Abstract:
A method of fabricating a semiconductor memory device, including a step of forming first and second side wall insulation films which includes the steps of: depositing a first insulation film on the gate electrode such that the first insulation film covers the first and second side walls of the gate electrode; applying a first anisotropic etching process proceeding generally perpendicularly to a principal surface of the substrate, to the first insulation film to form first and second lower side wall insulation films, respectively, on the first and second side walls of the gate electrode in an intimate contact therewith; exposing the first and second lower side wall insulation films to a nitriding atmosphere; depositing a second insulation film on the gate electrode such that the second insulation film covers the first and second lower side wall insulation films; and applying a second anisotropic etching process proceeding generally perpendicularly to the process proceeding generally perpendicularly to the principal surface of the substrate, to the second insulation film to form first and second upper side wall insulation films, respectively, on the first and second lower side wall insulation films.

Description:
This application is a Division of prior application Ser. No. 09/014,247 filed Jan. 27, 1998 now U.S. Pat. No. 6,392,310. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor devices and, more particularly, to a miniaturized semiconductor device and a fabrication process thereof in which leakage current is minimized. 
     2. Description of the Related Art 
     With the continuous progress of device miniaturization, the integration density of semiconductor integrated circuit devices, particularly the integration density of memory semiconductor integrated circuit devices, is increasing every year. These days, intensive investigations are being made on so-called sub-halfmicron devices having a minimum pattern width of 0.3 μm or less. 
     In a DRAM (dynamic random access memory) that stores information in a memory cell capacitor in the form of electric charges, the device miniaturization inevitably causes a decrease in the capacitance of the memory cell capacitor, and there is a tendency that the retention of information in the memory cell capacitor becomes unstable. Further, the read/write operation may become also unstable in such extremely miniaturized DRAMs. Similar problems also occur in so-called flash memory devices in which information is stored in a floating gate in the form of electric charges. Thus, various efforts are being made for stabilizing the operation of the miniaturized DRAMs and flash memories. 
     FIGS. 1A-1F show a conventional fabrication process of a DRAM. 
     Referring to FIG. 1A, an active region is defined on an Si substrate  1  typically doped to the p-type, by providing a field oxide film  2 A and SiO 2  on the Si substrate  1 , and a thin thermal oxide film  2 B also of SiO 2  is formed on the active region thus defined on the Si substrate  1  by the field oxide film  2 A. Further, a word line WL of polysilicon is provided on the Si substrate  1  so as to extend over the substrate  1  thus covered by the field oxide film  2 A and further the thermal oxide film  2 B, wherein the word line WL extends over the thermal oxide film  2 B in the active region and the word line WL thus extending over the thermal oxide film  2 B forms a gate electrode of a memory cell transistor. Thereby, the thermal oxide film  2 B forms a gate insulation film of the memory cell transistor. 
     In the step of FIG. 1A, an ion implantation process of P +  is conducted further into the Si substrate  1  while using the gate electrode  3  as a self-aligned mask, and there are formed diffusion regions  1 A and  1 B of the memory cell transistor in the Si substrate  1  at both lateral sides of the gate electrode  3 . 
     Next, in the step of FIG. 1B, an oxide film  4  of SiO 2  is deposited on the structure of FIG. 1A by a high temperature CVD process so as to cover the gate electrode  3 , and an anisotropic etching process acting generally perpendicularly to a principal surface of the substrate  1  is applied to the thermal oxide film  4  in the step of FIG. 1C by an RIE (reactive ion etching) process, to form side wall oxide films  4 A and  4 B covering both side walls of the gate electrode  3 . In the step of FIG. 1C, it is also possible, while not illustrated, to conduct an ion implantation process of P +  while using the gate electrode  3  and further the side wall oxide films  4 A and  4 B as a self-aligned mask, to form a so-called LDD (lightly-doped drain) structure. 
     Next, in the step of FIG. 1D, an interlayer insulation film  5  of BPSG (borophosophosilicate glass) is deposited on the structure of FIG. 1C, followed by a formation of a contact hole SA in the interlayer insulation film  5  so as to expose the diffusion region  1 A. Further, an electrode  6  is provided as a part of a bit line such that the electrode  6  fills the contact hole  5 A and achieves an electrical contact to the exposed diffusion region  1 A. 
     Further, in the step of. FIG. 1E, another interlayer insulation film  7  of BPSG is deposited on the structure of FIG. 1D, followed by a formation of a contact hole  7 A penetrating through the interlayer insulation films  7  and  5  such that the contact hole  7 A exposes the foregoing diffusion region  1 B. 
     Finally, in the step of FIG. 1F, an accumulation electrode  8 A of polysilicon is formed so as to fill the contact hole  7 A in electrical contact with the diffusion region  1 B, and a dielectric film  8 B having a so-called ONO structure, in which a thin SiN film is vertically sandwiched by a pair of thin SiO 2  films, is provided so as to cover the accumulation electrode  8 A. Further, an opposing electrode  8 C of polysilicon is provided so as to cover the foregoing dielectric film  8 B. Thereby, the electrode  8 A, the dielectric film  8 B and the opposing electrode  8 C form together a memory cell capacitor  8 . 
     In the DRAM of the foregoing conventional structure, it has been discovered that there are cases in which a leakage current flows between the accumulation electrode  8 A and the gate electrode  3  when the DRAM is miniaturized particularly to the degree in which the minimum pattern width is 0.3 μm or less. As the accumulation electrode  8 A forms a part of the memory cell capacitor  8  that holds the information in the form of electric charges, the leakage current occurring in the electrode  8 A causes a serious problem in the operation of the DRAM, particularly the stability of data retention. an enlarged scale. 
     Referring to FIG. 2A, it can be seen that the gate electrode  3  carries an anti-reflection film  3 A that has been used for patterning the gate electrode  3 . Further, a CVD oxide film  5 B is provided between the side wall oxide film  4 A or  4 B and the interlayer insulation film  5 . In order to secure a sufficient distance between the gate electrode  3  and the electrode  8 A in the contact hole  5 A, the contact hole  5 A is formed to have a tapered structure in which the diameter reduces gradually from a top surface to a bottom surface of the contact hole  5 A. 
     In such sub-halfmicron DRAMs having a minimum pattern width of 0.3 μm or less, it is actually difficult to form the contact hole  5 A in the ideally aligned state as shown in FIG. 2A, and actual devices generally have a structure shown in FIG. 2B, in which it will be noted that the contact hole  5 A is offset from the ideal state of FIG.  2 A. In the structure of FIG. 2B, the accumulation electrode  8 A filling the contact hole  5 A approaches the gate electrode  3 , and it is believed that such a reduction in the distance between the gate electrode  3  and the electrode  8 A causes the leakage current to flow between the accumulation electrode  8 A and the gate electrode  3 , although the exact current path of the leakage current is not fully explored yet. 
     As will be explained later, the problem of leakage current appears particularly conspicuous when an etching is applied to the diffusion region  1 B by a buffered HF solution for removing a oxide film from the surface of the diffusion region  1 B. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor device and a fabrication process thereof wherein the foregoing problems are eliminated. 
     Another and more specific object of the present invention is to provide a semiconductor device having a gate electrode in which leakage current to the gate electrode from an opposing electrode is successfully minimized, as well as a fabrication process thereof. 
     Another object of the present invention is to provide a semiconductor device., comprising: 
     a substrate; 
     a gate electrode provided on said substrate; 
     a side wall insulation film covering a side wall of said gate electrode; 
     a diffusion region formed in said substrate adjacent to said gate electrode; 
     an ohmic electrode formed on said diffusion region; and 
     a nitride film provided between said side wall insulation film and said wall of said gate electrode, such that said nitride film covers a part of a surface of said gate electrode facing said ohmic electrode. 
     Another object of the present invention is to provide a method of fabricating a semiconductor device, comprising the steps of: 
     forming a gate electrode on a substrate; 
     forming a diffusion region in said substrate adjacent to said gate electrode; 
     forming a side wall insulation film on a side wall of said gate electrode; 
     depositing an interlayer insulation film on said gate electrode provided with said side wall insulation film; 
     forming a contact hole in said interlayer insulation film so as to expose a surface of said diffusion region; and 
     forming an ohmic electrode so as to fill said contact hole; 
     wherein said method further comprises a step, before said step of depositing said interlayer insulation film., of depositing a nitride film such that said nitride film covers at least a part of said gate electrode that faces said ohmic electrode. 
     According to the present invention, the leakage current path between the gate electrode and the ohmic electrode is successfully interrupted by providing the nitride film such that the nitride film covers a part of the gate electrode that faces the ohmic electrode. 
     Another object of the present invention is to provide a DRAM, comprising: 
     a substrate; 
     a gate electrode provided on said substrate and forming a part of a word line; 
     a pair of side wall insulation films covering both lateral side walls of said gate electrode; 
     first and second diffusion regions formed in said substrate at both lateral sides of said gate electrode; 
     a first interlayer insulation film covering said gate electrode including said pair of side wall insulation films; 
     a first contact hole formed in said first interlayer insulation film so as to expose said first diffusion region; 
     a first electrode provided on said first interlayer insulation film so as to fill said first contact hole in contact with said first diffusion region, said first electrode thereby forming a part of a bit line; 
     a second interlayer insulation film provided on said first interlayer insulation film so as to cover said first electrode; 
     a second contact hole formed in said second interlayer insulation film so as to penetrate through said first interlayer insulation film, said second contact hole exposing said second diffusion region; 
     a second electrode provided on said second interlayer insulation film so as to fill said second contact hole in contact with said second diffusion region, said second electrode thereby forming an accumulation electrode of a memory cell capacitor; 
     a dielectric film provided on a surface of said second electrode as a capacitor electrode of said memory cell capacitor; 
     a third electrode provided on said dielectric film so as to sandwich said dielectric film therebetween together with said second electrode, said third electrode thereby forming an opposing electrode of said memory cell capacitor; and 
     a nitride film provided so as to cover at least a part of said gate electrode that faces said accumulation electrode. 
     Another object of the present invention is to provide a method of fabricating a DRAM, comprising the steps of: 
     forming a gate electrode on a substrate; 
     forming first and second diffusion regions in said substrate respectively adjacent to a first side wall and a second side wall of said gate electrode; 
     forming first and second side wall insulation films respectively on said first and second side walls of said gate electrode; 
     depositing a first interlayer insulation film such that said first interlayer insulation film covers said gate electrode and said first and second side wall insulation films; 
     forming a first contact hole in said first interlayer insulation film such that said first contact hole exposes said first diffusion region; 
     forming a bit line pattern on said first interlayer insulation film such that said bit line pattern fills said first contact hole in contact with said first diffusion region; 
     forming a second interlayer insulation film on said first interlayer insulation film such that said second interlayer insulation film fills said bit line pattern; 
     forming a second contact hole in said second interlayer insulation film such that said second contact hole penetrates through said first interlayer insulation film and exposes said second diffusion region; 
     forming an accumulation electrode of a memory cell capacitor such that said accumulation electrode fills said second contact hole and achieves an electrical contact with said second diffusion region; 
     forming a capacitor insulation film on said accumulation electrode; and 
     forming an opposing electrode on said capacitor electrode; 
     wherein said method further includes a step, after said step of forming said gate electrode but before said step of forming said first interlayer insulation film, of depositing a nitride film such that said nitride film covers a part of said gate electrode facing said accumulating electrode. 
     According to the present invention, the current path of the leakage current between the accumulating electrode and the gate electrode is interrupted by providing the nitride film, and the problem of loss of information caused by the dissipation of the electric charges held in the accumulating electrode of the memory cell capacitor is successfully eliminated. Thereby, the DRAM shows an excellent stability of data retention even when the device is miniaturized to a sub-halfmicron size. 
     Another object of the present invention is to provide a flash memory, comprising: 
     a substrate; 
     a gate electrode structure provided on said substrate, said gate electrode structure including: a floating electrode provided on said substrate, said floating electrode being isolated from said substrate by a tunnel insulation film intervening therebetween; and a control electrode provided on said floating electrode with a floating insulation film intervening between said control electrode and said floating electrode, said control electrode thereby forming a part of a word line, said gate electrode structure being defined by a pair of side walls; 
     a pair of side wall insulation films respectively covering said pair of side walls of said gate electrode structure; 
     first and second diffusion regions formed in said substrate at both lateral sides of said gate electrode structure; 
     an interlayer insulation film covering said gate electrode structure including said pair of side wall insulation films; 
     first and second contact holes formed in said interlayer insulation film so as to expose said first and second diffusion regions respectively; 
     a first electrode provided on said interlayer insulation film so as to fill said first contact hole in contact with said first diffusion region, said first electrode thereby forming a part of a bit line; 
     a second electrode provided on said interlayer insulation film so as to fill said second contact hole in contact with said second diffusion region; and 
     a nitride film provided on said gate electrode structure so as to cover at least one of said side walls such that said nitride film intervenes between said wall of said gate electrode structure and corresponding said side wall insulation film. 
     Another object of the present invention is to provide a method of fabricating a flash memory, comprising the steps of: 
     forming a tunnel insulation film on a substrate; 
     forming a gate structure by depositing a floating gate electrode, a floating insulation film and a control gate consecutively on said tunnel insulation film; 
     forming a diffusion region in said substrate while using said gate structure as a mask; 
     depositing an interlayer insulation film on said substrate such that said interlayer insulation film covers said gate structure; 
     forming a contact hole in said interlayer insulation film such that said contact hole exposes said diffusion region; and 
     forming an ohmic electrode on said interlayer insulation film such that said ohmic electrode fills said contact hole in contact with said diffusion region; 
     wherein said method further comprises a step, after said step of forming said gate electrode but before said step of depositing said interlayer insulation film, of forming a nitride film on said gate structure such that said nitride film covers at least a part of said gate structure facing said electrode. 
     According to the present invention, the problem of leakage of electric charges from the floating electrode of the gate structure is successfully eliminated by interrupting the leakage current path by providing the nitride film on the side wall of the gate structure. 
     Another object of the present invention is to provide a semiconductor device comprising: 
     a substrate, 
     a gate electrode formed on said substrate; 
     a diffusion region formed in said substrate adjacent to said gate electrode; 
     an ohmic electrode contacting said diffusion region; and 
     a side wall insulation film formed on a side wall of said gate electrode; 
     said side wall comprising a first insulation film contacting said side wall of said gate electrode at a side thereof facing said ohmic electrode, and a second insulation film formed on said first insulation film. 
     Another object of the present invention is to provide a method of fabricating a semiconductor device, comprising: 
     forming a gate electrode on a substrate; 
     forming a diffusion region in said substrate adjacent to said gate electrode; 
     forming a side wall insulation film on a side wall of said gate electrode; 
     depositing an interlayer insulation film on said gate electrode formed with said side wall insulation film; 
     forming a contact hole in said interlayer insulation film such that said contact hole exposes said diffusion region; and 
     forming an ohmic electrode such that said ohmic electrode fills said contact hole in electrical contact with said diffusion region; 
     wherein said step of forming said side wall insulation film comprises the steps of: 
     forming a first insulation film on said gate electrode such that said first insulation film covers said gate electrode including said side wall; 
     applying a first anisotropic etching process to said first insulation film such that said first anisotropic etching process proceeds generally perpendicularly to a principal surface of said substrate, a remaining part of said first insulation film forming thereby a first side wall insulation film covering said side wall of said gate electrode; 
     forming a second insulation film on said gate electrode such that said second insulation film covers said gate electrode including said first side wall insulation film; 
     applying a second anisotropic etching process to said second insulation film such that said second anisotropic etching process proceeds generally perpendicularly to said principal surface of said substrate, a remaining part of said second insulation film forming thereby a second side wall insulation film covering said first side wall insulation film laterally. 
     Another object of the present invention is to provide a method of fabricating a semiconductor memory device, comprising the steps of: 
     forming a gate electrode on a substrate; 
     forming first and second diffusion regions in said substrate respectively adjacent to first and second side walls of said gate electrode; 
     forming first and second side wall insulation films respectively on said first and second side walls of said gate electrode; 
     forming a first interlayer insulation film on said gate electrode such that said first interlayer insulation film covers said first and second side wall insulation films; 
     forming a first contact hole in said first interlayer insulation film such that said first contact hole exposes said first diffusion region; 
     forming a bit line pattern on said first interlayer insulation film so as to fill said first contact hole in electrical contact with said first diffusion region; 
     forming a second interlayer insulation film on said first interlayer insulation film so as to cover said bit line pattern; 
     forming a second contact hole in said second interlayer insulation film such that said second contact hole penetrates through said first interlayer insulation film and exposes said second diffusion region; 
     forming an accumulation electrode of a memory cell capacitor such that said accumulation electrode fills said second contact hole and contacts said second diffusion region; 
     forming a capacitor dielectric film on said accumulation electrode; and 
     forming an opposing electrode on said capacitor dielectric film, 
     wherein said step of forming said first and second side wall insulation films includes the steps of: 
     depositing a first insulation film on said gate electrode such that said first insulation film covers said first and second side walls of said gate electrode; 
     applying a first anisotropic etching process proceeding generally perpendicularly to a principal surface of said substrate, to said first insulation film to form first and second, lower side wall insulation films respectively on said first and second side walls of said gate electrode in an intimate contact therewith; 
     depositing a second insulation film on said gate electrode such that said second insulation film covers said first and second lower side wall insulation films; and 
     applying a second anisotropic etching process proceeding generally perpendicularly to said principal surface of said substrate, to said second insulation film to form first and second, upper side wall insulation films respectively on said first and second lower side wall insulation films. 
     According to the present invention, the leakage current from the gate electrode is suppressed successfully by forming the side wall insulation film of the gate electrode by two different side wall insulation films. 
     Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1F are diagrams showing a process of fabricating a conventional DRAM; 
     FIGS. 2A and 2B are diagrams explaining the problems pertinent to the conventional DRAM; 
     FIG. 3 is a diagram explaining the principle of the present invention; 
     FIGS. 4A and 4B are further diagrams explaining the principle of the present invention; 
     FIGS. 5A and 5B are further diagrams explaining the principle of the present invention; 
     FIG. 6 is a further diagram explaining the principle of the present invention; 
     FIGS. 7A-7G are diagrams showing the fabrication process of a DRAM according to a first embodiment of the present invention; 
     FIGS. 8A-8G are diagrams showing the fabrication process of a flash memory according to a second embodiment of the present invention; 
     FIGS. 9A and 9B are diagrams showing a modification of the first embodiment of the present invention; and 
     FIGS. 10A-10I are diagrams showing the fabrication process of a DRAM according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows the principle of the present invention, wherein those parts of FIG. 3 explained previously with reference to preceding drawings are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 3, the present invention uses a SiN film  9  on the gate electrode  3  such that the SiN film  9  covers a top surface of the gate electrode  3  (more precisely the anti-reflection film  3 A) as well as a side wall of the gate electrode  3 , and the side wall oxide film  4 B is provided on the SiN film  9 . It should be noted that the SiN film  9  is provided on a thermal oxide film forming an extension of the gate oxide film  2 B extending from the side wall of the gate electrode  3  to the contact hole  5 A. 
     By constructing the semiconductor device as indicated in FIG. 3, the leakage current between the accumulation electrode  8 A filling the contact hole  5 A and the gate electrode  3  is effectively suppressed. 
     FIG. 4A shows the experiments conducted on the leakage current occurring in a capacitor formed on a Si substrate  11 . 
     Referring to FIG. 4A, the Si substrate  11  is covered by a thick SiO 2  film  12  by a wet oxidation process for example, and a first polysilicon electrode pattern  13  is provided on the SiO 2  film  12 . Further, an SiO 2  film  14  is provided on the polysilicon electrode pattern  13  by a high temperature CVD process with a thickness of about 50 nm such that the SiO 2  film  14  covers the polysilicon electrode pattern  13 . Further, a second polysilicon pattern  15  is deposited on the SiO 2  film  14 . 
     FIG. 4B shows the result of the experiments of FIG. 4A, wherein the horizontal axis of FIG. 4B shows a leakage current while the vertical axis represents the number of specimens that showed a specific leakage current. In FIG. 4B, it should be noted that the solid circles represent the result for a case in which the polysilicon electrode pattern  15  is formed immediately after the formation of the SiO 2  film  14 , while the open circles represent the result for a case in which the SiO 2  film  14  is processed by a buffered HF solution prior to the formation of the electrode pattern  15 . 
     Referring to FIG. 4B, it should be noted that the leakage current is less than 10 −7 A for most of the specimens when the electrode pattern  15  is formed immediately after the formation of the SiO 2  film. On the other hand, when the SiO 2  film  14  is processed by a buffered HF solution, the number of the specimens that show a leakage current exceeding 10 −7 A increases significantly, in spite of the fact that the thickness of the SiO 2  film  14  reduces only 4%, from 50 nm to 48 nm, as a result of the treatment by the buffered HF solution. 
     In the structure of FIG. 3 or FIGS. 2A and 2B explained previously, it should be noted that the surface of the diffusion region  1 B exposed at the bottom of the contact hole  5 A is processed by a buffered HF solution for removing a oxide film therefrom prior to the formation of the accumulation electrode  8 A. Thus, the observed deterioration of the leakage current characteristic in the structure of FIGS. 2A and 2B is explained by the result of FIG.  4 B. 
     FIG. 5A shows a similar experiment on the leakage current occurring in a capacitor, in which a stacked structure of an SiO 2  film  16  and an SiN film  17  is interposed between the SiO 2  film  14  and the polysilicon electrode pattern  15 , wherein the SiO 2  film  16  and the SiN film  17  are formed by a high temperature CVD process. In the structure of FIG. 5A, a treatment by a buffered HF solution is applied after the formation of the SiN film  17  but before the formation of the polysilicon electrode pattern  15 . 
     FIG. 5B shows the result of the experiments conducted on the structure of FIG. 5A, wherein the x marks in FIG. 5B show the case in which the thickness of the SiO 2  film  16  is set to about 10 nm and the thickness of the SiN film  17  is set to about 6 nm, while the + marks represent the case in which thickness of the SiO 2  film  16  and the thickness of the SiN film  17  are both set to 10 nm. Further, the open circles in FIG. 5B represent the case in which no SiO 2  film  16  or SiN film  17  is provided. In other words, the open circles represent the result for the structure of FIG.  4 B. 
     Referring to FIG. 5B, it is clearly seen that the leakage current is reduced significantly by forming the SiN film  17  as compared with the case in which no SiN film  17  is formed. In other words, the result of FIG. 5B clearly indicates that the leakage current between the electrode  8 A and the electrode  3  is effectively suppressed by providing the SiN film  9 . 
     FIG. 6 is another diagram showing the principle of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 6, another side wall oxide film  4 C is provided on the structure of FIG. 2A or  2 B such that the side wall oxide film  4 C covers an outer lateral side of the side wall oxide film  4 B, wherein the side wall oxide film  4 C is formed similarly to the side wall oxide film  4 B by depositing an SiO 2  film on the gate oxide film  3  as well as on the side wall oxide film  4 B by a high temperature CVD process, followed by an anisotropic etching process acting substantially perpendicularly to the substrate principal surface. In the foregoing process of forming the structure of FIG. 6, it should be noted that the surface of the side: wall oxide film  4 B is processed in an N 2 O atmosphere prior to the deposition of the SiO 2  film, at a temperature substantially identical to the deposition temperature of the SiO 2  film. Thereby, a doped region doped by N is formed on the surface of the side wall oxide film  4 A as indicated in FIG. 6 by hatching. 
     It should be noted that the structure of FIG. 6, using a multilayered structure for the gate side wall oxide film, is also effective for suppressing the leakage current between the electrode  8 A and the electrode  3 . Of course, the multilayered structure of the gate side wall oxide film is not limited to the two-layer structure shown in FIG. 6 but three or more layer structure may also be employed. 
     First Embodiment 
     FIGS. 7A-7G show a fabrication process of a DRAM according to a first embodiment of the present invention. 
     Referring to FIG. 7A, an active region is defined on a Si substrate  21  typically doped to the p-type, by providing a field oxide film  22 A of SiO 2  on the Si substrate  21 , and a thin thermal oxide film  22 B also of SiO 2  is formed on the active region thus defined on the Si substrate  21  by the field oxide film  22 A. Further, a word line WL of polysilicon is provided on the Si substrate  21  so as to extend over the substrate  21  thus covered by the field oxide film  22 A and further the thermal oxide film  22 B, wherein the word line WL extends over the thermal oxide film  22 B in the active region and the word line WL thus extending over the thermal oxide film  22 B forms a gate electrode of a memory cell transistor. Thereby, the thermal oxide film  22 B forms a gate insulation film of the memory cell transistor. 
     In the step of FIG. 7A, an ion implantation process of P +  is conducted further into the Si substrate  21  while using the gate electrode  23  as a self-aligned mask, and there are formed diffusion regions  21 A and  21 B of the memory cell transistor in the Si substrate  21  at both lateral sides of the gate electrode  23 . 
     Next, in the step of FIG. 7B, an SiN film  24  is deposited on the structure of FIG. 7A by a CVD process or the like, such that the SiN film  24  covers the top surface as well as both side walls of the gate electrode  23 , typically with a thickness of about  30  nm, followed by a deposition of an SiO 2  film  25  in a step of FIG. 7C by a high temperature CVD process such that the SiO 2  film  25  covers the SiN film  24 . 
     Next, in the step of FIG. 7D, an anisotropic etching process acting generally perpendicularly to a principal surface of the substrate  21  is applied to the SiO 2    25  by an RIE (reactive ion etching) process, to form side wall oxide films  25 A and  25 B covering the both side walls of the gate electrode  23 , with the SiN film  24  intervening therebetween. Further, an ion implantation process of P +  is conducted in the step of FIG. 7D while using the gate electrode  23  and further the side wall oxide films  25 A and  25 B as a self-aligned mask, to form deeper diffusion regions  21 C and  21 D respectively in correspondence to the diffusion regions  21 A and  21 B, wherein the diffusion regions  21 A and  21 C or the diffusion regions  21 B and  21 D form together a so-called LDD (lightly-doped drain) structure. 
     Next, in the step of FIG. 7E, an interlayer insulation film  26  of BPSG (borophosophosilicate glass) is deposited on the structure of FIG. 7D by a CVD process, followed by a formation of a contact hole  26 A in the interlayer insulation film  26  so as to expose the diffusion regions  21 A and  21 C. Further, the exposed surface of the diffusion regions  21 A and  21 C thus exposed at the bottom of the contact hole  26 A is treated by a buffered HF solution to remove a oxide film therefrom, and an electrode  27  is provided as a part of a bit line BL such that the electrode  27  fills the contact hole  26 A. 
     Further, in the step of FIG. 7F, another interlayer insulation film  28  of BPSG is deposited on the structure of FIG. 7E, followed by a formation of a contact hole  28 A penetrating through the interlayer insulation films  26  and  28  such that the contact hole  28 A exposes the foregoing diffusion region  21 B (as well as the diffusion region  21 D). 
     Finally, in the step of FIG. 7G the surface of the diffusion region exposed at the bottom of the contact hole  28 A is processed by a buffered HF solution for removal of a oxide film therefrom, and a polysilicon electrode  29  forming the accumulation electrode of the memory cell capacitor is provided on the structure of FIG. 7F in contact with the diffusion regions  21 B and  21 D, such that the polysilicon electrode  29  fills the contact hole  28 A. Further, a dielectric film  30  having a so-called ONO structure, in which a thin SiN film is vertically sandwiched by a pair of thin SiO 2  films, is provided so as to cover the accumulation electrode  29 . Further, an opposing electrode  31  of polysilicon is provided so as to cover the foregoing dielectric film  30 . Thereby, the electrode  29 , the dielectric film  30  and the opposing electrode  31  form together a memory cell capacitor. 
     In the DRAM thus formed, the SiN film  24  interposed between the polysilicon electrode  29  filling the contact hole  28 A and the gate electrode  23  effectively prevents the leakage current flowing between the polysilicon electrode  29  and the gate electrode  23  even in such a case in which the exposed surface of the diffusion region is processed by an etching treatment caused by the buffered HF solution for removal of the oxide film therefrom. See the relationship of FIG.  5 B. Thereby, the SiN film  24 , having a very small thickness and extending only over the thermal oxide film that forms an extension of the gate insulation film  22 B, does not induce any stress or strain to the essential part of the semiconductor device such as the channel region or diffusion regions, and the operation of the semiconductor device is not deteriorated at all. 
     It should be noted that the present embodiment for fabricating a DRAM is effective also for a DRAM that has a self-aligned contact structure disclosed in the Japanese Laid-Open Patent Publication 8-274278). In such a case, the contact hole  28 A can be formed by using the side wall oxide films  25 A and  25 B as a self-aligned mask. 
     Second Embodiment 
     FIGS. 8A-8G show a fabrication process of a flash memory according to a second embodiment of the present invention. 
     Referring to FIG. 8A, a Si substrate  41  typically doped to the p-type is covered by a field oxide film  42 A of SiO 2  such that the field oxide film  42 A defines an active region on the surface of the Si substrate  41 , wherein the active region is covered by a thin thermal oxide film  42 B also of SiO 2 . 
     Next, in the step of FIG. 8B, a polysilicon pattern  43  is provided on the thermal oxide film  42 B such that the polysilicon pattern  43  extends over the foregoing active region on the Si substrate  41 . It should be noted that the thermal oxide film  42 B acts as a tunnel oxide film of the flash memory to be formed, while the polysilicon pattern  43  forms a part the floating gate. 
     Next, in the step of FIG. 8C, a dielectric film  44  of SiON is provided on the structure of FIG. 8B such that the dielectric film  44  covers the top surface as well as the side walls of the polysilicon pattern  43 , followed by consecutive depositions of a polysilicon film  45  and a WSi film  46  as indicated in FIG. 8C, wherein the structure of FIG. 8C is subjected to a patterning process in the step of FIG. 8D to form gate electrode structures G 1  and G 2  each having a stacked structure of polysilicon layer  43 , the SiON layer  44 , the polysilicon layer  45  and the WSi layer  46 . As noted already, the polysilicon layer  43  acts as the floating gate of the flash memory while the polysilicon layer  45  and the WSi layer  46  form the control electrode. 
     In the step of FIG. 8D, an ion implantation process of P +  or As +  is further conducted into the substrate  41  while using the gate electrode structures G 1  and G 2  as a self-aligned mask, and diffusion regions  41 A,  41 B and  41 C are formed in the substrate  41  as a result of the ion implantation process. 
     Next, in the step of FIG. 8E, an SiO 2  film  47  and an SiN film  48  are deposited consecutively on the structure of FIG. 8D by a high temperature CVD process, such that the SiO 2  film  47  and the SiN film  48  cover each of the gate electrode structure G 1  and the gate electrode structure G 2  continuously including the top surface and both side walls. 
     Next, in the step of FIG. 8F, an interlayer insulation film  49  of BPSG is deposited on the structure of FIG. 8E such that the interlayer insulation film  49  covers the gate electrode structures G 1  and G 2 , and contact holes  49 A- 49 C are formed in the interlayer insulation film  49  thus formed so as to expose the diffusion regions  41 A- 41 C respectively. 
     After the contact holes  49 A- 49 C are thus formed, a wet etching process is applied to the structure of FIG. 8F by using a buffered HF solution, and any oxide films remaining on the surface of the diffusion regions  41 A- 41 C exposed by the contact holes  49 A- 49 C are removed. After the foregoing etching process, ohmic electrodes  50 A- 50 C are formed on the interlayer insulation film  49  such that the ohmic electrodes  50 A- 50 C fill the contact holes  49 A- 49 C respectively, wherein the ohmic electrode  50 A or  50 C forms a part of a bit line BL while the ohmic electrode  50 B are connected to an erasing voltage source together with the corresponding ohmic electrodes of other memory cell transistors. Further, the suicide layer  46  in the gate electrode stricture G, or G 2  is connected to a word line WL as a part of the control electrode. 
     In the flash memory having such a structure, the gate electrode structure G 1  or G 2  is covered continuously by the SiN film  48 . Thereby, the problem of leakage of the electric charges from the floating gate electrode  43  to an adjacent electrode such as the electrode  50 A is effectively suppressed even in such a case in which the electrode  50 A is formed in the vicinity of the floating gate electrode  43 , and a reliable holding of information becomes possible for the flash memory. 
     Modifications 
     FIGS. 9A and 9B show modifications of the first embodiment that uses an SiN film in a DRAM for suppressing the leakage current, wherein those parts of FIGS. 9A and 9B corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     The structure of FIG. 9A is a modification of the construction of FIG.  3  and includes the SiN film  9  such that the SiN film  9  covers the side wall insulation film  4 B rather than contacting the side wall of the gate electrode  3  directly. In such a construction, too, it should be noted that the SiN film  9  covers a part of the gate electrode  3  facing the ohmic electrode  8  and effectively interrupts the leakage current between the electrode  8 A and the electrode  3 . 
     In the modification of FIG. 9B, the structure of the DRAM of FIG. 7G is modified such that the SiN film  24  extends on the side wall oxide films  25 A and  25 B rather than contacting the side wall of the gate electrode  23  directly. In such a structure, too, the SiN film  24  covers a part of the gate electrode  23  facing the accumulation electrode  29 , and the leakage current between the gate electrode  23  and the accumulation electrode  29  is effectively interrupted. 
     Third Embodiment 
     FIGS. 10A-10I are diagrams showing a fabrication process of a DRAM according to a third embodiment of the present invention. 
     Referring to FIG. 10A, a Si substrate  61  typically doped to the p-type is covered by a field oxide film  62 A of SiO 2  such that the field oxide film  62 A defines an active region on the surface of the substrate  61 , and a thermal oxide film is formed on the active region thus defined. Further, a word line WL of polysilicon is provided such that the polysilicon word line WL extends over the substrate  61  covered by the field oxide film  62 A or thermal oxide film  62 B. The word line WL extends over the surface of the substrate  61  thus covered by the field oxide film  62 A or the thermal oxide film  62 B. Thereby, the thermal oxide film  62 B acts,as a gate insulation film of the memory cell transistor. 
     In the step of FIG. 10A, an ion implantation process of P +  is conducted further into the substrate  61  while using the gate oxide film  63  as a self-aligned mask, to form diffusion regions  61 A and  61 B of the memory cell transistor at both lateral sides of the gate electrode  63 . 
     Next, in the step of FIG. 10B, an SiO 2  film  64  is deposited on the structure of FIG. 10A by a high temperature CVD process, such that the SiO 2  film  64  covers the top surface as well as both lateral side walls of the gate electrode  63 . 
     Next, in the step of FIG. 10C, an anisotropic etching process is applied to the SiO 2  film  64  in a direction generally perpendicularly to a principal surface of the substrate  61 , to form side wall oxide films  64 A and  64 B on both side walls of the gate electrode  63 . Further, an ion implantation process of P +  or As +  is conducted in the step of FIG. 10C to form deeper diffusion regions  61 C and  61 D in the substrate  61  respectively in a partially overlapped relationship with the shallower diffusion regions  61 A and  61 B. Thereby, the diffusion region  61 A and the diffusion region  61 C or the diffusion region  61 B and the diffusion region  61 D form an LDD structure. 
     It should be noted that the anisotropic etching process in the step of FIG. 10C is carried out in the same deposition apparatus used for depositing the SiO 2  film  64 , without taking out the substrate  61  from the deposition apparatus, wherein the structure of FIG. 10C thus obtained is then exposed, in a step of FIG. 10D, to an N 2 O atmosphere in the same deposition apparatus at a temperature substantially identical to the temperature used in the foregoing deposition apparatus for depositing an SiO 2  film by a high temperature CVD process. Thereby, the exposed surfaces of the side wall oxide films  64 A and  64 B are doped by N. 
     Next, in the step of FIG. 10E, an SiO 2  film (not shown) is deposited on the structure of FIG. 10D by a high temperature CVD process, followed by an anisotropic etching process generally perpendicularly to the principal surface of the substrate  61 , to form further side wall oxide films  64 C and  64 D respectively on the laterally outer sides of the side wall oxide films  64 A and  64 B. 
     In the step of FIG. 10E, an ion implantation process of P +  or As +  is conducted further into the substrate  61  while using the gate electrode  63  and the side wall oxide films  64 A- 64 D as a self-aligned mask, to form deeper diffusion regions  61 C and  61 D in the substrate  61 , such that the diffusion region  61 C overlaps the shallow diffusion region  61 A partially and such that the diffusion region  61 D overlaps the shallow diffusion region  61 B partially. Thereby, an LDD structure is formed similarly to the previous embodiments. 
     Next, in the step of FIG. 10F, an interlayer insulation film  65  of BPSG, or the like, is deposited on the structure of FIG. 10E by a CVD process, followed by a formation of a contact hole  65 A in the interlayer insulation film  65  thus formed, such that the contact hole  65 A exposes the diffusion region  61 A as well as the diffusion region  61 C. After the formation of the contact hole  65 A, a wet etching process is applied by using a buffered HF solution for removing a oxide film from the exposed surface of the diffusion region, and an ohmic electrode  66  is provided on the interlayer insulation film  65  such that the ohmic electrode  66  fills the contact hole  65 A. Thereby, the ohmic electrode  66  forms a pair of a bit line BL. 
     Next, in the step of FIG. 10G, a second interlayer insulation film  67  of BPSG is deposited on the interlayer insulation film  65  of FIG. 10E so as to cover the ohmic electrode  66 , and a contact hole  67 A is formed such that the contact hole  67 A extends through the first and second interlayer insulation films  65  and  67 . Thereby, the contact hole  67 A exposes the diffusion regions  61 B and  61 D. 
     In the present embodiment, the step of FIG. 10H is further conducted for removing a oxide film from the exposed surface of the diffusion regions  61 B and  61 D by applying a dry cleaning process using a hydrogen plasma through the contact hole  67 A. The dry cleaning process may be conducted at a temperature of about 200° C. by exciting an RF plasma in a mixed gas of H 2  and a gas containing oxygen such as H 2 O. For example, a dry cleaning process disclosed in the Japanese Laid-Open Patent Publication 6-140368 may be employed for this purpose. 
     After the dry cleaning process, a step of FIG. 10I is conducted in which a polysilicon electrode  68  constituting the accumulation electrode of a memory cell capacitor is provided in contact with the diffusion regions  61 B and  61 D such that the polysilicon electrode  68  fills the contact hole  67 A. Further, a capacitor dielectric film  69  of SiN is deposited on the electrode  68 , followed by a deposition of a polysilicon electrode  70  forming an opposing electrode of the memory cell capacitor on the dielectric film  69 . Preferably the SiN film  69  has an ONO structure in which the SiN film is sandwiched by a pair of thin oxide films, similarly to the previous embodiments. 
     In the DRAM of the present embodiment, the side wall oxide film on the gate electrode  63  is formed of two layers, the first layer  61 A or  61 B and the second layer  61 C or  61 D. Thereby, the problem of the leakage current flowing between the gate electrode  63  and the accumulation electrode  68  is effectively eliminated without using a nitride film. As noted already, the surface of the first layer  61 A or  61 B is annealed in the N 2 O atmosphere before the formation of the second layer, at a temperature substantially identical to the substrate temperature used in a high temperature CVD process for depositing an SiO 2  film. 
     In the present embodiment, it should further be noted that the removal of the oxide film is conducted prior to the deposition of the accumulation electrode  68  by applying a dry cleaning process conducted in a hydrogen plasma rather than applying a wet etching process conducted by a buffered HF solution. Thereby, the problem of deterioration of the leakage characteristics associated with the use of the wet etching treatment conducted by the buffered HF solution is successfully avoided. 
     In the present embodiment, the side wall oxide film is by no means limited to the foregoing two layer construction but may be formed in more than three layers. 
     Further, the dry cleaning process may be applied in the step of FIG. 10F to the substrate surface exposed by the contact hole  65 A for removing a oxide film from the exposed surface of the diffusion regions  61 A and  61 C, prior to the formation of the bit line electrode  66 . 
     Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.