be provided in different layers by using intermediate metal film. 
         [0059]    In this embodiment, the upper end of the first source-line contact portion SC 1  which comes into contact with the source diffusion layer  5  is located at a position lower than the lower end of the control gate electrode  11 . Here, the lower end direction is defined as the substrate direction, and the upper end direction is defined as the opposite direction to the substrate. “located at the lower position” is defined as “located in the substrate direction (at the substrate side) as compared with a comparison target member/side)”. 
         [0060]    The aspect ratio when the contact hole is formed is determined by the size ratio (D/W) of the depth (height) D to the opening width W. 
         [0061]    The aspect ratio of the contact hole in which the first source-line contact portion SC 1  is embedded is determined by the film thickness (height) of the floating gate electrode  3  and the size in the channel length direction of the source diffusion layer  5 . 
         [0062]    Accordingly, the size in the height direction of the contact hole is reduced, and thus the aspect ratio can be reduced. Even when the size in the width direction of the contact hole is reduced, the aspect ratio required to form the contact hole can be maintained. 
         [0063]    Therefore, it is unnecessary that the size in the channel length direction of the source diffusion layer  5  is increased to secure the aspect ratio of the contact hole in which the first source-line contact portion SC 1  is embedded. Accordingly, the size in the channel length direction of the source diffusion layer  5  can be reduced. 
         [0064]    As described above, according to the first embodiment of the present invention, the memory cell transistor can be miniaturized, and the size of the memory cell array portion can be reduced. 
         [0065]    A method of manufacturing the above structure will be described hereunder. 
       (ii) Manufacturing Method 
       [0066]    In order to attain the above structure, the first source-line contact portion SC 1  is formed before the control gate electrode  11  is formed. 
         [0067]    That is, the structures of the memory cell transistor and the source-line contact portion can be attained by forming the control gate electrode and the floating gate electrode separately from each other without using any self-alignment method of subjecting the control gate electrode and the floating gate electrode to gate processing at the same time. 
         [0068]    This manufacturing method will be described hereunder in detail. 
         [0069]    The manufacturing method according to the first embodiment will be described with reference to  FIGS. 5A to 10 . 
         [0070]    First, as shown in  FIGS. 5A and 5B , gate insulating film  2  formed of silicon oxide film is formed on the surface of the semiconductor substrate  1  by the thermal oxidation method, for example. Subsequently, polysilicon film  3 A is deposited on the gate insulating film  2  by the CVD (Chemical Vapor Deposition) method, for example. 
         [0071]    Then, the polysilicon film  3 A is subjected to patterning of line &amp; space so as to have a desired gate width. Thereafter, the polysilicon film  3 A and the semiconductor substrate  1  are successively etched by RIE (Reactive Ion Etching), for example, whereby the polysilicon film  3 A having a desired gate width and an element separating groove having STI (Shallow Trench Isolation) structure are formed, for example. 
         [0072]    Subsequently, silicon oxide is deposited on the whole surface of the semiconductor substrate  1 , for example by the HDP-CVD (High Density Plasma CVD) method, so as to be filled in the element separating groove. Thereafter, silicon oxide is flattened, for example by the CMP (Chemical Mechanical Polishing) method, so as to be coincident with the upper end of the polysilicon film  3 A. In this case, the element separating insulating layer  4  having the STI structure is formed in the semiconductor substrate  1 . 
         [0073]    The polysilicon film  3 A is subjected to patterning of line &amp; space so as to have a desired gate length. At this time, the patterning is performed so that the gate interval of an area in which a bit-line contact portion will be formed in a subsequent step is larger than the gate interval of an area in which a source-line contact portion will be formed, for example. Then, the polysilicon film  3 A is etched on the basis of the pattern concerned by RIE, for example. 
         [0074]    In this case, the floating gate electrode  3  is formed as shown in  FIG. 6 . The source diffusion layer  5  and the drain diffusion layer  6  are formed in self-alignment on the surface of the semiconductor substrate  1  by using the floating gate electrode  3  as a mask according to the ion implantation method, for example. Thereafter, for example, SiN is deposited so as to cover the whole surface of the semiconductor substrate  1  by the CVD method, for example, and then SiN is etched back. 
         [0075]    At this time, as shown in  FIG. 7 , the spacer insulating layer  7  is formed on the side wall of the floating gate electrode  3 . At this time, a recess portion defined by the spacer insulating layer  7  is formed on the source diffusion layer  5 . The spacer insulating layer  7  is not necessarily formed when the short margin between the source line contact and the gate electrode which will be afterwards formed is secured. 
         [0076]    Thereafter, for example, SiN film  8  which will become stopper film for CMP in a subsequent step is formed so as to cover the whole surface of the semiconductor substrate  1 . Furthermore, the first insulation layer  9  formed of BPSG (Boron Phosphorous Silicate Glass), TEOS or the like is formed by using the HDP-CVD method and the surface is planarized by the CMP method so that the upper end thereof is coincident with the upper end of the SIN film  8  as stopper film. 
         [0077]    Next, as shown in  FIGS. 8A and 8B , a contact hole X is formed in the insulating layer  9  by RIE so that the surface of the source diffusion layer  5  is exposed. An aspect ratio to form the contact hole X is determined in proportion to the film thickness (height) of the floating gate electrode  3 . Accordingly, the aspect ratio can be reduced as compared with the case where the aspect ratio is determined on the basis of the height of the stacked gate, and thus the size of the source diffusion layer  5  can be reduced. Furthermore, the contact hole X has a slit structure extending in the channel width direction. 
         [0078]    Subsequently, the first source-line contact portion SC 1  which is formed, for example, of tungsten (W) is embedded in the contact hole X so as to be coincident with the upper end of the SiN film  8  as the stopper film. 
         [0079]    After the SiN film  8  is removed by RIE, for example, as shown in  FIG. 9 , ONO film  10 A serving as the inter-electrode insulating film, for example, and polysilicon film  11 A serving as the control gate electrode, for example, is successively deposited on the whole surface of the semiconductor substrate  1 . 
         [0080]    Thereafter, as shown in  FIG. 10 , the polysilicon film and the ONO film is successively etched by the RIE method so as to have a desired gate width, and the inter-electrode insulating film  10  and the control gate electrode  11  are formed. 
         [0081]    As shown in  FIGS. 11A and 11B , barrier film  12  is formed so as to cover the surface of the control gate electrode  11 , and then a second insulating layer  13  is deposited on the whole surface of the semiconductor substrate  1 . Furthermore, the second source-line contact portion SC 2  is embedded, for example, in the tapered contact hole formed in the insulating layer  13  so as to come into contact with the first source-line contact SC 1 . 
         [0082]    Furthermore, the bit-line contact portion BC is embedded in an opening portion formed in the insulating layers  9 ,  13  so as to come into contact with the drain diffusion layer  6 . 
         [0083]    Thereafter, the bit line BL and the source line SL which are formed of metal material such as Al or Cu, for example, is formed on the upper surface of the insulating layer  13 . 
         [0084]    Through the above manufacturing process, the NOR type flash memory according to this embodiment is completed. 
         [0085]    In the memory cell transistor manufactured by the above manufacturing process, the first source-line contact portion coming into contact with the source diffusion layer can be formed at the aspect ratio based on the height of the floating gate electrode. Accordingly, the size of the source diffusion layer can be reduced, and the miniaturization of the memory cell transistor and the reduction of the size of the memory cell array portion can be performed. 
       (b) Application 
       [0086]    In this embodiment, from the viewpoint of lithography and the embedment performance of plug material, the miniaturization effect is larger in the source-line contact portion embedded in the slit-shaped contact hole than the bit-line contact portion. 
         [0087]    Therefore, in this embodiment, the upper end of the source-line contact portion is formed so as to be located at a lower position than the lower end of the control gate electrode, and the size in the channel length direction of the source diffusion layer can be reduced. 
         [0088]    However, this embodiment of the present invention is applied to not only the source-line contact portion, but also the bit-line contact portion, and thus the size of the memory cell transistor can be further reduced. 
         [0089]    In the following example, the embodiment of the present invention is applied to the source-line contact portion and the bit-line contact portion. 
       (i) Structure 
       [0090]      FIG. 12  is a plan view showing this application, and FIG.  13  is a cross-sectional view taken along the XIII-XIII line of  FIG. 11 . As shown in  FIG. 13 , the bit-line contact portion BC comprises a first bit-line contact portion BC 1  formed at the same time as the first source-line contact portion SC 1 , and a second bit-line contact portion BC 2  for connecting the first bit-line contact portion BC 1  to the bit line BL. 
         [0091]    In this application, the first bit-line contact portion BC 1  is formed simultaneously with the first source-line contact portion SC 1 . That is, the contact hole in which the first bit-line contact portion BC 1  will be embedded is formed at the aspect ratio based on the height (film thickness) of the floating gate electrode  3 . Therefore, the aspect ratio of the contact hole can be reduced, and the size in the channel length direction of the drain diffusion layer  6  can be reduced. Accordingly, the size of the source and drain diffusion layers can be reduced, so that the memory cell transistor can be miniaturized and the size of the memory cell array portion can be reduced. 
       (ii) Manufacturing Method 
       [0092]    A method of manufacturing the structure of this application will be described. 
         [0093]    First, in the same process as shown in  FIGS. 5A and 7 , the floating gate electrode  3  is formed on the gate insulating film  2  on the surface of the semiconductor substrate  1 . At this time, the size of an area in which the drain diffusion layer will be formed in a subsequent step is set to be narrower than the size shown in the embodiment (basic example) described above. 
         [0094]    Subsequently, the source and drain diffusion layers  5 ,  6  are formed in self-alignment on the surface of the semiconductor substrate  1  by using the floating gate electrode  3  as a mask. Thereafter, the spacer insulating layer  7 , the stopper film  8  and the first insulating layer  9  are successively formed. 
         [0095]    Then, as shown in  FIG. 14 , the first source-line contact portion SC 1  and the first bit-line contact portion BC 1  are embedded in the contact holes which are formed in the insulating layer  9  so as to come into contact with the diffusion layers  5 ,  6 , respectively. 
         [0096]    Subsequently, in the same process as shown in  FIGS. 9 to 11A ,  11 B, the inter-electrode insulating film  10  and the control gate electrode  11  are successively formed, and further the barrier film  12  and the second insulating layer  13  are formed. 
         [0097]    Then, the second source-line contact portion SC 2  and the second bit-line contact portion BC 2  are formed in the insulating layer  13  in the same step, for example. Thereafter, the source line SL and the bit line BL are formed on the insulating layer  13 , and the NOR type flash memory of this embodiment is completed. 
         [0098]    The memory cell transistor manufactured by the above manufacturing process can form the contact portion in contact with the diffusion layer at the aspect ratio based on the height of the floating gate electrode  3 . Accordingly, the size of the source and drain diffusion layers can be reduced, the memory cell transistor can be miniaturized and the size of the memory cell array portion can be reduced. 
       (d) Modification 
       [0099]    The spacer insulating layer  7  described above is not necessarily formed when the short margin between the contact portion and the gate electrode can be secured. Therefore, the memory cell of the embodiment of the present invention may be designed to have a structure shown in  FIG. 15 . In this case, only the first insulating layer  9  is embedded between the two floating gate electrodes  3  adjacent in the channel length direction. The first source-line contact portion SC 1  is embedded in the insulating layer  9  so as to be in contact with the source diffusion layer  5 . Accordingly, in the structure shown in  FIG. 15 , the size of the source diffusion layer  5  can be reduced, and the memory cell transistor can be miniaturized. Furthermore, the spacer insulating layer is not formed, so that the number of the manufacturing steps can be reduced and the manufacturing cost can be reduced. 
         [0100]    This modification may be applied to the structure of the application described above. 
       (2) Second Embodiment 
       [0101]    As described above, in the NOR type flash memory according to the first embodiment, the floating gate electrode and the control gate electrode are formed in different steps in order to reduce the aspect ratio for forming the source-line contact. Therefore, the floating gate electrode and the control gate electrode may be misaligned as compared with the case where the floating gate electrode and the control gate electrode are subjected to gate processing at the same time. 
         [0102]    In this embodiment, the structure of a memory cell transistor which can permit the misalignment (the positional displacement) between the control gate electrode and the floating gate electrode in addition to the effect of the first embodiment, and a method of manufacturing the structure concerned will be described. In the following description, the same elements as the first embodiment are represented by the reference numerals, and the detailed description thereof is omitted. 
       (a) Structure 
       [0103]      FIG. 16  is a cross-sectional view showing the structure in the channel length direction of this embodiment. 
         [0104]    This embodiment has a feature that the width in the channel length direction of the control gate electrode  11  is smaller than the width in the channel length direction of the floating gate electrode  3 . The misalignment between the control gate electrode  11  and the floating gate electrode  3  can be permitted by the above feature. 
         [0105]    Furthermore, even when the structure of the stacked gate electrode as described above is established, the structure of the first source-line contact portion SC 1  is not adversely affected. Accordingly, the size of the source diffusion layer  5  can be reduced. 
         [0106]    Furthermore, the short-circuit between the bit line contact portion BC and the control gate electrode  11  can be also prevented. Accordingly, according to this embodiment, the memory cell transistor can be miniaturized, and the size of the memory cell array portion can be reduced. In addition, the misalignment between the stacked gate electrodes can be permitted, and the manufacturing yield of the NOR type flash memory can be enhanced. 
       (b) Manufacturing Method 
       [0107]    A manufacturing method according to this embodiment will be described. 
         [0108]    First, the floating gate electrode  3  and the drain and source diffusion layers  5 ,  6  are successively formed by using the same process as shown in  FIGS. 5A to 9  of the first embodiment. Subsequently, the spacer insulating layer  7 , the stopper film  8  and the first insulating layer  9  are successively formed. 
         [0109]    Thereafter, the first source-line contact portion SC 1  is embedded in the insulating layer  9  so as to be in contact with the source diffusion layer  5 . Furthermore, the ONO film  10 A serving as the inter-electrode insulating film and the polysilicon film  11 A serving as the control gate electrode are deposited. 
         [0110]    Subsequently, the polysilicon film  11 A is subjected to the patterning for forming the control gate electrode. In this pattern, the size in the channel length direction of the control electrode is smaller than the size in the channel length direction of the floating gate electrode  3 . When etching is carried out on the basis of the pattern concerned by the RIE method, the size in the channel length direction of the control gate electrode  11  is smaller than the size in the channel length direction of the floating gate electrode  3  as shown in  FIG. 17 . 
         [0111]    Subsequently, as shown in  FIG. 16 , the barrier film  12  and the second insulating layer  13  are formed in the same process as shown in  FIGS. 10 ,  11 A and  11 B. Furthermore, the bit-line contact portion BC is formed in the insulating layers  9 ,  13 . Furthermore, the second source-line contact portion SC 2  is embedded in the insulating layer  13  so as to be in contact with the first source-line contact portion SC 1 . Thereafter, the source line SL and the bit line BL are formed on the insulating layer  13 , thereby completing the NOR type flash memory of this embodiment. 
         [0112]    In the memory cell transistor manufactured by the above manufacturing process, the contact portion which comes into contact with the diffusion layer can be formed at the aspect ratio based on the height of the floating gate electrode. Accordingly, the size of the diffusion layer can be reduced, the memory cell transistor can be miniaturized and the size of the memory cell array portion can be reduced. 
         [0113]    Furthermore, the size in the channel length direction of the control gate electrode is set to be smaller than the size in the channel length direction of the floating gate electrode, whereby the misalignment between the two stacked gate electrodes can be permitted. 
         [0114]    The structure and the manufacturing method of the stacked gate electrodes in this embodiment are also applicable to the application and modification of the first embodiment. 
       (3) Third Embodiment 
       [0115]    As described in the second embodiment, according to the embodiments of the invention, the floating gate electrode  3  and the control gate electrode are subjected to the gate processing in different steps, and thus they are misaligned. In the second embodiment, when the control gate electrode is subjected to the gate processing, the gate processing is conducted so that the size in the channel length direction of the control gate electrode is smaller than the size in the channel length direction of the floating gate electrode, thereby permitting the misalignment between the control gate electrode and the floating gate electrode. 
         [0116]    In this embodiment, a structure and a manufacturing method which can prevent the misalignment in a self-alignment style. The same elements as the first and second embodiments are represented by the same reference numerals, and the detailed description thereof is omitted. 
       (a) Structure 
       [0117]      FIG. 18  is a diagram showing the cross-section in the channel length direction of this embodiment. 
         [0118]    As shown in  FIG. 18 , the control gate electrode  11  is embedded in self-alignment in a recess portion defined by the side surface of the spacer insulating layer  7  and the upper surface of the floating gate electrode  3 . Therefore, no misalignment occurs between the floating gate electrode  3  and the control gate electrode  11 . 
         [0119]    The upper end of the first source-line contact portion SC 1  is covered by the inter-electrode insulating film  10 . That is, the inter-electrode insulating film is disposed on the area of the source diffusion layer  5 . The upper end of the first source-line contact portion SC 1  is located at a lower position than the upper end of the bit-line contact portion BC. 
         [0120]    The control gate electrode  11  is embedded in self-alignment in the recess portion by CMP. At this time, it is desired that the upper end of the control gate electrode  11  is perfectly coincident with the upper end of the inter-electrode insulating film  10  on the source-line contact portion SC 1 . However, in the surface polishing based on CMP, a dishing phenomenon occurs, and thus there occurs such a case that the upper end of the control gate electrode  11  is lower than the upper end of the inter-electrode insulating film  10  on the source-line contact portion SC 1 .  FIG. 18  shows the case where the upper end of the control gate electrode  11  is coincident with the upper end of the inter-electrode insulating film  10  on the source-line contact portion SC 1 . 
         [0121]    Furthermore, in this embodiment, the recess portion in which the control gate electrode  11  is embedded is defined by the side surface of the spacer insulating layer  7  and the upper surface of the floating gate electrode  3  as shown in  FIG. 18 , however, this embodiment is not limited to this structure. For example, as shown in  FIG. 15 , when no spacer insulating layer is formed, the reception portion may be defined by the side surface of the first insulating layer  9  and the upper surface of the floating gate electrode  3 . 
         [0122]    As described above, according to this embodiment, the size of the source diffusion layer can be reduced. Therefore, the memory cell transistor can be miniaturized, and the size of the memory cell array portion can be reduced. Furthermore, the misalignment between the two stacked gate electrodes can be prevented by a self-alignment method, and the manufacturing yield of the NOR type flash memory can be enhanced. 
         [0123]    The structure of this embodiment may be also applied to the application and modification of the first embodiment. 
       (b) Manufacturing Method 
       [0124]    The manufacturing method of this embodiment will be described. 
         [0125]    First, the gate insulating film  2 , the floating gate electrode  3  and the source and drain diffusion layers  5 ,  6  are successively formed by the same process as shown in  FIGS. 5A and 6 . At this time, the floating electrode  3  is formed to be thicker than a desired film thickness. 
         [0126]    Subsequently, the spacer insulating layer  7 , the stopper film  8  and the first insulating layer  9  are successively formed by the same step as shown in  FIGS. 7 and 8 . Then, the first source-line contact portion SC 1  is embedded in a contact hole formed in the first insulating layer  9  so as to be in contact with the source diffusion layer  5 . 
         [0127]    Thereafter, the floating gate electrode  3  is selectively etched by RIE as shown in  FIG. 19 , for example, whereby a recess portion Y defined by the upper surface of the floating gate electrode  3  and the side surface of the spacer insulating layer  7  is formed on the channel region. In this etching step, the floating gate electrode  3  is formed to have a desired film thickness, for example. 
         [0128]    Subsequently, as shown in  FIG. 20 , the inter-electrode insulating film  10  is formed on the floating gate electrode  3  and on the first source-line contact portion SC 1 . Then, polysilicon is embedded in the recess portion so as to be coincident with the upper end of the inter-electrode insulating film by CVD or CMP, for example. Accordingly, the control gate electrode  11  is formed in self-alignment on the floating gate electrode  3  through the inter-electrode insulating film  10 . 
         [0129]    Thereafter, the barrier film  12  and the second insulating layer  13  are successively formed by the same process as shown in  FIGS. 10 ,  11 A and  11 B. Furthermore, the second source-line contact portion SC 2 , the bit-line contact portion BC, the source line SL and the bit line BL are formed, thereby completing the NOR type flash memory of this embodiment shown in  FIG. 18 . 
         [0130]    In this embodiment, it is not necessarily to form the barrier film  12 . 
         [0131]    In the memory cell transistor manufactured by the above manufacturing process, the contact portion which is in contact with the diffusion layer can be formed at the aspect ratio based on the height of the floating gate electrode. Accordingly, the size of the source diffusion layer can be reduced, the memory cell transistor can be miniaturized and the size of the memory cell array portion can be reduced. 
         [0132]    Furthermore, according to the foregoing manufacturing method, the misalignment between the control gate electrode and the floating gate electrode can be prevented by the self-alignment method, and thus the manufacturing yield of the NOR type flash memory can be enhanced. 
         [0133]    The manufacturing method of this embodiment is also applicable to the application and modification of the first embodiment. According to the present invention, the memory cell transistor can be micro-structured (miniaturized), and the size of the memory cell array portion can be reduced. 
         [0134]    The present invention is not limited to the above-described embodiments, and various modifications may be made to the respective constituent elements without departing from the subject matter of the present invention. Various embodiments may be constructed by properly combining plural constituent elements disclosed in the above-described embodiments. For example, some constituent elements may be deleted from all the constituent elements disclosed in the above-described embodiments, or constituent elements of different embodiments may be properly combined. 
         [0135]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. 
         [0136]    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.