Abstract:
A micromachine successfully reduced in parasitic capacity between input and output electrodes, and having an oscillator configured as ensuring a high S/N ratio under operation at higher frequencies is disclosed. The micromachine comprises an insulating layer formed on a substrate; a first electrode for signal input formed on the insulating layer; a second electrode for signal output formed on the insulating layer; and an oscillator electrode formed as being opposed with the first electrode and the second electrode and as being spaced therefrom by an air gap, wherein the insulating layer has a groove formed therein at least between the first electrode and the second electrode.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     This is a divisional of co-pending U.S. patent application Ser. No. 10/835,769, filed Apr. 30, 2004, and claims benefit of parent of Japanese Patent Application No. JP 2003-133929, filed in the Japanese Patent Office on May 13, 2003, the entire contents of both of which being incorporated herein by reference to the extent permitted by law. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a micromachine and a method of fabricating the same, and in more detail, a micromachine having a frequency selection function and can be integrated with a semiconductor device, and a method of fabricating the same.  
         [0003]     A micro-resonator fabricated based on a semiconductor process is characterized by its small device occupational area, capability of realizing a high Quality-factor, and possibility of integration with other semiconductor devices, and its use as an IF filter and an RF filter, out of various wireless communication devices, has been proposed by several research institutes including Michigan University (see non-patent document 1, for example).  
         [0004]     The micro-resonator ever proposed and examined, however, has a resonance frequency only as high as not exceeding 200 MHz at maximum, and has been unsuccessful in providing its characteristic Quality-factor in a frequency range in GHz band, unlikely to a conventional gigahertz (GHz) filter based on surface acoustic wave (SAW) or a film bulk acoustic resonator (FBAR). At present, there is a general tendency of lowering in a peak of resonance frequency as an output signal in higher frequencies, so that improvement in signal-to-noise (S/N) ratio of the peak of resonance frequency is essential for obtaining a desirable filter characteristic.  
         [0005]     According to a disk type micro-resonator described in non-patent document 1, a noise component in an output signal is ascribable to a signal which directly transmits a parasitic capacitance formed between the input and output electrodes, and it is described that the noise component can be reduced by disposing an oscillator electrode applied with direct current (DC) between input and output electrodes. On the other hand, a DC voltage exceeding 30 V will be necessary for the disk type oscillator to obtain a sufficiently large output, and a preferable example of a practical configuration will be such as having a beam structure using a Clamp-Clamp beam. One typical beam structure applied with the above-described noise component reduction method will have an electrode arrangement as shown in  FIG. 6 . In  FIG. 6 , a stacked film  114  composed of a silicon oxide film  112  and a silicon nitride film  113  is formed on a silicon substrate  111 , an input electrode  115  and an output electrode  116  are formed thereon in parallel as being spaced from each other, and further thereon a beam resonator  117  is disposed across the input electrode  115  and the output electrode  116 , while being spaced by a micro air gap.  
         [0006]     [Non-Patent Document 1] 
         [0007]     Clark T. -C. Nguyen, Ark-Chew Wong, Hao Ding, “MP4.7 Tunable, Switchable, High-Q VHF Microelectromechanical Band Pass Filters”, 1999 IEEE International Solid-State Circuit Conference, P. 78-79  
         [0008]     Arrangement of the input electrode and the output electrode shown in  FIG. 2 , however, still suffers from parasitic capacitances C 1 , C 2 , as shown in  FIG. 7 , which reside between the input electrode  115  and the output electrode  116  in a space therebetween or through the underlying layer (stacked layer  114 ). In particular for an oscillator of a gigahertz design, S/N ratio will generally degrade due to shrinkage of the structure and narrowing of the distance between the input and output electrodes. This demands further reduction in the parasitic capacitance between the input and output electrodes. The present invention is therefore to provide a micromachine having a reduced parasitic capacitance between the input and output electrodes, and ensuring a large S/N ratio even under operation in higher frequencies.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention relates to a micromachine and a method of fabricating the same, accomplished in view of solving the above-described problems.  
         [0010]     A micromachine of the present invention comprises an insulating layer formed on a substrate; a first electrode for signal input (high-frequency signal input, for example) formed on the insulating layer; a second electrode for signal output (high-frequency signal output, for example) formed on the insulating layer; and an oscillator electrode formed as being opposed with the first electrode and the second electrode while being spaced therefrom by an air gap, wherein the insulating layer has a groove formed therein at least between the first electrode and the second electrode.  
         [0011]     Because the groove is formed in the insulating layer, which possibly configures a region having a large dielectric constant, at least between the first electrode and second electrode as the input/output electrodes, thus-configured micromachine is successful in reducing the capacitance between the first electrode and second electrode, in reducing noise component caused by a signal directly transmits between the first electrode and second electrode, and consequently in obtaining a large S/N ratio even under high-frequency operation. Generally, capacitance between the electrodes is mainly classified into that created while placing an air space in between, and that created while placing the underlying insulating layer in between. The present invention is to reduce the overall capacitance by forming the groove in the insulating layer in a portion between the first electrode and second electrode. The micromachine of the present invention can realize a higher S/N ratio and can more readily detect a signal at higher frequencies as compared with a micromachine having the same configuration of the oscillator electrode, first electrode and second electrode but having no groove between the first and second electrodes.  
         [0012]     A method of fabricating a micromachine of the present invention comprises the steps of forming an insulating layer on a substrate and forming a groove in the insulating layer; forming a first sacrificial layer so as to fill the groove; forming a first electrode for signal input (high-frequency signal input, for example) on the insulating layer on one side of the groove, forming a second electrode for signal output (high-frequency signal output, for example) on the insulting layer on the other side of the groove, and further forming wiring portions of an oscillator electrode as being opposed with the groove while placing the first electrode and the second electrode in between, and being spaced from the first electrode and the second electrode; filling a space between the first electrode and the second electrode with a second sacrificial layer; forming a third sacrificial layer on individual surfaces of the first electrode and the second electrode; forming an oscillator electrode electrically connected with the wiring portions as extending over the first electrode and the second electrode while placing the third sacrificial layer thereunder; and removing the first sacrificial layer, the second sacrificial layer and the third sacrificial layer.  
         [0013]     Because the groove is formed in the insulating layer, which possibly configures a region having a large dielectric constant, in a portion between the first electrode and second electrode as the input/output electrodes, the method of fabricating a micromachine is successful in reducing the capacitance between the first electrode and second electrode, in reducing noise component caused by a signal directly transmits between the first electrode and second electrode, and consequently in obtaining a large S/N ratio even under high-frequency operation. Generally, capacitance between the electrodes is mainly classified into that created while placing an air space in between, and that created while placing the underlying insulating layer in between. The present invention is to reduce the overall capacitance by forming the groove in the insulating layer in a portion between the first electrode and second electrode. The method of fabricating a micromachine of the present invention can provide a micromachine having a higher S/N ratio and capable of more readily detecting a signal at higher frequencies, as compared with a method of forming the same configuration of the oscillator electrode, first electrode and second electrode but forming no groove between the first and second electrodes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above and other objects, features and advantages of the present invention will become more apparent from the following description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:  
         [0015]      FIGS. 1A and 1B  are a schematic sectional view and a plan view, respectively, showing an overall configuration of a micromachine according to a first embodiment of the present invention;  
         [0016]      FIGS. 2A and 2B  are a schematic sectional view and a plan view, respectively, showing an overall configuration of a micromachine according to a second embodiment of the present invention;  
         [0017]      FIGS. 3A and 3B  are a schematic sectional view and a plan view, respectively, showing an overall configuration of a micromachine according to a third embodiment of the present invention;  
         [0018]      FIG. 4  is a plan view showing a modified example of a micromachine according to the third embodiment of the present invention;  
         [0019]      FIGS. 5A  to  5 H are schematic sectional views showing process steps of a method of fabricating a micromachine according to a first embodiment of the present invention;  
         [0020]      FIG. 6  is a schematic sectional view showing a configuration of a related art oscillator; and  
         [0021]      FIG. 7  is a schematic sectional view for explaining problems in a configuration of a related art oscillator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     A first embodiment of the micromachine of the present invention will be described referring to  FIGS. 1A and 1B .  FIG. 1A  is a sectional view of the overall configuration, and  FIG. 1B  is a plan view, where  FIG. 1A  is a sectional view taken along a line I-I in  FIG. 1B .  
         [0023]     As shown in  FIGS. 1A and 1B , an insulating layer  12  is formed on a substrate  11 . The substrate  11  is a semiconductor substrate, for which a silicon substrate or compound semiconductor substrate, for example, can be used. The insulating layer herein is formed typically using a silicon nitride film, where it is also allowable to adopt a stacked structure of a silicon oxide film and a silicon nitride film, or a silicon oxide film depending on materials composing sacrificial layers described later with respect to the fabrication method. The insulating layer  12  has a groove  13  formed therein. For a case where the insulating layer  12  is composed of a stacked structure, a surface of the groove  13  is preferably composed of a film same as that composing a surface of the insulating layer  12 . This is preferable in view of preventing the groove  13  from unnecessarily be widened in the direction of the insulating layer  12  in a removal process of the sacrificial layers.  
         [0024]     A first electrode  14  for signal input (high-frequency signal input, for example) is formed on the insulating layer  12  on one side of the groove  13 , and a second electrode  15  for signal output (high-frequency signal output, for example) is formed on the insulating layer  12  on the other side of the groove  13  so as to be spaced from the first electrode  14 . On the insulating layer  12 , wiring portions  16 ,  17  of an oscillator electrode are formed as being opposed with the groove  13  while placing the first electrode  14  and the second electrode  15  in between, and while being spaced from the first electrode  14  and the second electrode  15 .  
         [0025]     Over the first electrode  14  and the second electrode  15 , an oscillator electrode  19  connected to the wiring portions  16 ,  17  is formed while being opposed therewith and being spaced therefrom by an air gap  18 . The air gap  18  spacing the first electrode  14  and second electrode  15  from the oscillator electrode  19  is formed to a thickness of 10 nm to 100 nm, for example.  
         [0026]     Because the groove  13  is formed in the insulating layer  12  between the first electrode  14  and the second electrode  15 , thus-configured micromachine  1  of the first embodiment is successful in reducing the capacitance between the first electrode  14  and the second electrode  15 , and consequently in obtaining a large S/N ratio even under high-frequency operation. Generally, capacitance between the electrodes is mainly classified into one created while placing an air space in between, and the other created while placing an underlying insulating layer in between. The present embodiment is to reduce the overall capacitance by forming the groove  13  in the insulating layer  12  in a portion between the first electrode  14  and the second electrode  15 . Simulation study by the present inventor has revealed that the configuration of the first embodiment was successful in reducing the capacitance (DC) by as much as  31 % as compared with a configuration having no groove  13  formed therein.  
         [0027]     Next, operation of the micromachine  1  of the first embodiment will be briefed. In a case where the first electrode (input electrode)  14  is applied with voltage of a predetermined frequency, the oscillator electrode  19  spaced therefrom by the air gap  18  oscillates at a specific oscillation frequency, and a distance between the oscillator electrode  19  and the second electrode (output electrode)  15  formed as being opposed therewith and spaced therefrom by the air gap  18  varies at the specific oscillation frequency. This consequently varies capacitance of a capacitor ascribable to the air gap  18  which spaces the oscillator electrode  19  and the second electrode  15 , and a signal of the capacitance is output from the second electrode  15 . A high-frequency filter composed of this type of micro-resonator can realize a higher Quality-factor as compared with a high-frequency filter using surface acoustic wave (SAW) or a film bulk acoustic resonator (FBAR).  
         [0028]     Next, a second embodiment of the micromachine of the present invention will be described referring to  FIGS. 2A and 2B .  FIG. 2A  is a sectional view of the overall configuration, and  FIG. 2B  is a plan view, where  FIG. 2A  is a sectional view taken along a line II-II in  FIG. 2B .  
         [0029]     As shown in  FIGS. 2A and 2B , the insulating layer  12  is formed on the substrate  11 . The substrate  11  is a semiconductor substrate, for which a silicon substrate or a compound semiconductor substrate, for example, can be used. The insulating layer  12  herein is formed typically using a silicon nitride film, where it is also allowable to adopt a stacked structure of a silicon oxide film and a silicon nitride film, or a silicon oxide film depending on materials composing sacrificial layers described later in relation to the fabrication method. The insulating layer  12  has the groove  13  formed therein.  
         [0030]     The first electrode  14  for signal input (typically a signal input of a predetermined frequency, such as high-frequency signal input) is formed on the insulating layer  12  on one side of the groove  13 , and the second electrode  15  for signal output (typically a signal output of a predetermined frequency, such as high-frequency signal output) is formed on the insulating layer  12  on the other side of the groove  13  so as to be spaced from the first electrode  14 . The groove  13  herein is formed so as to get under side edges of the first electrode  14  and the second electrode  15 . It is essential that a degree of the getting-under of the groove  13  is not causative of vibration of the second electrode  15  which serves as an output electrode. On the insulating layer  12 , the wiring portions  16 ,  17  of the oscillator electrode are formed as being opposed with the groove  13  while placing the first electrode  14  and the second electrode  15  in between, and while being spaced from the first electrode  14  and the second electrode  15 .  
         [0031]     The oscillator electrode  19  which is electrically connected with the wiring portions  16 ,  17  is formed as extending over the first electrode  14  and the second electrode  15 , while being opposed therewith and spaced therefrom by the air gap  18 . The air gap  18  spacing the first electrode  14  and the second electrode  15  from the oscillator electrode  19  is formed to a thickness of 10 nm to 100 nm, for example.  
         [0032]     Because the groove  13  is formed in the insulating layer  12  between the first electrode  14  and the second electrode  15 , and because the groove  13  is formed so as to get under the first electrode  14  and the second electrode  15 , the micromachine of the second embodiment is more successful in reducing the capacitance between the first electrode  14  and the second electrode  15  as compared with the configuration of the first embodiment, and consequently in obtaining a large S/N ratio even under high-frequency operation.  
         [0033]     Operations of the micromachine  2  of the second embodiment are similar to those of the micromachine  1  described in the first embodiment.  
         [0034]     Next, a third embodiment of the micromachine of the present invention will be described referring to  FIGS. 3A and 3B .  FIG. 3A  is a sectional view of the overall configuration, and  FIG. 3B  is a plan view, where  FIG. 3A  is a sectional view taken along a line III-III in  FIG. 3B .  
         [0035]     As shown in  FIGS. 3A and 3B , the insulating layer  12  is formed on the substrate  11 . The substrate  11  is a semiconductor substrate, for which a silicon substrate or compound semiconductor substrate, for example, can be used. The insulating layer  12  herein is formed typically using a silicon nitride film, where it is also allowable to adopt a stacked structure of a silicon oxide film and a silicon nitride film, or a silicon oxide film depending on materials composing sacrificial layers described later in relation to the fabrication method. The insulating layer  12  has the groove  13  formed therein. The groove  13  is formed in the insulating layer  12  so as to extend around the side edges of the first electrode  14  and the second electrode  15  as described later.  
         [0036]     The first electrode  14  for signal input (typically a signal input of a predetermined frequency, such as high-frequency signal input) is formed on the insulating layer  12  on one side of the groove  13 , and the second electrode  15  for signal output (typically a signal output of a predetermined frequency, such as high-frequency signal output) is formed on the insulating layer  12  on the other side of the groove  13  so as to be spaced from the first electrode  14 . The groove  13  herein is formed so as to get under side edges of the first electrode  14  and the second electrode  15 . It is also allowable that the groove  13  is formed so as to get under the side edges of the first electrode  14  and the side edges of the second electrode  15 . Further on the insulating layer  12 , the wiring portion  16  of the oscillator electrode is formed so as to oppose with the first electrode  14  while being spaced by the groove  13  opposite to the second electrode  15 , and the wiring portion  17  of the oscillator electrode is formed so as to oppose with the second electrode  15  while being spaced by the groove  13  opposite to the first electrode  14 .  
         [0037]     The oscillator electrode  19  which is electrically connected with the wiring portions  16 ,  17  is formed as extending over the first electrode  14  and the second electrode  15 , while being opposed thereto and spaced therefrom by the air gap  18 . The air gap  18  spacing the first electrode  14  and the second electrode  15  from the oscillator electrode  19  is formed to a thickness of 10 nm to 100 nm, for example.  
         [0038]     Because the groove  13  is formed in the insulating layer  12  so as to extend around the side edges of the first electrode  14  and the second electrode  15 , the micromachine  3  of the third embodiment is more successful in reducing the capacitance between the first electrode  14  and the second electrode  15  as compared with the configuration of the first embodiment, and consequently in obtaining a large S/N ratio even under high-frequency operation. Simulation study by the present inventor has revealed that the configuration of the first embodiment was successful in reducing the capacitance (DC) by as much as 39% as compared with a configuration having no groove  13  formed therein.  
         [0039]     Operations of the micromachine  3  of the third embodiment are similar to those of the micromachine  1  described in the first embodiment.  
         [0040]     Next, a modified example of the micromachine according to the third embodiment of the present invention will be explained referring to a plan view shown in  FIG. 4 .  
         [0041]     As shown in  FIG. 4 , it is also allowable to form the first groove  13  around a part of the side edges of the first electrode  14  and around a part of the side edges of the second electrode  15 . In this example, the groove  13  is formed in the insulating layer  12  between the first electrode  14  and the second electrode  15 , and so as to extend towards the direction of the edges of the first electrode  14  and the second electrode  15 .  
         [0042]     Also the micromachine  4  of the above-described modified example of the third embodiment is successful in further reducing the capacitance between the first electrode  14  and the second electrode  15  as compared with the configuration of the first embodiment, and consequently in obtaining a large S/N ratio even under high-frequency operation.  
         [0043]     Operations of the micromachine  4  of the modified example of the third embodiment are similar to those of the micromachine  1  described in the first embodiment.  
         [0044]     Next, a method of fabricating a micromachine according to the first embodiment of the present invention will be described referring to sectional views of  FIGS. 5A  to  5 H for explaining process steps for the fabrication.  
         [0045]     As shown in  FIG. 5A , the insulating layer  12  is formed on the substrate  11 , and the groove  13  is formed in the insulating layer  12 . The substrate  11  is a semiconductor substrate, for which a silicon substrate or compound semiconductor substrate, for example, can be used. The insulating layer  12  is formed typically using a silicon oxide film or a silicon nitride film, or a stacked film of these, where a silicon nitride film was used herein. The insulating layer  12  can typically be formed by the LPCVD (Low Pressure CVD) process. The groove  13  is formed by forming a resist film (not shown) on the insulating layer  12 , an opening for forming the groove  13  is formed in the resist film by a photolithographic technique, and the insulating layer  12  is etched under masking by the resist film. The resist film is removed thereafter.  
         [0046]     In an exemplary process in which the insulating layer  12  is formed using a stacked film, first the surface of the substrate (silicon substrate, for example)  11  is thermally oxidized to thereby form a silicon oxide film. The silicon oxide film herein is formed in order to relax stress possibly exerted on the substrate  11  by a silicon nitride film formed later. Next the silicon nitride film is formed by the LPCVD process.  
         [0047]     Next, as shown in  FIG. 5B , a first sacrificial layer  31  is formed so as to fill the groove  13 . The first sacrificial layer  31  is necessarily formed using a film having an etching selectivity against the insulating layer  12 , and is typically formed using a silicon oxide film for the insulating layer  12  composed of silicon nitride, and using a silicon nitride film for the insulating layer  12  composed of silicon oxide.  
         [0048]     In an exemplary case where the first sacrificial layer  31  is formed using a silicon oxide film, a hot-wall-type CVD apparatus can be used. Reaction gases typically used herein are silane (e.g., monosilane (SiH.sub.4)) gas and nitrogen monoxide (N.sub.2O) gas, where a flow rate thereof are typically adjusted to 50 cm.sup.3/min and 1000 cm.sup.3/min, respectively. Although a high-temperature oxide (HTO) film formed in the hot-wall-type CVD apparatus was used herein in consideration of the step coverage and denseness of the film, it is also allowable to use any other oxide films obtained by using TEOS gas, or any glass films composed of NSG, PSG, BSG, BPSG and so forth.  
         [0049]     An excessive portion of the first sacrificial layer  31  on the insulting layer  12  is then removed. The removal can typically be carried out by CMP or etchback. It is particularly preferable to planarize a surface of the insulating layer  12  (including the first sacrificial layer  31 ) by CMP. Conditions for CMP are typically adjusted to a head load of 30 kPa and a number of rotations of as slow as 23 rpm. Pre-treatment and post-treatment before and after CMP were carried out using a dilute hydrofluoric acid (DHF).  
         [0050]     Next, an electrode-making film (not shown) for making the electrodes is formed. The electrode-making film is formed using a polysilicon film containing some impurity for obtaining electro-conductivity. The impurity available herein is an n-type impurity such as phosphorus, arsenic, antimony or the like. Phosphorus was used herein, so as to produce phosphorus-doped polysilicon. Method of forming the phosphorus-doped polysilicon was the CVD process generally used for forming polysilicon using phosphine (PH.sub.3) as an impurity gas.  
         [0051]     It is preferable to carry out thermal oxidation and annealing thereafter to thereby allow the impurity phosphorus atoms reside in a crystal boundary of the phosphorus-doped polysilicon film to diffuse into the grains, to thereby activate the impurity and lower the resistivity. The thermal oxidation was carried out typically under an oxygen atmosphere at 1,000.degree. C. for 12 minutes, and the annealing was carried out typically under a nitrogen atmosphere at 1,000.degree. C. for 6 minutes.  
         [0052]     A resist film (not shown) was then formed on a electrode-making film; the resist film was patterned by a lithographic technique to thereby form geometry of the first electrode, the second electrode and the wiring portion of the oscillator electrode; and the electrode-making film was etched under masking by the resist film to thereby form, as shown in  FIG. 5C , the first electrode  14  which serves as an input electrode on one side of the groove  13  on the insulating layer  12 , the second electrode  15  which serves as an output electrode on the other side of the groove  13  on the insulating layer  12 , and the wiring portions  16 ,  17  of the oscillator electrode on the insulating layer  12  are formed as being opposed with the groove  13  while placing the first electrode  14  and the second electrode  15  in between, and while being spaced from the first electrode  14  and the second electrode  15 .  
         [0053]     Next, as shown in  FIG. 5D , a space individually between the first electrode  14 , the second electrode  15  and the wiring portions  16 ,  17  are filled with a second sacrificial layer  32 . The second sacrificial layer  32  used herein is a film similar to that used for the first sacrificial layer  31 . Method of forming the second sacrificial layer  32  may be similar to that for forming the first sacrificial layer  31 , or may differ therefrom. The excessive portion of the second sacrificial layer  32  on the individual electrodes is then removed so as to expose the individual surfaces of the first electrode  14 , the second electrode  15  and the wiring portions  16 ,  17 . The removal can be carried out by CMP or etchback. It is preferable to planarize the surface of the first sacrificial layer  31  (including the first electrode  14 , the second electrode  15  and wiring portions  16 ,  17 ).  
         [0054]     Next, as shown in  FIG. 5E , a mask  41  is formed on the second sacrificial layer  32  (including the first electrode  14 , the second electrode  15  and the wiring portions  16 ,  17 ). The mask  41  herein is formed using a silicon nitride film. It is to be noted that the mask  41  is formed using a silicon oxide film for a case where the first and second sacrificial layers  31 ,  32  are formed using a silicon nitride film. The silicon nitride film is then patterned based on a general lithographic technique using a resist film and based on an etching technique, to thereby form an opening  42  in which surfaces of the first electrode  14 , the second electrode  15  and parts of the individual wiring portions  16 ,  17  are exposed. The resist film is then removed.  
         [0055]     Next, as shown in  FIG. 5F , oxidation under masking with the mask  41  is carried out, to thereby form a third sacrificial layer  33  composed of a silicon oxide film on the surfaces of the first electrode  14 , the second electrode  15  and parts of the individual wiring portions  16 ,  17 . Method of forming the silicon oxide film was typically a thermal oxidation method, which was actually carried out under an oxygen atmosphere at 1,000.degree. C. for 12 minutes. For a case where the mask  41  is formed using a silicon oxide film, the first and second sacrificial layers  31 ,  32  are formed using a silicon nitride film, so that the third sacrificial layer  33  composed of silicon nitride is formed by nitrification on the surfaces of the first electrode  14 , the second electrode  15  and parts of the individual wiring portions  16 ,  17 . The third sacrificial layer  33  is formed typically to a thickness of 10 nm to 100 nm on the first electrode  14  and the second electrode  15 .  
         [0056]     Next, although not shown, a resist mask having openings on the wiring portions  16 ,  17  is formed by a general lithographic technique, and the third sacrificial layer  33  was selectively removed on the wiring portions  16 ,  17  by etching under masking by the resist mask. The resist mask is then removed.  
         [0057]     Next, an electrode-making film (not shown) for making the oscillator electrode is formed. The electrode-making film is carried out using a polysilicon film containing some impurity for obtaining electro-conductivity. The impurity available herein is an n-type impurity such as phosphorus, arsenic, antimony or the like. Phosphorus was used herein, so as to produce phosphorus-doped polysilicon. Formation of the electrode-making film can be formed similarly to the method of forming the electrode-making film for forming the first and second electrodes  14 ,  15  and so forth. A resist film (not shown) is then formed on the electrode-making film, the resist film is patterned by a lithographic technique to thereby form a geometry of the oscillator electrode, and the electrode-making film is etched (typically by dry etching) under masking with thus-patterned resist film to thereby form, as shown in  FIG. 5G , the oscillator electrode  19  electrically connected with the wiring portions  16 ,  17  as extending over the first electrode  14  and the second electrode  15  while placing the third sacrificial layer  33  thereunder.  
         [0058]     Next, the first, second and third sacrificial layers  31 ,  32 ,  33  are selectively removed. In a case where the first, second and third sacrificial layers  31 ,  32 ,  33  are formed using silicon oxide films, the removal can be carried out by wet etching using an etching solution capable of selectively removing silicon oxide without affecting polysilicon and silicon nitride. The etching solution used herein was a buffered hydrofluoric acid (BHF) (50:1 mixture of HF (1.0 wt %) and N-H.sub.4F (39.2 wt %)) solution. For a case where the first, second and third sacrificial layers  31 ,  32 ,  33  are formed using silicon nitride films, they are removed by wet etching using an etching solution capable of selectively removing silicon nitride without affecting polysilicon and silicon oxide (e.g., hot phosphoric acid solution). By these procedures, as shown in  FIG. 5H , the groove  13  is formed in the insulating layer  12  between the first electrode  14  and the second electrode  15 , and over the first electrode  14  and the second electrode  15 , the oscillator electrode  19  connected to the wiring portions  16 ,  17  is formed while being opposed therewith and being spaced therefrom by the air gap  18 .  
         [0059]     Because the groove  13  is formed in the insulating layer  12  between the first electrode  14  and the second electrode  15 , the method of fabricating a micromachine of the first embodiment is successful in reducing the capacitance between the first electrode  14  and the second electrode  15 , and consequently in obtaining a large S/N ratio even under high-frequency operation.  
         [0060]     Next, the method of fabricating a micromachine according to a second embodiment of the present invention will be explained referring to  FIGS. 2A and 2B .  
         [0061]     The second embodiment is a fabrication method characterized by having, after the removal of the first sacrificial layer  31 , the second sacrificial layer  32  and the third sacrificial layer  33  as described in the first embodiment explained referring to  FIGS. 5A  to  5 H, the insulating layer  12  is etched in the groove  13  at portions under the side edges of the first electrode  14  and the second electrode  15  so as to allow the groove  13  to get under the first electrode  14  and the second electrode  15 . The etching in this process step can be carried out by wet etching using a hot phosphoric acid solution as an etching solution for a case where the insulating layer  12  is composed of a silicon nitride film, and using a buffered hydrofluoric acid solution as an etching solution for a case where the insulating layer  12  is composed of a silicon oxide film. That is, an etching solution capable of selectively etching the insulating layer  12  without affecting the first and second electrodes  14 ,  15 , the oscillating electrode  19  and the wiring portions  16 ,  17  is used.  
         [0062]     Next, a method of fabricating a micromachine according to the third embodiment of the present invention will be described again referring to  FIGS. 3A and 3B .  
         [0063]     The third embodiment can be accomplished by forming the groove  13  so as to extend around the sides edges of the first electrode  14  and the second electrode  15 , in the process step of forming the groove  13  in the first embodiment as previously described referring to  FIGS. 5A  to  5 H. Other process steps are similar to those in the first embodiment.  
         [0064]     It is also allowable, as shown in  FIG. 4 , to form the groove in the insulating layer  12  around a part of the side edges of the first electrode  14  and around a part of the side edges of the second electrode  15 . In this example, the groove  13  is formed in the insulating layer  12  between the first electrode  14  and the second electrode  15 , and so as to extend towards the direction of the edges of the first electrode  14  and the second electrode  15 .  
         [0065]     Also the fabrication methods according to the second and third embodiments are successful in reducing capacitance between the first electrode  14  and the second electrode  15 , similarly to the fabrication method according to the first embodiment.