Patent Publication Number: US-6342716-B1

Title: Semiconductor device having dot elements as floating gate

Description:
This application is a Divisional of application Ser. No. 09/208,753, filed Dec. 10, 1998 now U.S. Pat. No. 5,445,981. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for forming a dot element, a semiconductor device using the dot element and a method for fabricating the device. More particularly, the present invention relates to a method for forming a dot element out of an ultrafine particle of the size of several nanometers and functioning as a quantum dot element, a semiconductor device using the dot element and a method for fabricating the device. 
     Currently, a ULSI is formed by integrating a great number of MOS devices on a single chip. In general, as an MOS device is miniaturized, the performance thereof is enhanced correspondingly. However, if the gate length thereof is 0.1 μm or less, then the device can hardly operate normally as a transistor, because such a size is a physical limit for the device. A single-electron tunneling device, called a “coulomb blockade”, has attracted much attention, recently as a candidate for breaking through such a limit (Kenji Taniguchi et al., FED Journal, Vol. 6, No. 2, 1995). In principle, a single-electron tunneling device performs logical operations and storing operations by controlling the movement of individual electrons, and is very effective in reducing power consumption. However, in order to form a single-electron tunneling device, semiconductor or metal fine particles of the size of several nanometers, called “quantum dot elements”, are required. As disclosed in Japanese Laid-Open Publication No. 9-69630, for example, if a large number of Au dot elements are formed out of Au fine particles by sputtering or the like between metal electrodes formed on a substrate, then the Au dot elements form multiple bonds with each other, thereby realizing single-electron effects. In accordance with this method, however, it is very difficult to accurately control the positions where the Au dot elements are formed. 
     Thus, Sato et al. proposed another method for forming a dot element. In accordance with the method of Sato et al., 3-(2-aminoethylamino)propyltrimethoxy silane (APTS) is deposited on a substrate on which a PMMA resist pattern has been formed. Then, APTS on the PMMA resist is partially lifted off together with an unnecessary portion of the PMMA resist, thereby selectively leaving APTS at desired positions on the substrate. Thereafter, Au fine particles are attached onto only APTS, thereby forming Au dot elements. 
     Aside from the single-electron tunneling device, a different method for breaking through the limit of a device size using dot elements was also proposed. For example, S. Tiwari et al. disclosed in IEDEM Tech. Digest, 521 (1995) that an operating voltage would be lowered by using dot elements of silicon fine particles for the floating gate of a nonvolatile memory or the like. Tiwari et al. suggested that silicon dot elements could be formed directly on a substrate by performing a CVD process on accurately controlled conditions. 
     However, the methods of T. Sato et al. and Tiwari et al. have the following problems. 
     To control the positions of dot elements on a substrate by the method of T. Sato et al., the process steps of forming a PMMA resist pattern or the like on the substrate and then lifting off APTS with unnecessary portions of the PMMA resist pattern are required. Thus, the fabrication process is adversely complicated. In addition, in this method, the Au dot elements are formed onto APTS by utilizing the polarization of charges. Accordingly, if charges have been polarized at other sites on the semiconductor substrate, then Au fine particles are unintentionally attached to such sites. Therefore, it is not always possible to selectively form the Au dot elements only at desired sites. 
     On the other hand, in accordance with the method of Tiwari et al., silicon dot elements are directly formed on a substrate by a CVD technique. Thus, it is very difficult to control the sizes and positions of such dot elements on the substrate. 
     Because of these inconveniences, it is now hard to use dot elements, formed by the conventional methods, as a member of a semiconductor device or as quantum dot elements, in particular. That is to say, in accordance with the conventional methods, a semiconductor device, including dot elements formed with the sizes and positions thereof accurately controlled, is very much less likely to be realized. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for forming dot elements while accurately controlling the sizes and positions thereof by taking various measures to precisely control the positions and sizes of fine particles over a substrate. Another object of the present invention is to provide semiconductor device of various types, each including the dot elements functioning as quantum dot elements as a component. 
     A first method for forming a dot element according to the present invention includes the steps of: a) forming a first compound on a part of a substrate; b) attaching a second compound to the surface of a fine particle, the second compound having such a nature as to be combined with the first compound formed on the substrate; c) combining the first and second compounds together and selectively placing the fine particle only on the part of the substrate where the first compound has been formed, thereby forming a dot element out of the fine particle. 
     In accordance with the first method, the positional accuracy of the dot elements can be controlled based on the position of the first compound formed on the substrate. In addition, only by selecting fine particles of a desired uniform size from the beginning, the sizes of the dot elements can be easily controlled. Accordingly, the positions and sizes of dot elements can be accurately controlled by performing a simple process, without any need for complicated process steps. As a result, dot elements, functioning as quantum dot elements in a device, can be practically formed. 
     In one embodiment of the present invention, both the first and second compounds are preferably organic compounds. 
     In another embodiment of the present invention, one of the first and second compounds may be an antigen and the other may be an antibody of the antigen. 
     In such an embodiment, a dot element can be formed such that the fine particle is fixed at a desired position, not undesired position, with a lot more certainty by taking advantage of the high selectivity of an antigen-antibody reaction. 
     In still another embodiment, at least one of the first and second compounds may be a protein or an enzyme. 
     In such an embodiment, the above effects can be attained because a protein or an enzyme is generally likely to react with a particular material. 
     In still another embodiment, in the step a), an energy wave is preferably irradiated onto only a part of the substrate after the first compound has been formed on the substrate. 
     In such an embodiment, the first compound can be easily left only at a particular site on the substrate by appropriately selecting the first compound and the energy wave. 
     In still another embodiment, the energy wave may be selected from the group consisting of: light; X-rays; and electron beams. 
     In still another embodiment, the dot elements may be formed in matrix by using an interference pattern of the energy wave as the energy wave. 
     In such an embodiment, a matrix of regularly arranged dot elements can be provided as a component of a device. 
     In still another embodiment, an electron beam irradiated by an atomic force microscope or a scanning tunneling microscope may be used as the energy wave. 
     In still another embodiment, a gold fine particle may be used as the fine particle. 
     In such an embodiment, a dot element functioning as a quantum dot element can be formed particularly easily, because gold fine particles have already been practically used as ultrafine particles of the size in the range from 1 to 10 nm. 
     In still another embodiment, the first method may further include, posterior to the step c), the step of d) directly fixing the dot element onto the substrate by removing the first and second compounds. 
     In such an embodiment, a useful dot element can be formed with inconveniences avoided, even when the existence of the first and second compounds is unfavorable for the operation of the device. 
     In still another embodiment, the step d) may be per formed by bringing the first and second compounds into contact with oxygen plasma or carbon dioxide in a super-critical state. 
     In such an embodiment, part of the dot elements can be removed without displacing the dot elements from the fixed positions thereof. Accordingly, the final positions of the dot elements fixed can be more accurate. 
     A second method for forming a dot element according to the present invention includes the steps of: a) forming a protein thin film on a substrate, the protein thin film including a plurality of shells, each having an, inner hollow, and conductor or semiconductor fine particles encapsulated in the inner hollows of the shells; b) removing the shells from the protein thin film on the substrate, thereby leaving only the fine particles in the thin film like a layer on the substrate; and c) patterning the layer of the fine particles, thereby forming dot elements out of the fine particles on the substrate. 
     In accordance with the second method, dot elements can be formed by using a protein containing a conductor or a semiconductor. 
     In one embodiment of the present invention, the step a) may include the sub-steps of: i) preparing a solution containing the protein and a film-forming material having an affinity with the protein; ii) forming an affinitive film out of the film-forming material on the surface of the solution; iii) attaching the protein to the affinitive film, thereby forming a single-layered film of the protein; and iv) immersing the substrate in the solution and then lifting the substrate out of the solution, thereby attaching the single-layered film of the protein and the overlying affinitive film to the substrate. 
     In such an embodiment, dot elements can be easily formed by using a so-called Langmuir-Blodgett film. 
     In another embodiment of the present invention, the protein may be ferritin and the film-forming material may be polypeptide, for example. 
     In still another embodiment, in the step b), the fine particles may be left at a pitch determined by selecting a type of the protein shell or by adding, substituting or eliminating a group. 
     A semiconductor device according to the present invention functions as a nonvolatile memory cell. The semiconductor device includes: a semiconductor substrate; a tunnel insulating film, which is formed on the semiconductor substrate and has a such a thickness as to allow electrons to be tunneled therethrough; dot elements, which are formed out of semiconductor or conductor fine particles on the tunnel insulating film and function as a floating gate; a control gate for controlling the movement of electrons between the dot elements and the semiconductor substrate; an interelectrode insulating film interposed between the dot elements and the control gate; and source/drain regions formed in the semiconductor substrate so as to sandwich the dot elements therebetween. 
     In the semiconductor device, the floating gate of the nonvolatile memory cell is composed of the dot elements formed out of fine particles. Thus, the level of current or power consumption for injecting charges into the floating gate or taking out electrons therefrom can be reduced. 
     In one embodiment of the present invention, the dot elements are preferably formed only under the control gate. 
     In such an embodiment, it is possible to prevent without fail an electric shortcircuit from being generated between the floating gate and the source/drain regions or between the source/drain regions themselves. 
     In another embodiment of the present invention, the dot elements may be asymmetrically formed under the control gate to be closer to one of the source/drain regions. 
     In such an embodiment, since the number of dot elements can be reduced, the power consumption can be reduced during an erase operation. In addition, if the dot elements are selectively formed at such a region that the dot elements can function as a floating gate most effectively during write, read and erase operations, power consumption and operating voltage can be even more reduced. 
     In still another embodiment, the dot elements are preferably formed under the control gate to be closer to a region functioning as a drain during writing. 
     In such an embodiment, while a write operation is per formed using channel hot electrons, the dot elements are located over a region where electrons, moving from the source to drain region, are most accelerated. As a result, write current can be reduced and power consumption can be reduced. 
     In still another embodiment, the control gate may be formed over the semiconductor substrate with a gate insulating film interposed therebetween. And the device may further include: a protective insulating film covering a side face of the control gate and including a part functioning as the interelectrode insulating film; and a sidewall insulating film formed over the side face of the control gate with the protective insulating film interposed therebetween. And the dot elements may be buried in the sidewall insulating film so as to be located over the semiconductor substrate through the tunnel insulating film. 
     In such an embodiment, the above effects can also be attained, because the dot elements can be formed in the vicinity of the source or drain region. Also, the nonvolatile memory cell may be formed as a transistor of a so-called LDD type by using the sidewall insulating film. Thus, a structure advantageous to miniaturization can be obtained. 
     In such a case, the dot elements may be formed only in a part of the sidewall insulating film closer to the drain or source region. 
     In still another embodiment, the semiconductor device may further include: a select gate formed over the semiconductor substrate with a gate insulating film interposed therebetween; a protective insulating film covering a side face of the select gate; and a sidewall insulating film formed over the side face of the select gate with the protect tive insulating film interposed therebetween. The dot elements may be buried in the sidewall insulating film so as to be located over the semiconductor substrate through the tunnel insulating film. And the control gate may be formed so as to cover the sidewall insulating film through an interelectrode insulating film. 
     In such an embodiment, since the device also includes the select gate functioning as a select transistor, a highly reliable nonvolatile memory cell consuming even small power can be obtained. 
     In still another embodiment, an inclined portion having a level difference may be formed in part of the principal surface of the semiconductor substrate. The gate insulating film may be formed so as to overlap the inclined portion. And the dot elements may be formed on either a slope or a lower-level portion of the inclined portion, the lower-level portion being located adjacent to the slope. 
     In such an embodiment, the dot elements functioning as a floating gate are located just in the direction toward which channel hot electrons are moving during writing. Accordingly, write efficiency can be improved and power consumption can be further reduced. 
     In still another embodiment, a stepped portion having a level difference may be formed in part of the principal surface of the semiconductor substrate. The gate insulating film may be formed so as to overlap the stepped portion. And the dot elements may be formed to be self-aligned with a part of the gate insulating film on a side face of the stepped portion. 
     In such an embodiment, the dot elements can be formed to be self-aligned with only the side face of the stepped portion in accordance with the first or second method for forming a dot element of the present invention. Accordingly, if the dot elements are used as a charge storage such as a floating gate, a memory device, which can be satisfactorily controlled during writing and reading, is obtained. 
     In still another embodiment, the substrate may be a silicon substrate, the principal surface of which is a {111} plane, and the side face of the stepped portion may be a {100} plane. 
     In such an embodiment, electrons can be injected into the dot elements even more easily by using channel hot electrons, because a thermally oxidized film is thicker on the {111} plane but thinner on the {100} plane. 
     In still another embodiment, the semiconductor substrate may be an SOI substrate including an insulator layer under a semiconductor layer. 
     In such an embodiment, a high-speed operating nonvolatile memory cell can be obtained. 
     In still another embodiment, the dot elements may be formed out of silicon or metal fine particles., 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 ( a ) through  1 ( d ) are cross-sectional views illustrating respective process steps for forming a dot element in the first embodiment. 
     FIG. 2 is a schematic representation illustrating a molecular structure of ferritin in the second embodiment. 
     FIGS.  3 ( a ) through  3 ( d ) are cross-sectional views illustrating respective process steps for forming a dot element in the second embodiment. 
     FIG. 4 is a copy of an SEM photograph showing the surface of a silicon substrate on which a polypeptide film and a ferritin film have been formed and which is subjected to a heat treatment. 
     FIG. 5 is a cross-sectional view illustrating a state of the substrate subjected to the heat treatment in the second embodiment. 
     FIG. 6 is a cross-sectional view of a nonvolatile memory cell of the third embodiment including dot elements, formed in accordance with the method of the present invention, as a floating gate. 
     FIGS.  7 ( a ) through ( d ) are cross-sectional views illustrating respective process steps for fabricating a memory cell in the third embodiment. 
     FIG. 8 is a cross-sectional view of a nonvolatile memory cell of the fourth embodiment, in which dot elements are formed only in the vicinity of one end of a diffusion layer in accordance with the method of the present invention. 
     FIG. 9 is a cross-sectional view of a nonvolatile memory cell of the fifth embodiment, in which dot elements are formed only in a sidewall oxide film on both sides in accordance with the method of the present invention. 
     FIGS.  10 ( a ) through  10 ( d ) are cross-sectional views illustrating respective process steps for fabricating a memory cell in the fifth embodiment. 
     FIG. 11 is a cross-sectional view of a nonvolatile memory cell of the sixth embodiment, in which dot elements are formed only in a sidewall oxide film on either one side in accordance with the method of the present invention. 
     FIG. 12 is a cross-sectional view of a nonvolatile memory cell of the seventh embodiment, in which dot elements are formed only in a sidewall oxide film on either one side in accordance with the method of the present invention. 
     FIG. 13 is a cross-sectional view of a nonvolatile memory cell of the eighth embodiment, in which dot elements are formed over an SOI substrate in accordance with the method of the present invention. 
     FIG. 14 is a cross-sectional view of a nonvolatile memory cell of the ninth embodiment, in which dot elements are formed only on a lower-level portion of an oxide film overlapping an inclined portion in accordance with the method of the present invention. 
     FIG. 15 is a cross-sectional view of a nonvolatile memory cell of the tenth embodiment, in which dot elements are formed only on a corner portion of a stepped portion in accordance with the method of the present invention. 
     FIGS.  16 ( a ) through  16 ( c ) are cross-sectional views illustrating respective process steps for fabricating a nonvolatile memory cell in the tenth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EMBODIMENT 1 
     Hereinafter, the first embodiment of the present invention will be described with reference to the drawings. FIGS.  1 ( a ) through  1 ( d ) are cross-sectional views illustrating respective process steps for forming a dot element in the first embodiment. 
     First, in the step shown in FIG.  1 ( a ), a Rat IgG antibody film  102  is formed on a p-type Si substrate in accordance with a spin coating method or a submerged adsorption method. Specifically, the Rat IgG antibody film  102  may be formed easily by spin-coating the substrate with a thin film of acetyl cellulose containing the Rat IgG antibody, for example. 
     Next, in the step shown in FIG.  1 ( b ), a photo mask  103  for blocking light from reaching only a part of the p-type Si substrate  101  is prepared. And ultraviolet rays  104  are selectively irradiated from above the photo mask  103  onto the Rat IgG antibody film  102  except for that part. A portion of the Rat IgG antibody film  102  that has been irradiated with the ultraviolet rays  104  loses its activity as an antibody owing to the energy of the ultraviolet rays  104  to be a deactivated Rat IgG antibody film  105 . The other portion of the Rat IgG antibody film  102  that has not been irradiated with the ultraviolet rays  104  still retains its activity as an antibody. 
     Then, in the step shown in FIG.  1 ( c ), a solution containing Au fine particles  106  combined with a Rat IgG antigen  107  is prepared. And the p-type Si substrate  101 , on which the Rat IgG antibody film  102  has been formed, is immersed in the solution for five to ten hours (a container is not shown in FIG.  1 ( c )). As a result of this process, the Rat IgG antigen  107 , combined with the Au fine particles  106 , is selectively combined with the Rat IgG antibody film  102  on the p-type Si substrate  101 . Consequently, the Rat IgG antigen  107 , combined with the Au fine particles  106 , is fixed on the Rat IgG antibody film  102 . On the other hand, since the deactivated Rat IgG antibody film  105  has lost its activity as an antibody owing the irradiated ultraviolet rays  104 , the Rat IgG antigen  107  is not fixed onto the deactivated Rat IgG antibody film  105 . Accordingly, the Rat IgG antigen  107 , combined with the Au fine particles  106 , is fixed only on the Rat IgG antibody film  102  on the p-type Si substrate  101 . 
     Subsequently, in the step shown in FIG.  1 ( d ), the p-type Si substrate  101  is exposed to oxygen plasma for twenty minutes. As a result of this process, the Rat IgG antibody film  102 , the deactivated Rat IgG antibody film  105  and the Rat IgG antigen  107  on the p-type Si substrate  101  are all dissolved by the oxygen plasma. In other words, the Rat IgG antibody film  102  and the Rat IgG antigen  107 , which have been interposed between the Au fine particles  106  and the p-type Si substrate  101 , are dissolved to disappear. Consequently, dot elements  110  of a desired size are formed out of the Au fine particles at desired positions on the p-type Si substrate  101 . 
     In accordance with the method of this embodiment, the positions, where the dot elements  110  are formed, are determined by the pattern and position of the photo mask  103  during a single photolithography process. Thus, in this embodiment, accurate control is realized without performing the complicated process of T. Sato et al. Also, unlike the method of Tiwari et al., Au fine particles  106 , which have already been formed separately to have a uniform particle size, can be used. Accordingly, the sizes of the dot elements  110  can also be controlled accurately. Moreover, since the antigen-antibody reaction has very high selectivity, the dot elements  110  are not formed at undesired positions unlike the method of T. Sato et al. Therefore, the positions and sizes of the dot elements  110  can be controlled accurately, and a semiconductor device, including quantum dot elements as a component and attaining various functions and excellent characteristics, is realized. 
     In this embodiment, a p-type Si substrate is used as a substrate on which the dot elements are formed. Alternatively, any other material may be used so long as an antigen or antibody can be formed on the surface thereof. Also, any material other than silicon, such as silicon oxide or silicon nitride, may be naturally formed on the p-type Si substrate  101 . 
     Ultraviolet rays are used in this embodiment for partially deactivating the antibody. Alternatively, any other energy wave such as X-rays, electron beams or ion beams may also be used so long as the energy wave can partially deactivate the antibody. The irradiation of electron beams may be performed not only in vacuum, but also by using an atomic force microscope (AFM), a scanning tunneling microscope (STM) or the like in the atmosphere. 
     Also, assume an interference pattern is formed on the Rat IgG antibody film  102 , formed on the p-type Si substrate  101 , by irradiating the film  102  with F 2  vacuum ultraviolet laser light at a tilt angle. The laser light has been divided into two luminous fluxes and then superimposed with each other. Then, part of the interference pattern on the Rat IgG antibody film  102 , which has been exposed to the light at a higher intensity, is deactivated to form a striped Rat IgG antibody film  102 . Furthermore, the same two luminous fluxes of the F 2  vacuum ultraviolet laser light may be superimposed with each other and irradiated at a tilt angle onto the p-type Si substrate  101  after the substrate  101  has been rotated by 90 degrees horizontally. Then, a Rat IgG antibody film  102  can be finally formed in the shape of a matrix of several square nanometers. In forming an interference pattern of the laser light, an island is formed at a pitch determined by a wavelength and an incident angle. Thus, the size of the Rat IgG antibody film  102  can be arbitrarily set. 
     In this embodiment, Au fine particles commercially available from British Bio Cell Corporation (particle size is 1, 2, 5 or 10 nm) are used. Though Au fine particles are used in this embodiment, any other fine particles such as Si, Ti or GaAs may also be used so long as the fine particles can be combined with an antigen or antibody. 
     In this embodiment, a Rat IgG antibody film is first formed on a substrate and then a Rat IgG antigen, combined with Au fine particles, is used. Alternatively, the antigen and the antibody may be interchanged. That is to say, an antigen may be formed first on a substrate and then Au fine particles may be modified with an antibody. It is noted that an “antigen” is herein a generic term for various substances capable of inducing antigen-antibody reactions and immune response. In the world of nature, the “antigen” includes proteins, polysaccharides, compounds thereof and compound lipids, each having a molecular weight of about 1,000 or more. On the other hand, an “antibody” is herein a generic term for proteins produced in a living body in response to the stimulation of an antigen and combined specifically to the antigen. 
     In this embodiment, a combination of a Rat IgG antibody and a Rat IgG antigen is used. Alternatively, any antibody selected from the group consisting of: a Rabbit IgG antibody; a Mouse IgG antibody; a Human IgG antibody; a Guinea Pig IgG antibody; a Chicken IgG antibody; a Goat IgG antibody; and a Sheep IgG antibody may be used in combination with an associated antigen thereof. 
     However, it is noted that, according to the present invention, a combination of a first compound combined with fine particles and a second compound selectively combined with the first compound is not limited to the combination of an antigen and an antibody. Alternatively, any other combination of compounds, e.g., a combination of a protein and an enzyme, may also be used so long as the pair can be selectively combined with each other. 
     Furthermore, the film of the first compound formed on the substrate may be made of any other material so long as the activity of the film is selectively variable in response to light, electron beams or the like. Thus, a film containing a silane coupling agent including a vinyl group (such as bis(dimethylamino)methylvinyl silane, tris(1-methylvinyloxy)vinyl silane) or the like) may be used. In such a case, an antigen or antibody combined with fine particles is sometimes unnecessary. 
     In this embodiment, oxygen plasma is used for removing the antigen and antibody from the substrate. However, if any substance, which is likely to be affected by oxygen plasma, exists on the substrate, then the organic substances such as an antigen or an antibody may be removed from the substrate by bringing the surface of the substrate into contact with carbon dioxide in a super critical state. In such a case, the removal efficiency can be naturally improved if any other solvent is mixed therein as an entrainer. 
     EMBODIMENT 2 
     Next, the second embodiment of the present invention will be described with reference to the drawings. In this embodiment, a method for forming a dot element using a metalprotein compound such as ferritin will be described. 
     First, ferritin powder is prepared as a source material. FIG. 2 is a schematic representation illustrating the molecular structure of ferritin. As shown in FIG. 2, ferritin  120  is a metal-protein compound, in which a core  121  of Fe 2 O 3  is surrounded by protein shells  122 , and can be extracted from an animal viscus such as equine or bovine spleen or liver. The diameter of the core  121  is about 6 nm, and the total number of iron atoms thereof is in the range from about 2,000 to about 3,000. The shell  122  is a *24 monomers of a protein having a molecular weight of about 20,000 and the outer diameter R of the *24 monomers is about 12 nm. 
     Next, a method for forming a dot element using ferritin  120  will be described. FIGS.  3 ( a ) through  3 ( d ) are cross-sectional views illustrating respective process steps for forming a dot element in this embodiment. 
     First, as shown in FIG.  3 ( a ), a buffer solution  124  is reserved in a water tank  123  made of Teflon, ferritin  120  is dispersed in the buffer solution  124  and a polypeptide film (i.e., a film for forming a Langmuir-Blodgett film) is formed on the surface of the buffer solution  124 . The pH of the buffer solution  124  is adjusted at about six by using an appropriate acid such as very rare hydrochloric acid. 
     Before long, as shown in FIG.  3 ( b ), ferritin  120  attaches to the polypeptide film  125 , because the polypeptide film  125  is positively charged, whereas ferritin  120  is negatively charged. As a result, two-dimensional crystals of ferritin  120  are formed. 
     Thereafter, as shown in FIG.  3 ( c ), a p-type Si substrate  101 , on which a silicon dioxide film  108  has been formed, is floated on the surface of the buffer solution  124 , thereby attaching the polypeptide film  125  and the two-dimensional crystals of ferritin  120  to the surface of the p-type Si substrate  101 . And then the substrate  101  is taken out of the water tank  123 . 
     As a result, as shown in FIG.  3 ( d ), the two-dimensional crystals of ferritin  120  are formed over the p-type Si substrate  101  with the silicon dioxide film  108  and the polypeptide film  125  interposed therebetween. Thereafter, the substrate is placed within an inert gas (e.g., nitrogen plasma) which is less likely to react with silicon, and then subjected to a heat treatment at 500° C. Consequently, the protein in ferritin  120  and the polypeptide film  125  disappear almost entirely, whereby dot elements are left as an assembly of ferrous oxides included in the inner hollows of the ferritin molecules, which formed the two-dimensional crystals. That is to say, a large number of mutually isolated dot elements are obtained. If it is hard to conduct a heat treatment, the protein and the polypeptide film may be removed by bringing the surface of the substrate in contact with silicon dioxide in a super-critical state or the like. The efficiency of the removal can be naturally improved if any other solvent is mixed as an entrainer. 
     FIG. 4 is a copy of an SEM photograph showing the surface of the silicon substrate on which the polypeptide film and the ferritin film were formed and which was subjected to a heat treatment at 500° C. for an hour (the scale is 1: 100,000). In FIG. 4, a large number of white dots are dot elements of a ferrous oxide, while the other dark parts are a very small amount of residual protein and the silicon substrate. As can be estimated from FIG. 4, the dot elements of ferrous oxide are located at respective sites where the cores  121  of the ferritin molecules as two-dimensional crystals used to exist. 
     Thereafter, the entire substrate, on which the ferrous oxide dot elements are formed, is subjected to a heat treatment again within hydrogen at a temperature in the range from about 300 to about 500° C. for about 60 minutes. As a result, the ferrous oxide dot elements are reduced to be iron dot elements. 
     FIG. 5 is a cross-sectional view illustrating a state of the substrate that has been subjected to the heat treatment. As shown in FIG. 5, the Fe dot elements  128  are arranged two-dimensionally over the p-type Si silicon substrate  101  with the silicon dioxide film  108  interposed therebetween. The two-dimensional arrangement of the Fe dot elements  128  was confirmed by an AFM analysis. As a result of the analysis, the height of the dot elements  128  was either 5.3 nm or 10.6 nm, and the majority of the dot elements  128  were 5.3 nm high. 
     The diameter of the dot element  128  is about 6 nm, which is substantially equal to that of the ferrous oxide core in ferritin. A pitch between dot elements  128  is about 12 nm, which is substantially equal to the diameter R of the protein shell  122  in ferritin  120 . 
     In this embodiment, the dot elements are formed out of Fe fine particles. Alternatively, the dot elements may be ferrous oxide dot elements. 
     Also, the protein for forming dot elements may be any of various kinds of proteins including a metal such as hemoglobin, adenovirus or globular virus such as T4 phage, instead of ferritin. Since the sizes of these proteins are different from each other (some of them have a particle size as large as about 100 nm) and can also be artificially changed by adding, substituting or eliminating various groups to the shells, dot elements can be arranged at a desired pitch. 
     The size of a dot element itself can also be controlled. For example, if the shell thickness of a protein such as adenovirus or polyoma is changed in accordance with genetic engineering technologies, then the size of the inner hollow of the shell may also be changed. And it is possible in. turn to change the diameter of a metal (or metal oxide) fine particle included in the shell. In other words, the diameter of a fine particle may be changed not only by substituting a different kind of protein, but also by an artificial adjustment. 
     The absolute magnitude (i.e., the number of atoms or molecules) of a metal (or metal oxide) encapsulated in the inner hollow of a protein shell is determined by the size of the inner hollow. However, it was reported that an Fe core surround by Mn can be formed in the inner hollow of ferritin (see, for example, F. C. Meldrum, T. Douflas, S. Levi, P. Arosio and S. Mann, “Reconstitution of Manganese Oxide Cores in Horse Spleen and Recombinant Ferritins”, 1995 J. Inorg. Biochem. 58:59-68). Thus, if such a technique is used, a dot element having a concentric structure of two or three kinds of metals may also be formed. 
     Also, it was reported that ferritin may include the following metals (or metal oxides) other than Fe: Al (J. Fleming, 1987 Proc. Natl. Acad. Sci. USA 84: 7866-7870); Be (D. J. Price, 1983 J. Biol. Chem. 258: 10873-10880); Ga (R. E. Weiner, 1985 J. Nucl. Med. 26: 908-916); Pb (J. Kochen, 1975 Prediatr. Res. 9: 323 (abst. #399)); Mn (P. Mackle, 1993 J. Amer. Chem. Soc. 115: 8471-8472, etc.); P (A. Treffy, 1978 Biochem. J. 171: 313-320, etc.); U (J. F. Hainfeld, 1992 Proc. Natl. Acad. Sci. USA 89: 11064-11068); and Zn (D. Price, 1982 Proc. Natl. Acad. Sci. USA 79: 3116-3119). 
     It is noted that in order to form a single-layered film of a protein such as ferritin, any method other than the method of this embodiment using a Langmuir-Blodgett film may be employed. 
     EMBODIMENT 3 
     Next, the third embodiment of the present invention will be described with reference to the drawings. In the following third to tenth embodiments, examples of nonvolatile memory cells including the dot elements, formed by the method for forming a dot element of the present invention as described in the first and second embodiments, as a floating gate will be described. 
     FIG. 6 is a cross-sectional view illustrating a structure of a nonvolatile memory cell using dot elements as a floating gate. As shown in FIG. 6, a polysilicon electrode  206 ; dot elements  204 ; a gate oxide film  203 ; and a silicon dioxide film  205  are provided over a p-type Si substrate  201 . The polysilicon electrode  206  functions as a control gate. The dot elements  204  are formed out of Au, Fe or Si fine particles by the method of the first and second embodiments, have a particle size of several nanometers and functions as a floating gate electrode. The gate oxide film  203  is interposed between the p-type Si substrate  201  and the floating gate and functions as a tunnel insulating film. The silicon dioxide film  205  is formed between the control gate and the floating gate and functions as an interelectrode insulating film for transmitting a voltage applied to the control gate to the floating gate. In the p-type Si substrate  201 , first and second n-type diffusion layers  207   a  and  207   b , functioning as source/drain regions, are formed. The region between the first and second n-type diffusion layers  207   a  and  207   b  in the p-type Si substrate  201  functions as a channel region. An element-isolating oxide film  202  is formed by a selective oxidation technique or the like between the memory cell illustrated and an adjacent memory cell in order to electrically isolate these cells from each other. The first and second n-type diffusion layers  207   a  and  207   b  are connected through tungsten plugs  210  to first and second aluminum interconnects  211   a  and  211   b , respectively. Though not shown in FIG. 6, the polysilicon electrode  206  and the p-type Si substrate  201  are also connected to the aluminum interconnects  211   a  and  211   b . This memory cell is configured such that voltages at respective parts thereof can be controlled through the aluminum interconnects and so on. 
     Such a structure can be easily formed by performing the process steps shown in FIGS.  7 ( a ) through  7 ( d ). 
     First, in the step shown in FIG.  7 ( a ), the element-isolating oxide film  202  is formed by a LOCOS technique on the p-type Si substrate  201  so as to surround an active region, and the gate oxide film  203  is formed on the substrate. Thereafter, the dot elements  204  are formed over the entire surface of the substrate in accordance with the method of the first or second embodiment. 
     Next, in the step shown in FIG.  7 ( b ), a silicon dioxide film and a polysilicon film are deposited in this order over the substrate by a CVD technique so as to fill in the gaps between the dot elements  204 . 
     Then, in the step shown in FIG.  7 ( c ), the silicon dioxide and polysilicon films are patterned using a photoresist mask Pr 1 , thereby forming the silicon dioxide film  205  as an interelectrode insulating film and the polysilicon electrode  206  as a control gate electrode. In this process step, part of the gate oxide film  203 , not covered with the photoresist mask Pr 1 , is removed and the dot elements  204  located over the part are also removed at the same time. Thereafter, dopant ions are implanted using the photoresist mask and the polysilicon electrode  206  as a mask, thereby forming the first and second n-type diffusion layers  207   a  and  207   b.    
     Thereafter, in the step shown in FIG.  7 ( d ), an interlevel dielectric film  208  is formed. Contact holes  209  are opened in the interlevel dielectric film  208 . The tungsten plugs  210  are formed by filling in the contact holes  209  with tungsten. And the first and second aluminum interconnects  211   a  and  211   b  are formed. All of these parts may be formed by known techniques. 
     Next, the operation of this memory cell will be described. This memory cell is provided with an,MOS transistor (memory transistor) including the polysilicon electrode  206  as a control gate and the first and second n-type diffusion layers  207   a  and  207   b  as source/drain regions. And this is a nonvolatile memory cell for performing read, write and erase operations by sensing a variation in threshold voltages of the memory transistor in accordance with the amount of charges stored in the dot elements  204  functioning as a floating gate. 
     First, a read operation will be described. The first aluminum interconnect  211   a  and the p-type Si substrate  201  are grounded and the second aluminum interconnect  211   b  is allowed to be floating. When an appropriate voltage (e.g., 5 V) is applied to the polysilicon electrode  206  functioning as a control gate, a channel is formed over the p-type Si substrate  201  if charges are not stored in the dot elements  204 . Then, the transistor is turned ON, electrons flow from the first to the second aluminum interconnect  211   a  to  211   b  and these interconnects will soon have a potential of 0 V. On the other hand, if electrons, i.e., negative charges, are stored in the dot elements  204 , then the threshold voltage of the transistor has virtually increased, because electrons exist in the dot elements  204 . Therefore, even if a voltage of 5 V is applied to the polysilicon electrode  206  as a control gate, no channel is formed over the p-type Si substrate  201  and no current flows. Accordingly, the potential in the second aluminum interconnect  211   b  does not become 0 V., By. regarding these two states having mutually different voltages as “0” and “1”, a the memory can store binary values. 
     If the amount of charges stored in the dot elements  204  is also controlled in addition to sensing the existence thereof, a multivalued memory storing tertiary or more values may be realized. In such a case, a circuit for detecting the variation in threshold voltages of the transistor with fine steps is preferably provided. 
     Next, an erase operation will be described. Fowler-Nordheim (FN) current flowing through the oxide film or direct tunneling current is used for erasure. Assume a negative voltage (e.g., −12 V; this polarity is described with respect to the p-type Si substrate  201 ) is applied to the polysilicon electrode  206  as a control gate. Then, the electrons stored in the dot elements  204  as a floating gate are tunneled through the gate oxide film  203  to move into the p-type Si substrate  201 . As a result, data is erased. 
     Next, a write operation will be described. Fowler-Nordheim (FN) current flowing through the oxide film, direct tunneling current or injection of channel hot electrons (CHE) is used for writing. In the case of using the FN current or the direct tunneling current, a positive voltage (e.g., +12 V; this polarity is also described with respect to the p-type Si substrate  201 ) is applied to the polysilicon electrode  206  as a control gate. As a result, electrons are attracted toward the polysilicon electrode  206  facing the p-type Si substrate  201  through the dot elements  204  and tunneled through the gate oxide film  203  to be stored in the dot elements  204  as a floating gate. On the other hand, in the case of injection CHE&#39;s, the second aluminum interconnect  211   b  and the p-type Si substrate  201  are grounded. An appropriate positive voltage (e.g., 5 V) is applied to the first aluminum interconnect  211   a . And the voltage in the polysilicon electrode  206  as a control gate is controlled at a value where CHE&#39;s are more likely to be generated (e.g., 2.5 V, which is half of the drain voltage). By setting the voltages at such values, most electrons move through the channel formed in the p-type Si substrate  201  from the second to the first n-type diffusion layer  207   b  to  207   a . Some channel hot electrons, however, have got huge energy to be tunneled through the gate oxide film  203  and stored in the dot elements  204  as a floating gate. 
     In the nonvolatile memory cell of this embodiment, the floating gate is composed of Si fine particles having so small a particle size as to function as quantum dot elements. Thus, a very small amount of charges are stored. Accordingly, the level of current may be reduced during writing and erasing. As a result, a nonvolatile memory cell consuming less power can be formed. 
     In a floating gate made of fine particles, if only a small number of (typically one) electrons entered ensure a stable state, then the electrons are less likely to be released (i.e., coulomb blockade effect). Thus, if the thickness of the tunnel insulating film is very much reduced to realize a write operation at a high speed and with a low voltage, electrons can still be retained just as expected. 
     In the nonvolatile memory cell of this embodiment, it is sufficient for the dot elements  204  constituting the floating gate to exist only in the silicon dioxide film  204  interposed between the polysilicon electrode  206  and the p-type Si substrate  201 . If the dot elements exist at other sites, an electrically erroneous operation is sometimes caused. For example, assume the dot elements  204  also exist over the first and second n-type diffusion layers  207   a  and  207   b . In such a case, the dot elements  204 , which should function as a floating gate, might be electrically short-circuited with the source/drain regions to lose its function of storing charges. Or the first and second n-type diffusion layers  207   a  and  207   b  might be electrically continuous with each other. In order to prevent such phenomena, the dot elements  204  are preferably formed only in the region interposed between the polysilicon electrode  206  and the p-type Si substrate  201  in accordance with the method of the first embodiment. 
     The dot elements  204  may be formed to be continuous or in contact with each other. That is to say, the dot elements  204  may form a single film as a whole. Alternatively, the dot elements  204  may be dispersed and spaced apart from each other as described above. 
     If the dot elements  204  are dispersed, then the silicon dioxide film  205  exists in the gaps between the dot elements  204  as shown in FIG.  6 . Stated otherwise, the dot elements  204  are buried in the silicon dioxide film  205 . In such a case, the following effects can be attained. In a floating gate where a conductor exists continuously like a conventional floating gate made of polysilicon, if only a part of an insulating film is broken down, then the charges in the entire floating gate flow into the substrate, whereby the function of the floating gate is lost. By contrast, in a floating gate composed of mutually isolated and insulated dot elements in this embodiment, even if charges are lost from some of the dot elements in the floating gate owing to the deterioration of a part of the tunnel insulating film, charges are still retained in the other dot elements. That is to say, if dot elements such as those of this embodiment are used, the reliability of the semiconductor memory device is also improved. 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, a substrate made of a compound semiconductor such as GaAs or any other semiconductor may also be used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used for the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     EMBODIMENT 4 
     Next, the fourth embodiment of the present invention, in which the dot elements are locally formed under the control gate electrode, will be described. FIG. 8 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the fourth embodiment. In FIG. 8, the components having the same structures as the counterparts in FIG. 6 are identified by the same reference numerals, and the structures and functions thereof are just as already described. 
     As shown in FIG. 8, the structure of the nonvolatile memory cell in this embodiment is almost the same as that of the nonvolatile memory cell shown in FIG.  6 . This embodiment is different from the third embodiment in that the dot elements  204  constituting a floating gate are formed only in the vicinity of the first n-type diffusion layer  207   a.    
     Such a structure can be easily formed by performing the following process steps. First, the gate oxide film  203  is formed on the p-type Si substrate  201 . Thereafter, the dot elements  204  are formed in a region over the gate oxide film  203  and in the vicinity of one end of the first n-type diffusion layer  207   a  in accordance with the method of the first embodiment. Next, a silicon dioxide film and a polysilicon film are deposited thereon and patterned, thereby forming the silicon dioxide film  205  as an interelectrode insulating film and the polysilicon electrode  206  as a control gate electrode. Thereafter, dopant ions are implanted using the polysilicon electrode  206  as a mask, thereby forming the first and second n-type diffusion layers  207   a  and  207   b . And then the same process steps as those of the third embodiment are performed to form the interlevel dielectric film  208 , the contact holes  209 , the tungsten plugs  210  and the first and second aluminum interconnects  211   a  and  211   b.    
     In the third embodiment, the dot elements  204  constituting the floating gate are formed substantially uniformly over the entire region interposed between the polysilicon electrode  206  and the p-type Si substrate  201 . However, in the case of using CHE&#39;s for writing, a larger number of CHE&#39;s are generated in the vicinity of an end of the drain just under the polysilicon electrode  206 . Thus, the dot elements  204 , constituting the floating gate, are only required to exist over a region to be the drain during writing. Accordingly, if the dot elements  204  to be the floating gate are formed only over the end of the first n-type diffusion layer  207   a  and voltages are set such that the first n-type diffusion layer  207   a  functions as the drain during writing, a write operation can be performed efficiently. That is to say, in the same way as in the write operation of the third embodiment using CHE&#39;s, the second aluminum interconnect  211   b  and the p-type Si substrate  201  are grounded, an appropriate positive voltage is applied to the first aluminum interconnect  211   a  and the voltage in the polysilicon electrode  206  is controlled at a value where CHE&#39;s are more likely to be generated. 
     By employing such a memory cell structure and such a write operation, the number of dot elements  204  required to constitute the floating gate can be reduced. Accordingly, the number of electrons to be injected by using the CHE&#39;s can also be reduced. As a result, power consumption can be reduced and a write speed can be increased. 
     On the other hand, it is when the dot elements  204  exist in the vicinity of one end of the source under the polysilicon electrode  206  that the threshold value of the transistor is changed most effectively with the existence/absence of charges in the dot elements  204  to constitute the floating gate. Accordingly, in performing a read operation on the memory cell shown in FIG. 8, the potential level relationship between the first and second n-type diffusion layers  207   a  and  207   b  are inverted compared to the case of writing such that the first and second n-type diffusion layers  207   a  and  207   b  function as source and drain, respectively. As a result, the read performance can be further improved. In order to realize such a relationship, the first aluminum interconnect  211   a  and the p-type Si substrate  201  are grounded, the second aluminum interconnect  211   b  is allowed to be floating and an appropriate voltage is applied to the polysilicon electrode  206  as in the read operation of the third embodiment. 
     Alternatively, the first and second n-type diffusion layers  207   a  and  207   b  may also function as drain and source, respectively, during a read operation. In such a case, the second aluminum interconnect  211   b  and the p-type Si substrate  201  are grounded, the first aluminum interconnect  211   a  is allowed to be floating and an appropriate voltage is applied to the polysilicon electrode  206  in an opposite manner to the read operation of the third embodiment. 
     In an erase operation of this embodiment, Fowler-Nordheim (FN) current flowing through the oxide film or direct tunneling current may also be used. In such a case, a negative voltage, which polarity is described with respect to the p-type Si substrate  201  or the first n-type diffusion layer  207   a , is applied to the polysilicon electrode  206 . Then, the electrons stored in the dot elements  204  as a floating gate are tunneled through the gate oxide film  203  to move into the p-type Si substrate  201  or the first n-type diffusion layer  207   a . As a result, data is erased. 
     Thus, in the memory cell of this embodiment, information can be written, read out or erased even more effectively depending on the type or application of the semiconductor memory device by locating the dot elements  204  as a floating gate at a desired position between the source and the drain. 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, a substrate made of a compound semiconductor such as GaAs or any other semiconductor may also be used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used for the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     EMBODIMENT 5 
     Next, the fifth embodiment of the present invention, in which dot elements are buried in a sidewall insulating film of the control gate electrode, will be described. FIG. 9 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the fifth embodiment. In FIG. 9, the components having the same structures as the counterparts in FIG. 6 are identified by the same reference numerals, and the structures and functions thereof are just as already described. 
     As shown in FIG. 9, the polysilicon electrode  206  to be a control gate electrode is formed over the p-type Si substrate  201  with the gate oxide film  203  interposed therebetween. A covering oxide film  220  is formed to extend from the side faces of the polysilicon electrode  206  to the surface of the substrate. Over the side faces of the polysilicon electrode  206 , a sidewall oxide film  221  and the dot elements  204  to be the floating gate are formed with the covering oxide film  220  interposed therebetween. The dot elements  204  are covered with the sidewall oxide film  221 . Part of the covering oxide film  220 , which is not in contact with the polysilicon electrode  206  but located between the dot elements  204  and the p-type Si substrate  201 , functions as a tunnel insulating film. In the p-type Si substrate  201 , the first and second n-type diffusion layers  207   a  and  207   b  of the third and fourth embodiments are not formed. Instead, first and second n-type lightly doped layers  227   a  and  227   b , each reaching the vicinity of an end of the polysilicon electrode  206 , and first and second n-type heavily doped layers  237   a  and  237   b , each reaching the vicinity of an outer end of the sidewall oxide film  221 , are formed. That is to say, the substrate has a so-called “LDD” structure. 
     Such a structure can be easily formed by performing the process steps shown in FIGS.  10 ( a ) through  10 ( d ). 
     First, in the step shown in FIG.  10 ( a ), the gate oxide film  203  and the polysilicon electrode  206  are formed over the p-type Si substrate  201 . Then, n-type dopant ions (e.g., arsenic ions) are implanted by using these as a mask, thereby forming the first and second n-type lightly doped layers  227   a  and  227   b.    
     Next, in the step shown in FIG.  10 ( b ), the upper surface of the p-type Si substrate  201  and the side faces of the polysilicon electrode  206  are oxidized to form a silicon dioxide film  220   a  to be the covering oxide film  220 . Thereafter, the dot elements  204  to be the floating gate are formed over the entire silicon dioxide film  220   a  in accordance with the method of the first or second embodiment. Furthermore, another silicon dioxide film  221   a  is deposited by a CVD technique over the substrate, thereby filling in the gaps of the dot elements  204  with silicon dioxide as an insulator. 
     Subsequently, in the step shown in FIG.  10 ( c ), the two silicon dioxide films  221   a  and  220   a  are etched anisotropically, thereby forming the covering oxide film  220  and the sidewall oxide film  221  around the polysilicon electrode  206  to be the control gate electrode. Thereafter, n-type dopant ions (e.g., arsenic ions) are implanted at a high doping level using the polysilicon electrode  206  and the sidewall oxide film  221  as a mask, thereby forming the first and second n-type heavily doped layers  237   a  and  237   b.    
     Finally, in the step shown in FIG.  10 ( d ), an interlevel dielectric film  208  is formed. Contact holes  209  are opened in the interlevel dielectric film  208 . The tungsten plugs  210  are formed by filling in the contact holes  209  with tungsten. And the first and second aluminum interconnects  211   a  and  211   b  are formed. All of these parts may be formed by known techniques. 
     It is noted that in the anisotropic etching between the process steps shown in FIGS.  10 ( b ) and  10 ( c ), only the upper silicon dioxide film  221   a  may be removed and the lower silicon dioxide film  220   a  may be left. In such a case, the dot elements  204  remain on the lower silicon dioxide film  220   a . However, if only the exposed dot elements  204  are selectively etched (using an acid, for example), then the sidewall oxide film  221  covering the dot elements  204  can be formed over the side faces of the polysilicon electrode  206  while removing the exposed dot elements  204 . 
     While the nonvolatile memory cell of this embodiment performs a write, read or erase operation, the voltages are set as described in the third embodiment. 
     In the nonvolatile memory cell of this embodiment, the dot elements  204  to be the floating gate can be formed to be self-aligned with the side faces of the polysilicon electrode  206  as a control gate by etching the two silicon dioxide films  220   a  and  221   a  anisotropically. Thus, this structure is suitable for miniaturization. 
     Moreover, the first and second n-type heavily doped layers  237   a  and  237   b  can be formed in a self-aligned manner by using the sidewall oxide film  221  provided for protecting the dot elements  204 . Accordingly, an LDD structure, suitable for miniaturization and advantageous to the suppression of short channel effects, can be obtained. 
     Only the gate oxide film  203  exists between the polysilicon electrode  206  functioning as a control gate and the p-type Si substrate  201  functioning as a channel. In such a case, no tunneling current needs to be supplied through the gate oxide film  203 . Thus, the memory transistor of this embodiment may have the same structure and performance as those of a transistor used for an ordinary logical device. 
     On the other hand, the dot elements  204  to be the floating gate face the first and second n-type lightly doped layers  227   a  and  227   b  vertically and the polysilicon electrode  206  horizontally through the covering oxide film  220 . Assume the polysilicon electrode  206  is made of polysilicon doped with a dopant such as phosphorus at a high level. Then, during the oxidation process step for forming the covering oxide film  220 , the polysilicon electrode  206  is oxidized about three times faster than the n-type lightly doped layer  227   a . Accordingly, the thickness of the covering oxide film  220  becomes about three times larger on the side faces of the polysilicon electrode  206  than on the first and second n-type lightly doped layers  227   a  and  227   b . Thus, while charges are stored in the dot elements  204  by using CHE&#39;s during writing, it is possible to prevent the charges, which have reached the dot elements  204 , from being laterally tunneled through the covering oxide film  220  to reach the polysilicon electrode  206 . And at the same time, charges can be easily tunneled through the thin covering oxide film  220  over the first and second n-type lightly doped layers  227   a  and  227   b  and injected into the dot elements  204 . 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, a substrate made of a compound semiconductor such as GaAs or any other semiconductor may also be used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used for the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     EMBODIMENT 6 
     Next, the sixth embodiment of the present invention, in which dot elements are buried in a sidewall insulating film of the control gate electrode, will be described. FIG. 11 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the sixth embodiment. In FIG. 11, the components having the same structures as the counterparts in FIG. 6 are identified by the same reference numerals, and the structures and functions thereof are just as already described. 
     As shown in FIG. 11, the polysilicon electrode  206  to be a control gate electrode is formed over the p-type Si substrate  201  with the gate oxide film  203  interposed therebetween. A covering oxide film  220  is formed to entirely surround the polysilicon electrode  206  and reach the surface of the substrate. Over the side faces of the polysilicon electrode  206 , a sidewall oxide film  221  is formed with the covering oxide film  220  interposed therebetween. The dot elements  204  to be the floating gate are formed in the sidewall insulating film  221  on the covering oxide film  220 . Part of the covering oxide film  220 , which is not in contact with the polysilicon electrode  206  but located between the dot elements  204  and the p-type Si substrate  201 , functions as a tunnel insulating film. In the p-type Si substrate  201 , the first and second n-type diffusion layers  207   a  and  207   b  of the third and fourth embodiments are not formed. Instead, first and second n-type lightly doped layers  227   a  and  227   b , each reaching the vicinity of an end of the polysilicon electrode  206 , and first and second n-type heavily doped layers  237   a  and  237   b , each reaching the vicinity of an outer end of the sidewall oxide film  221 , are formed. That is to say, the substrate has a so-called “LDD” structure. 
     Such a structure can be easily formed by performing the following process steps. First, the gate oxide film  203  and the polysilicon electrode  206  are formed over the p-type Si substrate  201 . Then, dopant ions are implanted by using these as a mask, thereby forming the first and second n-type lightly doped layers  227   a  and  227   b . Next, the upper surface of the p-type Si substrate  201  and the side faces of the polysilicon electrode  206  are oxidized to form the covering oxide film  220 . Thereafter, the dot elements  204  to be the floating gate are formed only over one end of the first n-type lightly doped layer  227   a  in accordance with the method of the first embodiment. Furthermore, a silicon dioxide film or the like is deposited over the substrate, thereby filling in the gaps of the dot elements  204  with an insulator. Subsequently, the silicon dioxide film is etched anisotropically, thereby forming the sidewall oxide film  221 . 
     Thereafter, dopant ions are implanted at a high doping level using the sidewall oxide film  221  as a mask, thereby forming the first and second n-type heavily doped layers  237   a  and  237   b.    
     While the nonvolatile memory cell of this embodiment per forms a write, read or erase operation, the voltages are set as described in the fourth embodiment. 
     In the nonvolatile memory cell of this embodiment, the same effects as those attained by the fourth embodiment can also be attained. In addition, the dot elements  204  functioning as the floating gate can be formed to be self-aligned with the polysilicon electrode  206  functioning as a control gate. Moreover, the first n-type lightly doped layer  227   a  is formed to be self-aligned with the polysilicon electrode  206  and the first n-type heavily doped layer  237   a  is formed to be self-aligned with the sidewall oxide film  221 . Thus, the respective layers  227   a  and  237   a  can be regarded as being self-aligned with the dot elements  204 . As can be understood, the dot elements can be located to be self-aligned with a desired region between the drain and the source in the memory cell of this embodiment. Accordingly, information can be written, read out or erased even more effectively depending on the type or application of the semiconductor memory device. 
     Furthermore, the first and second n-type heavily doped layers  237   a  and  237   b  can be formed in a self-aligned manner by using the sidewall oxide film  221  provided for protecting the dot elements  204 . Accordingly, an LDD structure, suitable for miniaturization and advantageous to the suppression of short channel effects, can be obtained. Also, compared with the third embodiment, relative positional accuracy between the dot elements  204  to be the floating gate and the first n-type heavily doped layer  237   a  can be improved. 
     Only the gate oxide film  203  exists between the polysilicon electrode  206  functioning as a control gate and the p-type Si substrate  201  functioning as a channel. In such a case, no tunneling current needs to be supplied through the gate oxide film  203 . Thus, the memory transistor of this embodiment may have the same structure and performance as those of a transistor used for an ordinary logical device. 
     On the other hand, the dot elements  204  to be the floating gate face the first n-type lightly doped layer  227   a  vertically and the polysilicon electrode  206  horizontally through the covering oxide film  220 . Assume the polysilicon electrode  206  is made of polysilicon doped with a dopant such as phosphorus at a high level. Then, during the oxidation process step for forming the covering oxide film  220 , the polysilicon electrode  206  is oxidized about three times faster than the n-type lightly doped layer  227   a . Accordingly, the thickness of the covering oxide film  220  becomes about three times larger around the polysilicon electrode  206  than on the first n-type lightly doped layer  227   a . Thus, while charges are stored in the dot elements  204  by using CHE&#39;s during writing, it is possible to prevent the charges, which have reached the dot elements  204 , from being laterally tunneled through the covering oxide film  220  to reach the polysilicon electrode  206 . And at the same time, charges can be easily tunneled through the thin covering oxide film  220  over the first n-type lightly doped layer  227   a  and injected into the dot elements  204 . 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, a substrate made of a compound semiconductor such as GaAs or any other semiconductor may also be used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used as the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     EMBODIMENT 7 
     Next, the seventh embodiment of the present invention, in which dot elements are buried in a sidewall insulating film of a select gate electrode, will be described. FIG. 12 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the seventh embodiment. In FIG. 12, the components having the same structures as the counterparts in FIG. 11 are identified by the same reference numerals, and the structures and functions thereof are just as already described. 
     As shown in FIG. 12, a polysilicon electrode  239  is formed over the p-type Si substrate  201  with the gate oxide film  203  interposed therebetween. This polysilicon electrode  239  functions as a select gate electrode, not a control gate electrode. A covering oxide film  220  is formed to entirely surround the polysilicon electrode  239  and reach the surface of the substrate. Over the side faces of the polysilicon electrode  239 , a sidewall oxide film  221  is formed with the covering oxide film  220  interposed therebetween. The dot elements  204  to be the floating gate are formed in the sidewall insulating film  221  on the covering oxide film  220 . A control gate electrode  242  is further formed to be capacitively coupled to the dot elements  204  with an interelectrode insulating film  241  interposed therebetween. The structures of the other parts are the same as those in the sixth embodiment. 
     Such a structure can be easily formed by performing the process steps of forming the interelectrode insulating film  241  and the control gate electrode  242  in addition to the fabrication process steps of the sixth embodiment. Thus, the detailed description thereof will be omitted herein. 
     In this embodiment, the select gate electrode  239 , i.e., a select transistor, is formed. Thus, not only the effects of the sixth embodiment can also be attained, but also a highly reliable nonvolatile memory cell, which can be driven with lower power consumption and at a lower voltage, can be obtained. 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, a substrate made of a compound semiconductor such as GaAs or any other semiconductor may also be used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used as the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     EMBODIMENT 8 
     Next, the eighth embodiment of the present invention, in which an SOI substrate is used instead of the Si substrate, will be described. FIG. 13 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the eighth embodiment. In FIG. 13, the components having the same structures as the counterparts in FIG. 6 are identified by the same reference numerals, and the structures and functions thereof are just as already described. 
     As shown in FIG. 13, a buried oxide layer  250  is formed in this embodiment in a surface region of the p-type Si substrate  201  to reach a predetermined depth. And on the buried oxide layer  250 , the first and second n-type diffusion layers  207   a  and  207   b  and a channel region  291  interposed therebetween are formed. The structures of the other parts are the same as those shown in FIG.  6 . 
     In this embodiment, the write, read and erase operations are performed in fundamentally the same way as in the third embodiment. In this embodiment in particular, since the potential in the channel region  291  of each nonvolatile memory cell can be controlled, the write, read and erase operations may be advantageously performed more accurately and more rapidly. 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used for the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     In the memory cell structures of the foregoing fourth to seventh embodiments and the ninth embodiment to be described below, a buried oxide layer may also be formed in a surface region of the p-type Si substrate  201  to reach a predetermined depth. And a channel region may also be formed between these two layers on the buried oxide layer. 
     EMBODIMENT 9 
     Next, the ninth embodiment of the present invention, in which the dot elements are formed on a lower-level portion of a gate insulating film having an inclined portion, will be described. FIG. 14 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the ninth embodiment. In FIG. 14, the components having the same structures as the counterparts in FIG. 8 are identified by the same reference numerals, and the structures and functions thereof are just as already described. 
     As shown in FIG. 14, an inclined portion is provided in the upper surface of the p-type Si substrate  201  in this embodiment. The gate oxide film  203 , the silicon dioxide film  205  and the polysilicon electrode  206  functioning as a control gate electrode are formed so as to overlap the inclined portion and the regions on right- and left-hand sides thereof. And the dot elements  204  to be the floating gate are formed only on the lower-level portion of the gate oxide film  203  (i.e., the left-hand part of the hatched gate oxide film  203 ) in the vicinity of one end of the first n-type diffusion layer  207   a.    
     Such a structure can be easily formed by performing the step of forming the inclined portion in the p-type Si substrate  201  in addition to the fabrication process steps of the fourth embodiment, and thus the description thereof will be omitted herein. The inclined portion may be formed in the p-type Si substrate  201  by crystalline anisotropic etching (e.g., using an aqueous solution of ethylenediamine and catechol) for aligning the edge of the substrate to a particular plane orientation, for example. 
     The nonvolatile memory cell of this embodiment performs the write, read and erase operations. in fundamentally the same way as in the fourth embodiment. 
     In this embodiment, not only the effects of the fourth embodiment, but also the following effects can be attained. 
     In the case of using CHE&#39;s for writing, in the memory cell structures shown in FIGS. 6 and 8, the direction of the CHE&#39;s moving from the second to the first n-type diffusion layer  207   b  to  207   a  in the channel formed in the p-type Si substrate  201  is ordinarily vertical to the direction of the CHE&#39;s passing through the gate oxide film  203  and being injected into the dot elements  204 . Thus, the injection probability of the CHE&#39;s is very low. However, in the structure shown in FIG. 14, the channel is inclined. Thus, the angle formed between the direction of the CHE&#39;s moving from the second to the first n-type diffusion layer  207   b  to  207   a  in the channel and the direction of the CHE&#39;s passing through the gate oxide film  203  and being injected into the dot elements  204  is less than 90 degrees. As a result, the injection probability of CHE&#39;s is increased. Consequently, power consumption can be reduced and write speed can, be increased. 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, an SOI substrate formed by epitaxially growing an Si layer on an insulating substrate may be naturally used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used as the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     EMBODIMENT 10 
     Next, the tenth embodiment of the present invention, in which the dot elements are formed on a corner portion of a gate insulating film having a stepped portion, will be described. FIG. 15 is a cross-sectional view illustrating the structure of a nonvolatile memory cell in the tenth embodiment. 
     As shown in FIG. 15, in the memory cell of this embodiment, a stepped portion  260 , the side face of which is a {100} plane, is formed in a p-type Si substrate  201   x , the principal surface of which is a {111} plane. A first gate oxide film  251 , a sidewall oxide film  252 , a second gate oxide film  253  and a polysilicon electrode  206  functioning as a control gate electrode are formed in this order so as to overlap the stepped portion  260  and right- and left-hand side regions thereof. And the dot elements  204  functioning as a floating gate are formed only on the corner portion of the first gate oxide film  251  so as to be located in the vicinity of one end of the first n-type diffusion layer  207   a . In this case, the first gate oxide film  251  is relatively thick on the {111} plane, in which Si single crystals are most densely arranged, and is relatively thin on the side face of the stepped portion  260 , which is a {100} plane. The thickness of the second gate oxide film  253  is almost as large as that of the first gate oxide film  251  at the relatively thick portion thereof. 
     Such a structure can be easily formed by performing the process steps shown in FIGS.  16 ( a ) through  16 ( c ). 
     First, in the step shown in FIG.  16 ( a ), the p-type Si substrate  201   x , the principal surface of which is a {111} plane, is prepared. And the stepped portion  260 , the side face of which is a {100} plane, is formed in the active region of the p-type Si substrate  201   x . The stepped portion  260  may be formed in the p-type Si substrate  201   x  by crystalline anisotropic etching (e.g., using an aqueous solution of ethylenediamine and catechol) for aligning the edge of the substrate to a particular plane orientation, for example. 
     Then, a silicon dioxide film  251   a  to be a gate oxide film is formed on the substrate by thermal oxidation. In this case, the oxide film is formed to be relatively thick on the principal surface, because the principal surface is the densest {111} plane. On the other hand, since the side face of the stepped portion  260  is a {100} plane where the density of Si atoms is low, the oxide film formed thereon becomes relatively thin. That is to say, the silicon dioxide film  251   a , which is thick on the principal surface of the substrate but thin on the side face of the stepped portion  260 , is obtained. Thereafter, in accordance with the method of the first or second embodiment, the dot elements  204 , to constitute a floating gate, are formed over the entire surface of the substrate. Furthermore, another silicon dioxide film  252   a  is deposited over the substrate by a CVD technique, thereby filling in the gaps of the dot elements  204  with an insulator. 
     Next, in the step shown in FIG.  16 ( b ), the two silicon dioxide film  251   a  and  252   a  are etched anisotropically, thereby forming the first gate oxide film  251  and the sidewall oxide film  252  around the polysilicon electrode  206  to be a control gate electrode. In this process step, the lower-level silicon dioxide film  251   a  is entirely removed except for the portion on the side face of the stepped portion  260  and on the surrounding regions thereof. Thus, the dot elements  204 , which existed on the removed portion of the silicon dioxide film  251   a , are also removed simultaneously. 
     Thereafter, in the step shown in FIG.  16 ( c ), another silicon dioxide film  253   a  to be the second gate oxide film, and a polysilicon film  206   a  to be the polysilicon electrode are deposited in this order by a CVD technique. 
     In the subsequent process steps (not shown), the polysilicon film  206   a  and the silicon dioxide film  253   a  are patterned, thereby forming the polysilicon electrode  206  functioning as a control gate electrode and the second gate oxide film  253 . Then, n-type dopant ions (such as arsenic ions) are implanted at a high doping level by using the polysilicon electrode  206  as a mask to form n-type diffusion layers  237   a  and  237   b . Thereafter, an interlevel dielectric film  208  is formed. Contact holes  209  are opened in the interlevel dielectric film  208 . Tungsten plugs  210  are formed by filling in the contact holes  209  with tungsten. And the first and second aluminum interconnects  211   a  and  211   b  are formed. All of these parts may be formed by known techniques. As a result, the memory cell structure shown in FIG. 15 is obtained. 
     It is noted that in the anisotropic etching between the process steps shown in FIGS.  16 ( a ) and  16 ( b ), only the upper-level silicon dioxide film  252   a  may be removed and the lower-level silicon dioxide film  251   a  may be left. In such a case, the dot elements  204  remain on the lower-level silicon dioxide film  251   a . However, if only the exposed dot elements  204  are selectively etched (using an acid, for,example), then the sidewall oxide film  252  covering the dot elements  204  can be formed on the side face of the polysilicon electrode  206  while removing the exposed dot elements  204 . 
     The nonvolatile memory cell of this embodiment performs the write, read and erase operations in fundamentally the same way as in the ninth embodiment. 
     In this embodiment, not only the effects of the ninth embodiment, but also the following effects can be attained. 
     In the case of using CHE&#39;s for writing, in the structure shown in FIG. 15 having a stepped channel, the direction of CHE&#39;s moving from the second to the first n-type diffusion layer  207   b  to  207   a  in the channel is substantially parallel to the direction of CHE&#39;s passing through the first gate oxide film  251  and being injected into the dot elements  204 . Thus, the injection probability of CHE&#39;s can be even more increased. In addition, since the first gate oxide film  251  is relatively thin on the side face of the stepped portion  260 , which is a {100} plane, the injection probability of CHE&#39;s can be increased noticeably. Consequently, power consumption can be remarkably reduced and write speed can be tremendously increased. 
     In this embodiment, the substrate is made of p-type silicon. Alternatively, an n-type Si substrate may be used. Furthermore, an SOI substrate formed by epitaxially growing an Si layer on an insulating substrate may be naturally used. 
     Also, any fine particles other than Si, Au, Fe fine particles may also be used for the dot elements. For example, semiconductor fine particles other than Si fine particles or metal, semiconductor or semi-insulating fine particles such as Ti and GaAs fine particles having a function of storing charges may also be used. 
     If a single-layer protein film is formed to produce dot elements for the memory cell of the third, fifth, eighth, ninth or tenth embodiment, the dot elements may be formed in accordance with any method other than the method of the second embodiment. Also, instead of a single-layer protein film, a multilayer thin film may also be used.