Abstract:
Provided is a method of fabricating an oxide film on a substrate. The method includes: (a) placing an electrode adjacent to a first region of the substrate on which the oxide film is to be formed under a humid atmosphere; (b) contacting the electrode with the substrate while applying voltage between the electrode and the substrate; and (c) applying pressure to the electrode while applying voltage between the electrode and the substrate and forming the oxide film on the surface of the substrate due to anodic oxidation. Accordingly, the oxide film fabrication method may be used to fabricate an electronic device with a nanometer scale.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims the benefit of Russian Patent Application No. 2004123698, filed on Aug. 2, 2004, in the Russian Patent &amp; Trademark Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Embodiments of the present disclosure may relate to a method of fabricating an oxide film, and more particularly, to a method of fabricating an oxide film whose thickness (nm) at a predetermined position of a substrate can be precisely adjusted.  
         [0004]     2. Description of the Related Art  
         [0005]     According to a known oxide film creation method (J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek, and J. Bennett, Appl.Phys.Lett.    56 ,  pp. 2001-2003 (1990)), an electrode is placed adjacent to a substrate surface. A negative electrostatic potential is applied between the electrode and the substrate surface long enough for electrode-substrate current flow to reach a magnitude near 3 nA, while the substrate surface is in an oxygen or air atmosphere to activate anodic oxidation. Then the electrode placement is varied with regard to the position on the surface of the substrate. That is, the electrode is placed adjacent to the surface in other positions, forming oxide film relief, which comprises detached dielectric areas with a resolution of 0.1 micron or less. The electrode is placed close to the substrate surface but does not make physical contact. The magnitude of electrostatic potential used is 1.7 to 3.5 V. After anodic oxidation of the substrate, the substrate is etched with the oxide film serving as a mask for lithography. In this method semiconductor Si substrates and HNO 3 :HOAc:HF (5:3:3) etchant are used.  
         [0006]     This method has a disadvantage in that the obtained oxide films have a shallow depth, less than  5  nm penetration. There are two factors which can explain this disadvantage. First, the natural oxide film layer on the semiconductor or metal surface limit the electrode-substrate current and anodic oxidation process because of their dielectric nature. The second factor is mechanical stresses in the substrate volume which may form during the oxide growth because of differences between the stoichiometry of reactants in the anodic oxidation reaction. The mechanical stresses limit ion diffusion and ion drift in the local anodic oxidation reaction zone. Increases of the stress may result in the activation energy of ion diffusion increasing too. This brings about a decrease of the ion diffusion coefficient, which determines anodic oxide growth rate in the substrate volume. So, the oxide growth process toward the substrate volume is stunted. The shallow depth of dielectric oxide film penetration in the substrate limits the practical use of the method because of ultra thin film creation demand. Dielectric oxide film penetration in the substrate determines the variation of structure properties for practical application.  
         [0007]     There is also oxide film creation (U.S. Pat. No. 5,785,838) wherein an electrode with a tip radius of less than 100 nm is placed adjacent to a substrate surface with a negative electrostatic potential between the electrode and a position on the substrate surface for a time interval. The electrode is placed close enough to the substrate surface for electrode-substrate current flow to begin, while the substrate surface is surrounded by an oxygen-bearing gas, wherein the oxygen-bearing gas is adsorbed on said substrate surface to activate anodic oxidation. Then the electrode placement is varied with regard to the surface of the substrate, the electrode is adjacent to the surface in other positions, forming oxide film relief which comprises detached dielectric areas with a resolution of 0.1 micron or less. The electrode is placed close to the substrate surface but does not make physical contact. The magnitude of current through the electrode-substrate system is governed by varying the electrode-substrate distance during the anodic oxidation process.  
         [0008]     The electrode tip material may be a diamond. The electrode is placed close to the substrate surface but does not make physical contact. The magnitude of electrostatic potential is 8 V. After anodic oxidation of the substrate material the substrate material is etched with the oxide film serving as a mask for lithography. Substrate materials include Si, Ge, GaAs or InP semiconductor substrates.  
         [0009]     This method has a disadvantage in that the obtained oxide films have a shallow depth, less than 6-8 nm penetration. There are two factors which can explain this disadvantage. First, the natural oxide film layer on the semiconductor or metal surface limit the electrode-substrate current and anodic oxidation process because of their dielectric nature. The second factor is mechanical stresses in the substrate volume which form during the oxide growth because of differences between the stoichiometry of reactants in the anodic oxidation reaction. The mechanical stresses limit ion diffusion and ion drift in the local anodic oxidation reaction zone. Increases of the stress may result in the activation energy of ion diffusion increasing too. This brings about a decrease of ion diffusion coefficient, which determines anodic oxide growth rate in the substrate volume. So, the oxide growth process toward the substrate volume is stunted. The shallow depth of dielectric oxide film penetration in the substrate limits the practical use of the method because of ultra thin film creation demand. Dielectric oxide film penetration in the substrate determines the variation of structure properties for practical application.  
       OBJECTS AND SUMMARY  
       [0010]     Embodiments of the present disclosure provide a method of fabricating an oxide film whose thickness may be arbitrarily adjusted according to a region on the substrate, and whose thickness at a predetermined region may be greater than that of other nearby regions.  
         [0011]     According to an aspect of the present disclosure, there may be provided a method of fabricating an oxide film on a substrate, the method comprising: (a) placing an electrode adjacent to a first region of the substrate on which the oxide film is to be formed and applying voltage between the electrode and the substrate under a humid atmosphere; (b) contacting the electrode with the substrate in the state where the voltage is applied between the electrode and the substrate; and (c) applying pressure to the electrode to be pressed to the substrate in the state where the voltage is applied between the electrode and the substrate, and forming an oxide film on the surface of the substrate due to anodic oxidation.  
         [0012]     Operation (c) may comprise: applying pressure, which is less than the pressure for creation of inelastic deformation of the substrate, to the electrode and oxidizing the surface of the substrate; and applying pressure, which is greater than the pressure for creation of inelastic deformation of the substrate, to the electrode and oxidizing the surface of the substrate.  
         [0013]     The method may further comprise: moving the electrode to a second region on the surface of the substrate and performing operations (a) through (c).  
         [0014]     The substrate may be made of semiconductor or metal.  
         [0015]     The semiconductor may be selected from the group consisting of Si, GaAs, Ge, InAs, GaN, and SOI, and the metal may be selected from the group consisting of Al, Ti, Ru, and Cu.  
         [0016]     The voltage may be less than 25 V.  
         [0017]     The voltage may range from 4 to 15 V.  
         [0018]     The pressure high enough to cause the inelastic deformation in operation (c) may range from 10 5  to 10 9 N/m 2 .  
         [0019]     The electrode may be a conductive tip.  
         [0020]     The tip may be an atomic force microscope (AFM) probe tip. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other features and advantages of embodiments of the present disclosure will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0022]      FIGS. 1A through 1C  are diagrams illustrating sequential operations for fabricating an oxide film according to an embodiment of the present disclosure;  
         [0023]      FIG. 2  is an image illustrating an oxide film formed on a substrate according to an embodiment of the present disclosure;  
         [0024]      FIG. 3  is a diagram of an apparatus for fabricating an oxide film according to an embodiment of the present disclosure; and  
         [0025]      FIG. 4  is a graph illustrating a profile of an oxide film fabricated according to an embodiment of the present disclosure. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Embodiments of the present disclosure will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the disclosure are shown.  
         [0027]      FIGS. 1A through 1C  are diagrams illustrating methods of fabricating an oxide film according to embodiments of the present disclosure. The method of fabricating the oxide film according to the present embodiment is characterized in that negative potential may be applied through a sharp tip to a surface of a substrate to generate an electrostatic field and thus promote local anodic oxidation on the surface of the substrate. Also, an oxide film fabrication method according to the present embodiment is characterized in that the thickness of the oxide film may be adjusted by applying pressure to the surface of the substrate using the tip.  
         [0028]     Referring to  FIG. 1A , a tip is used as an electrode. In detail, an atomic force microscope (AFM) tip may be used as the electrode. First, the electrode, that is, the tip ( 1 ) is placed adjacent to a substrate ( 2 ). A natural oxide film ( 3 ) is typically formed on a surface of the substrate ( 2 ). As a distance between the tip ( 1 ) and the substrate ( 2 ) decreases, predetermined current flows between the tip ( 1 ) and the substrate ( 2 ) due to voltage applied from a power source ( 5 ). Reference numeral  4  denotes a humid layer, which is referred to as a water layer hereinafter. As the water layer ( 4 ), for example, H 2 O or C 2 H 5 OH may be used. When the current flows between the tip ( 1 ) and the substrate ( 2 ), a positive ion M +  of the substrate ( 2 ) that may be made of semiconductor or metal is moved toward the tip ( 1 ), and an oxygen ion  0 - is moved toward the substrate ( 2 ), such that an oxide film ( 6 ) is formed. Although the natural oxide film ( 3 ) may be formed in  FIG. 1A , the natural oxide film ( 3 ) is not an essential element.  
         [0029]     The substrate ( 2 ) may be made of any material which can generate a negative ion when voltage is applied thereto. Specifically, the substrate ( 2 ) may be made of a material selected from the group consisting of semiconductors Si, GaAs, Ge, InAs, GaN, and SOI. The substrate ( 2 ) may be made of a material selected from the group consisting of metal Al, Ti, Ru, and Cu. The tip ( 1 ) functions as an electrode in the present embodiment, and may be made of any conductive material having an end portion that can apply pressure to a predetermined region of the substrate ( 2 ). For example, an AFM tip may be used as the tip ( 1 ).  
         [0030]     Referring to  FIG. 1B , after the tip ( 1 ) is placed adjacent to the surface of the substrate ( 2 ), pressure is applied to the surface of the substrate ( 2 ). Here, the pressure is less than the pressure required to create inelastic deformation of the surface of the substrate ( 2 ). The height of the oxide film formed around the tip ( 1 ) increases, and the depth of the oxide film ( 6 ) penetrating into the substrate ( 2 ) increases.  
         [0031]     Referring to  FIG. 1C , the tip ( 1 ) is pressed against the surface of the substrate ( 2 ). Here, the pressure applied to press the tip ( 1 ) against the surface of the substrate ( 2 ) is high enough to create inelastic deformation of the surface of the substrate ( 2 ). The height of the oxide film formed around the tip ( 1 ) increases further, and the depth of the oxide film ( 6 ) penetrating into the substrate ( 2 ) contacting the tip ( 1 ) increases further.  
         [0032]     As results of the processes shown in  FIGS. 1A through 1C  demonstrate, as the pressure applied to the tip ( 1 ) increases and the voltage applied to the tip ( 1 ) increases, the height of the oxide film formed around the tip ( 1 ) increases and the depth of the oxide film ( 6 ) penetrating into the substrate ( 2 ) increases. Consequently, the thickness of the oxide film formed at a predetermined region can be adjusted easily. Here, the applied voltage may preferably be less than approximately 25 V. Since the processes may be unstable under too high voltage environment, it is preferable that the voltage range from 4 to 15 V. The pressure applied to the substrate ( 2 ) may be varied according to the material of the substrate ( 2 ). For example, a pressure high enough to create inelastic deformation of the substrate ( 2 ) may range from 10 5  to 10 9 N/m 2 . The predetermined pressure may be applied after a simple contact between the substrate ( 2 ) and the tip ( 1 ), or the predetermined pressure may be periodically applied for a predetermined time. For example, the pressure may be applied at a 0.1 Hz time interval, or the pressure may be applied at a 10 6  Hz time interval. After the oxide film ( 6 ) is formed using the tip ( 1 ) at a predetermined region of the substrate ( 2 ), an oxidation process may be performed by moving the tip ( 1 ) in non-contact with the surface of the substrate ( 2 ) or continuously performed by moving the tip ( 1 ) in contact with the surface of the substrate ( 2 ).  
         [0033]     The electrochemical change of the surface of the substrate ( 2 ) may depend on the tip ( 1 ), for example, the sharp AFM probe tip. That is, the tip ( 1 ) is a desirable and variable element for fabricating the oxide film in the present embodiment. The electrode is placed adjacent to the surface of the substrate ( 2 ), and voltage is applied under a humid atmosphere to form an oxide film on the surface of the substrate ( 2 ) due to anodic oxidation. At the same time when the voltage is applied, the electrode may be pressed to the substrate ( 2 ). The depth of the dielectric oxide layer penetrating into the substrate ( 2 ) increases according to the magnitude of the pressure applied to the electrode and the time period for which the electrode is pressed to the substrate ( 2 ).  
         [0034]      FIG. 2  demonstrates an AFM-image of topographical relief of a GaAs substrate surface with local anodic oxide lines, which were formed under similar electrostatic potential between a tip and the substrate surface and for similar durations in each line, but different pressure was applied to the tip against the surface (pressure action). Reference numeral  9  denotes the local oxide line obtained without applying any additional pressure (no pressure method), reference numeral  7  denotes the local anodic oxide line obtained from applying a pressure in excess of that required to create inelastic deformation (extra pressure action), reference numeral  8  denotes the local anodic oxide line obtained from applying a pressure less than that required to create inelastic deformation (pressure action).  
         [0035]     The highest 10 nm local anodic oxide line  7  is obtained at a regime of surface scratching, wherein the electrode is pressed against the surface with a pressure in excess of that required to create inelastic deformation (extra pressure action) ( FIG. 1C ). In this case, the electrode is pressed against the surface with such pressure that the pressure would create grooves with a 1 to 2 nm depth at the surface through destroying natural oxide when no potential voltage for anodic oxidation activation is applied. A second local oxide line  8  is obtained at regime of simultaneous mechanical and electrochemical surface modification, wherein there is a negative potential between the tip and the surface and the electrode is pressed against the surface with pressure less than that required to create inelastic deformation (pressure action) ( FIG. 1B ). The lowest local anodic oxide line  9  is obtained from a no pressure method ( FIG. 1A ), wherein additional pressure is not applied to the electrode in relation to the surface. In this case the height of local anodic oxide line  9  is 3.4 times less than line  7  and is 1.6 times less than line  8 . These local anodic oxide lines ( FIG. 2 ) were obtained under similar oxidation activation voltage and time intervals of applying this voltage in each point of said surface. From detailed direct and indirect analysis, the depth of the local anodic oxidation may have a direct ratio to the height of the local anodic oxidation and may obtain a depth of more than 30 nm. In embodiments of the present disclosure the electrode placement is varied with regard to the principal surface of the semiconductor or metal substrate, the electrode is placed adjacent to the principal surface in other positions, forming oxide line relief (by local anodic oxidation in humid atmosphere) which comprises detached dielectric areas, wherein mechanical pressure was applied on said electrode towards the said surface simultaneously with applying of electrical potential. The depth of dielectric oxide film penetration in the substrate may be increased as result.  
         [0036]      FIG. 3  shows an apparatus for embodiments of the present disclosure which may be positioned on the base of a scanning probe microscope, for example, an atomic force microscope, wherein a probe comprising a sharp tip may be used as the electrode.  1  denotes a sharp tip,  5  denotes an electrical potential source,  10  denotes a probe,  11  denotes a probe positioning system,  12  denotes a laser,  13  denotes a photodetector,  14  denotes a feedback system,  15  denotes a data processing box,  16  denotes a humidity controller,  17  denotes a substrate,  18  denotes a humidity control area,  19  denotes an electromagnetic noise shield area, and  20  denotes a vibration and noise damper area. A method of embodiment of the present disclosure is executed by using of the sharp tip of a SPM-probe, for example, an AFM-probe may be used as an electrode.  
         [0037]     A part of the probe ( 10 ), i.e., the sharp tip ( 1 ) may be placed adjacent to the principal surface of substrate ( 17 ). The probe ( 10 ) may be fixed by positioning system ( 11 ), which allows varying the probe placement in lateral and vertical directions in relation to principal surface ( 17 ). The placement of sharp tip ( 1 ) in relation to surface ( 17 ) may be determined by laser ( 12 ), which may reflect from the probe ( 10 ) surface and to the photodetector ( 13 ). The signal of the photodetector ( 13 ) may be directed to the information processing box ( 15 ) through the feedback system ( 14 ). The information processing box ( 15 ) may be used for governing of positioning system ( 11 ).  
         [0038]     The matched voltage source ( 5 ) and humidity controller ( 16 ) may be used for controlling and measuring electrostatic potentials between sharp tip ( 1 ) and principal surface ( 16 ) and the humidity in the humidity control area ( 18 ), respectively. In order to remove electromagnetic and mechanical noises, a grounded metal area ( 19 ) and a vibroisolation area ( 20 ) may be used. The humidity in humidity control area ( 18 ) may be maintained from 10 to 95% for achieving requirements of existence of adsorbed water film on the principal surface. In a method of embodiment of the present disclosure, a regime of permanent pressure of sharp tip ( 1 ) against the surface ( 17 ) may be obtained by using the positioning system ( 11 ) of probe ( 10 ). A regime of periodical pressure of sharp tip ( 1 ) against the surface ( 17 ) may be obtained by using the positioning system ( 11 ) of probe ( 10 ) which is then vibrated on a frequency.  
         [0039]     There are two main factors for limiting depth during practice of the no pressure method of oxide film creation ( FIG. 1A ). First, the natural oxide film layer ( 3 ) on the semiconductor or metal surface ( 2 ) limit the electrode-substrate current and anodic oxidation process because of their dielectric nature. The second factor results from mechanical stresses in the substrate volume which may form during the oxide growth because of differences between the stoichiometry of reactants in the anodic oxidation reaction. So the anodic oxide ( 6 ) growth may be limited 5 to 8 nm.  
         [0040]     Mechanical pressure applied by a sharp electrode against a substrate surface applied simultaneously with electrical potential allows embodiments of the present disclosure to overcome the above limiting oxidation factors. During the anodic oxide growth process the direct mechanical action on the said substrate surface by sharp tip may increase the natural oxide conductivity by deteriorating the oxide structure, from one point of view, and may allow removing mechanical stresses in the anodic oxide volume, from another point of view.  
         [0041]     While in a humid atmosphere, the sharp tip electrode ( 1 ) is placed adjacent to the semiconductor or metal substrate surface ( 2 ) covered by natural oxide film ( 3 ) and adsorbed water film ( 4 ), then the electrode is pressed against the surface a pressure less than that required to create inelastic deformation (pressure action), wherein the electrode is wetted in said water film ( FIG. 1B ). The negative electrostatic potential is applied between the electrode, the sharp tip ( 1 ), and the surface position by a matched voltage source ( 5 ) for a determined time interval, for example, 0.1 Hz. As a result, the initiation of anodic oxidation process may be activated in the area of said surface which is situated directly under the sharp tip ( 1 ). In this process the natural oxide film may be thicker in the area directly under the sharp tip toward the tip and in the substrate volume ( FIG. 1B ). Formed local anodic oxide ( 6 ) may have parameters which may be superior to those in known technical inventions. The formed local anodic oxide film may have the height of tens of nanometers.  
       EXAMPLES  
       [0042]     The following examples are provided to more fully illustrate embodiments of the present disclosure.  
       Example I  
       [0043]     A surface of a GaAs substrate covered by natural oxide film is placed in a humidity control area where a humid atmosphere is maintained. An adsorbed water film is formed on the surface. Then an electrode, a sharp tip of an AFM-probe, is placed adjacent to the surface so that the sharp tip is wetted in the adsorbed water film. Then the electrode is placed into contact with the surface. Then, by matched voltage source a negative electrostatic potential of 10V is applied between the electrode and the surface for a 0.1 Hz time interval. Simultaneously the electrode is pressed to the surface by a pressure force regime, wherein the timing of the physical contacts between the electrode and position on the principal surface during said time interval is equal to 0.1 Hz. The action on the surface is made by pressing the electrode against the surface with a pressure force in excess of that required to create inelastic deformation (extra pressure action). In this case, the electrode is pressed against the surface with such pressure that the pressure would create grooves with a 1 to 2 nm depth at the surface through destroying natural oxide when no potential voltage for anodic oxidation activation is applied. Here, the pressure force is equal to 10 9  N/m 2 . The electrode placement is varied with regard to the principal surface of the substrate, and the electrode is placed adjacent to the principal surface in other positions, forming oxide film relief which comprise detached dielectric areas. The thickness of formed local anodic oxide film is equal to 50 nm.  
       Example II  
       [0044]     A surface of a GaAs substrate covered by natural oxide film is placed in a humidity control area where a humid atmosphere is maintained. An adsorbed water film is formed on the surface. Then an electrode, the sharp tip of an AFM-probe, is placed adjacent to the surface so that the sharp tip is wetted in the adsorbed water film. Then the electrode is placed into contact with the surface. Then, by matched voltage source, a negative electrostatic potential of 4V is applied between the electrode and the surface. Simultaneously the electrode is pressed to the surface by a pressure force regime, wherein the timing of physical contacts between the electrode and a position on the principal surface during said time interval is to equal 0.1 Hz. The action on the surface is made by pressing the electrode against the surface with a pressure force less than that required to create inelastic deformation (pressure action). Here, the pressure force is equal to 10 7  N/m 2 . The electrode placement is varied with regard to the principal surface of the substrate, and the electrode is placed adjacent to the principal surface in other positions, forming oxide film relief which comprise detached dielectric areas. The thickness of formed local anodic oxide film is equal to 10 nm.  
       Example III  
       [0045]     A surface of a GaAs substrate covered by natural oxide film is placed in a humidity control area where a humid atmosphere is maintained. An adsorbed water film is formed on the surface. Then an electrode, the sharp tip of an AFM-probe is placed adjacent to the surface so that the sharp tip is wetted in the adsorbed water film. Then the electrode is placed into contact with the surface. Then, by matched voltage source, a negative electrostatic potential of 7V is applied between the electrode and the surface. Simultaneously the electrode is pressed to the surface by a pressure force regime, wherein the timing of physical contacts between the electrode and positions on the principal surface during said time interval is equal to 10 6  Hz The action on the surface is made by pressing the electrode against the surface with a pressure force less than that required to create inelastic deformation (pressure action). Here, the pressure force is equal to 10 7  N/m 2 . The electrode placement is varied with regard to the principal surface of the substrate, and the electrode is placed adjacent to the principal surface in other positions, forming oxide film relief which comprise detached dielectric areas. The thickness of formed local anodic oxide film is equal to 20 nm.  
       Example IV  
       [0046]     A surface of a Ti substrate covered by natural oxide film is placed in a humidity control area where a humid atmosphere is maintained. An adsorbed water film is formed on the surface. Then an electrode, the sharp tip of an AFM-probe is placed adjacent to the surface so that the sharp tip is wetted in the adsorbed water film. Then the electrode is placed into contact with the surface. Then, by matched voltage source, a negative electrostatic potential of 7V is applied between the electrode and the surface. Simultaneously the electrode is pressed to the surface by a pressure force regime, wherein the physical contacts between the electrode and position on the principal surface during said time interval are equal to 0.1 Hz. The action on the surface is made by pressing the electrode against the surface with a pressure force in excess of that required to create inelastic deformation (extra pressure action). In this case, the electrode is pressed against the surface with such pressure that the pressure would create grooves with a 1 to 2 nm depth at the surface through destroying natural oxide when no potential voltage for anodic oxidation activation is applied. Here, the pressure force is equal to 10 9  N/m 2 . The electrode placement is varied with regard to the principal surface of the substrate, and the electrode is placed adjacent to the principal surface in other positions, forming oxide film relief which comprise detached dielectric areas. The thickness of formed local anodic oxide film is equal to 30 nm.  
       Example V  
       [0047]     A surface of the Al substrate covered by natural oxide film is placed in a humidity control area into a humid atmosphere. An adsorbed water film is formed on the surface. Then an electrode, the sharp tip of an AFM-probe is placed adjacent to the surface so that the sharp tip is wetted in the adsorbed water film. Then the electrode is placed into contact with the surface. Then, by matched voltage source, a negative electrostatic potential of 10V is applied between the electrode and the surface. Simultaneously the electrode is pressed to the surface by a pressure force regime, wherein the physical contacts between the electrode and position on the principal surface during said time interval are equal to 10 4  Hz. The action on the surface is made by pressing the electrode against the surface with a pressure force less than that required to create inelastic deformation (pressure action). Here, the pressure force is equal to 10 7  N/m 2 . The electrode placement is varied with regard to the principal surface of the substrate, and the electrode is placed adjacent to the principal surface in other positions, forming oxide film relief which comprise detached dielectric areas. The thickness of formed local anodic oxide film is equal to 15 nm.  
         [0048]     As described above, the thickness of the oxide film may be adjusted according to the position on the surface of the substrate. That is, a thin oxide film may be formed at a first point on the surface of the substrate and a thick oxide film may be formed at a second point on the surface of the substrate. Further, the thickness of the oxide film formed on the substrate can increase, and the depth of the oxide film penetrating into the substrate can increase.  
         [0049]     While embodiments of the present disclosure have been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the following claims.