Patent Publication Number: US-2010112743-A1

Title: Method of manufacturing semiconductor device including vibrator which is provided with side insulating film and insulating separation region formed by thermal oxidation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application P2008-284626 filed on Nov. 5, 2008 and prior Japanese Patent Application P2008-321014 filed on Dec. 17, 2008; the entire contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor device including beam-shaped vibrators. 
     2. Description of the Related Art 
     Semiconductor devices such as Micro-Electro-Mechanical system (MEMS) devices including beam-shaped vibrators, surface acoustic wave (SAW) devices, film bulk acoustic resonators (FBAR) made of piezoelectric material are used in accelerometers, gyrosensors, and the like. For example, capacitive accelerometers detecting acceleration by sensing a change in electrostatic capacity between two vibrators and the like have been put into practical use. 
     In order to form beam-shaped vibrators each having a free end, generally, an SOI substrate including insulating film layers and silicon layers stacked on a silicon substrate is used. The insulating film layers are used as sacrificial layers, and the silicon layers are used as free ends. On the other hand, some manufacturing methods which do not use the sacrificial layers are proposed. In a manufacturing method not using the sacrificial layers, a trench for isolation is formed between the free end of each vibrator and the semiconductor substrate to which the vibrator is fixed, and then the trench for isolation is filled with an insulating material to form an insulating separation region between the free end of the vibrator and the semiconductor substrate. 
     However, in such a proposed manufacturing method not using the sacrificial layers, side insulating films are formed on the side surfaces of each vibrator by chemical vapor deposition (CVD). Accordingly, the side insulating films decrease in thickness toward the bottom of each vibrator and have non-uniform thickness. When the side insulating films of the vibrators have non-uniform thickness, the semiconductor device has an unstable feature in sensing changes in electrostatic capacity between the vibrators. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is a method of manufacturing a semiconductor device which includes a plurality of beam-shaped vibrators each having a free end extended in a recess formed in an upper surface of a semiconductor substrate and a fixed end fixed to the semiconductor substrate. The method includes partially etching the upper surface of the semiconductor substrate to form side grooves and expose side surfaces of the vibrators; partially etching the upper surface of the semiconductor substrate to form separation grooves where insulating separation regions between the vibrators and the semiconductor substrate are to be formed; thermally oxidizing surfaces of the separation grooves to form the insulating separation region composed of oxidized films filled in the separation grooves; thermally oxidizing the side surfaces of the vibrators to form side insulating film; and performing release etching of the semiconductor substrate using the side insulating film as a mask to expose bottom surfaces of the vibrators and form the vibrators arranged in the recess formed in the semiconductor substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a structure of a semiconductor device manufactured by a method of manufacturing a semiconductor device according to a first embodiment of the present invention. 
         FIGS. 2A to 2D  are cross-sectional views of the semiconductor device shown in  FIG. 1 . 
         FIGS. 3A to 12E  are process views for illustrating the method of manufacturing a semiconductor device according to the first embodiment of the present invention. 
         FIG. 13  is a process cross-sectional view for illustrating the method of manufacturing a semiconductor device according to a modification of the first embodiment of the present invention. 
         FIG. 14  is a schematic view showing a vibrator of the semiconductor device according to the modification of the first embodiment of the present invention. 
         FIG. 15  is a schematic view showing free ends of vibrators of the semiconductor device according to the modification of the first embodiment of the present invention. 
         FIG. 16  is a schematic view showing a structure of a semiconductor device manufactured by a method of manufacturing a semiconductor device according to a second embodiment of the present invention. 
         FIGS. 17A to 17C  are cross-sectional views of the semiconductor device shown in  FIG. 16 . 
         FIGS. 18A to 26B  are process views for illustrating the method of manufacturing a semiconductor device according to the second embodiment of the present invention. 
         FIG. 27  is a perspective view for illustrating a method of forming a recess of the semiconductor device according to the second embodiment of the present invention. 
         FIG. 28  is a process cross-sectional view illustrating a method of manufacturing a semiconductor device according to a modification of the second embodiment of the present invention. 
         FIG. 29  is a schematic view showing a vibrator of the semiconductor device according to the modification of the second embodiment of the present invention. 
         FIG. 30  is a schematic view showing a structure example of the semiconductor device according to the modification of the second embodiment of the present invention. 
         FIG. 31  is a schematic view showing a structure of a semiconductor device according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The first and second embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the drawings, the same or similar reference numerals are applied to the same or similar parts and elements. It is to be noted that the drawings are schematic and have different relationship between thickness and planer dimensions, proportions of thickness of layers, and the like from the real ones. Accordingly, specific thicknesses and dimensions should be determined with reference to the following description. Moreover, it is obvious that some parts have different dimensional relationships or proportions throughout the drawings. 
     The following embodiments just shows devices and methods to embody the technical idea of the present invention, and the technical idea of the present invention does not specify materials, shapes, structures, and arrangements of the constituent components and the like to the following description. The technical idea of the present invention can be variously modified in the scope of claims. 
     First Embodiment 
     A method of manufacturing a semiconductor device according to a first embodiment of the present invention is a method of manufacturing a semiconductor device  1  including beam vibrators  21  and  22  which have free ends within a recess  100  formed in the upper surface of a semiconductor substrate  10  and have fixed ends fixed to the semiconductor substrate  10 . Specifically, the manufacturing method includes: partially etching the upper surface of the semiconductor substrate  10  to form side grooves and expose side surfaces of the vibrators  21  and  22  and simultaneously forming separation grooves in which insulating separation regions  30  between the semiconductor substrate  10  and individual vibrators  22  are to be formed; thermally oxidizing surfaces of the separation grooves to form the insulating separation regions  30  composed of oxidized films filled in the separation grooves; thermally oxidizing the side surfaces of the vibrators  21  and  22  to form the side insulating films; and performing release etching for the semiconductor substrate  10  using the side insulating films as a mask to expose bottom surfaces of the vibrators  21  and to form the vibrators  21  and  22  arranged within the recess  100 , which is formed in the semiconductor substrate  10 . 
     The vibrators  21  and  22  are beam-shaped vibrators. The semiconductor device  1  is a capacitive accelerometer detecting acceleration based on fluctuations in electrostatic capacity which are dependent on distance between the vibrators  21  and  22 . The semiconductor substrate  10  can be a semiconductor substrate whose surface is oxidized by thermal oxidation, for example such as a silicon (Si) substrate. 
     The vibrator  21  is a fishbone-shaped beam vibrator including a central stripe section and a plurality of beam sections. The central stripe section has both ends fixed to the semiconductor substrate  10  and is arranged within the recess  100 . The beam sections which have fixed ends fixed to the central stripe portion and have free ends extended from the fixed ends in the recess  100 . Each of the vibrators  22  is a beam vibrator which has a fixed end fixed to the edge of the semiconductor substrate  10  surrounding the recess  100  and has a free end extending within the recess  100 . Between the fixed and free ends of each vibrator  22 , the insulating separation region  30  is formed. As shown in  FIG. 1 , the vibrators  22  which are provided with the insulating separation regions  30  between the free ends thereof and the semiconductor substrate  10  and the beams of the vibrator  21  whose free ends are electrically connected to the semiconductor substrate  10  are alternately arranged, and the free ends of the vibrator  21  are interdigitated with the free ends of the vibrators  22  within the recess  100 . 
       FIGS. 2A to 2C  show cross-sectional views along directions IIa-IIa, IIb-IIb, and IIc-IIc of  FIG. 1 , respectively.  FIG. 2D  shows a perspective view of a region D of  FIG. 1 . 
     As shown in  FIG. 2A , the side insulating films  40  are formed on the side surfaces of the vibrators  21  and  22 . On the upper surfaces of the vibrators  21  and  22 , upper oxidized films  51  are formed, and on the upper oxidized films  51 , PSG films  52  are formed. The bottom surfaces of the vibrators  21  and  22  are exposed in the recess  100 . 
     As shown in  FIG. 2B , the upper oxidized film  51  and PSG film  52  on each vibrator  22  are partially removed to form an opening  55 . A metallic electrode  62 , which is electrically connected to the vibrator  22  in the opening  55 , is formed on the PSG film  52 . In  FIG. 1 , the openings  55  are indicated by dashed lines. 
     As shown in  FIG. 2C , the upper oxidized film  51  and PSG film  52  on the semiconductor substrate  10  are partially removed to form another opening  55 . A metallic electrode  61 , which is in contact with the semiconductor substrate  10  in the opening  55 , is formed on the PSG film  52 . The metallic electrode  61  is electrically connected to the vibrator  21  through the semiconductor substrate  10 . 
     The vibrators  21  and  22  are beam vibrators having the free ends extended in the recess  100 , and the free ends of the vibrators  21  and  22  change positions thereof according to external cause including external impact. Since the free ends of the vibrator  21  are interdigitated with the free ends of the vibrators  22 , the changes in positions of the vibrator  21  and vibrators  22  change the electrostatic capacity between the vibrator  21  and vibrators  22 . 
     A description is given of an example of the operation of the semiconductor device  1  below. When an external force is applied to the semiconductor device  1 , the distances between the vibrator  21  and vibrators  22  vary because of the influence of the external force. When an external force is applied to the semiconductor device  1  with voltage being applied to between the vibrators  21  and  22 , the changes in distance between the vibrator  21  and vibrators  22  are sensed as a change in electrostatic capacity. The semiconductor device  1  transmits the sensed change in electrostatic capacity as a detection signal to a signal processing circuit (not shown). The signal processing circuit processes the detection signal to detect an acceleration produced in the semiconductor device  1 . In other words, the semiconductor device  1  is a part of an accelerometer detecting an acceleration based on changes in electrostatic capacity between the vibrators  21  and  22 . The signal processing circuit may be arranged in a same chip as the semiconductor device  1  or may be arranged in a chip different from the chip where the semiconductor device  1  is arranged. 
     The insulating separation regions  30 , which electrically separate the vibrators  22  and the semiconductor substrate  10 , are individually provided between the vibrators  22  and the semiconductor substrate  10 . The vibrator  21  and each vibrator  22  therefore serve as capacitor plates. Electrical signals from the vibrators  22  are outputted through the metallic electrodes  62  to the outside of the semiconductor device  1 . Electrical signals from the vibrator  21  are outputted through the metallic electrode  61 , which is in contact with the semiconductor substrate  10 , to the outside of the semiconductor device  1 . 
     In the semiconductor device  1  shown in  FIG. 1 , in order to obtain good sensitivity, connections between the vibrator  21  and semiconductor substrate  10  are composed of springs which allow the vibrator  21  to easily vibrate. The semiconductor device  1  is therefore configured so that the electrical signals from the vibrator  21  are transmitted to the metallic electrode  61  through the semiconductor substrate  10 . However, the vibrator  21  and metallic electrode  61  may be directly connected with the insulating separation regions  30  being provided between the semiconductor substrate  10  and the vibrator  21 . 
     With reference to  FIGS. 3A to 12E , a description is given of a method of manufacturing a semiconductor device according to the first embodiment of the present invention.  FIGS. 3A ,  4 A,  5 A,  6 A,  7 A,  8 A,  9 A,  10 A,  11 A, and  12 A are cross-sectional views taken in a same direction as that of  FIG. 2A ;  FIGS. 3B ,  4 B,  5 B,  6 B,  7 B,  8 B,  9 B,  10 B,  11 B, and  12 B are cross-sectional views taken in a same direction as that of  FIG. 2B ; and  FIGS. 3C ,  4 C,  5 C,  6 C,  7 C,  8 C,  9 C,  10 C,  11 C, and  12 C are cross-sectional views taken in a same direction as that of  FIG. 2C .  FIG. 2D  is a perspective view of a region same as the region D, and  FIGS. 3E to 12E  are top views of the same. The direction of the cross sections of  FIGS. 3A to 12C  and the positions of the region D of  FIGS. 3D ,  4 D,  5 D,  6 D,  7 D,  8 D,  9 D,  10 D,  11 D, and  12 D are shown in  FIGS. 3E ,  4 E,  5 E,  6 E,  7 E,  8 E,  9 E,  10 E,  11 E, and  12 E. The later-described method of manufacturing a semiconductor device is an example, and it is obvious that the present invention can be implemented by other various manufacturing methods including modifications thereof. 
     (a) As shown in  FIGS. 3A to 3E , the upper surface of the semiconductor substrate  10 , which is a silicon substrate, is thermally oxidized to form the upper oxidized film  51 . The upper oxidized film  51  has a thickness of about 500 nm, for example. As shown in  FIGS. 4A to 4E , on the upper oxidized film  51 , the PSG film  52  with a thickness of about 1 μm is formed by CVD or the like, thus forming an upper insulating film including the upper oxidized film  51  and PSG film  52 . 
     (b) The upper insulating film is partially removed by selective etching trough a photoresist mask using a photolithography technique to form the openings  55  as shown in  FIGS. 5A to 5E . 
     (c) In order to prevent crystal defects due to a nitride silicon (SiN) film  54  later described, the upper surface of the semiconductor substrate  10  exposed in each opening  55  is thermally oxidized to about  50  nm to form a sacrificial oxidized film  56  as shown in  FIGS. 6A to 6E . 
     (d) All over the semiconductor substrate  10 , the SiN film  54  with a thickness of about 150 to 350 nm is formed by low pressure CVD or the like. Subsequently, using a photolithography technique or the like, the SiN, PSG, and upper oxidized films  54 ,  52 , and  51  are patterned to partially expose the upper surface of the semiconductor substrate  10 . In other words, as shown in  FIGS. 7A to 7E , openings  410 , in which the upper surface of the semiconductor substrate  10  in a region where the recess  100  is to be formed is exposed, are formed. Moreover, openings  310 , in which the upper surface of the semiconductor substrate  10  in regions where the insulating separation regions  30  are to be formed are exposed, are formed. Width W 1  of the openings  410 , which is a distance between each beam section of the vibrator  21  and the vibrator  22  adjacent thereto, is 2 μm, for example. Width W 2  of each opening  310  in a direction perpendicular to the direction that the insulating separation region  30  extends is 0.5 μm, for example. 
     (e) As shown in  FIGS. 8A to 8E , upper part of the semiconductor substrate  10  is partially etched using the patterned SiN film  54  as a mask. Specifically, upper portions of the semiconductor substrate  10  exposed in the openings  410  are etched to form the side grooves  400  and expose the side surfaces of the vibrators  21  and  22 . Simultaneously with the formation of the side grooves  400 , upper portions of the semiconductor substrate  10  which are exposed in the openings  310  are etched to form the separation grooves  300 . Depths of the side grooves  410  and separation grooves  300  are about 25 μm, for example. The etching to form the side surface grooves  410  and separation grooves  300  can be Bosch process using deep reactive ion etching (D-RIE) or the like. 
     (f) As shown in  FIGS. 9A to 9E , the surface of each side groove  400  is thermally oxidized to form the side insulating films  40  on the side surfaces of the side groove  400  and form a bottom insulating film  42  on the bottom surface of the side surface groove  400 . The side and bottom insulating films  40  and  42  have thicknesses of about 1 to 2 μm, for example. The surface of each separation groove  300  is thermally oxidized simultaneously with the thermal oxidization of the surfaces of the side grooves  400  to fill the separation grooves  300  with thermal oxidized films, thus forming the insulting separation regions  30 . After the separation grooves  300  are filled with the oxidized films, the thermal oxidization process at the surfaces of the separation grooves  300  stops. 
     (g) As shown in  FIGS. 10A to 10E , the SiN film  54  is removed. Subsequently, the sacrificial oxidized films  56  exposed in the bottom surfaces of the openings  55  are removed using buffered hydrofluoric acid or hydrofluoric acid. 
     (h) As shown in  FIGS. 11A to 11E , the metallic electrodes  61  and  62  are formed into predetermined patterns using a lift-off process or the like. The metallic electrodes  61  and  62  can be made of aluminum (Al) films, copper (Cu) films, or the like. For example, a photoresist film for the lift-off is formed over the upper surface of the semiconductor substrate  10  and is then patterned using a photolithography technique or the like. Subsequently, an Al film with a thickness of about 1 to 3 μm is formed over the upper surface of the semiconductor substrate  10  by sputtering or vapor deposition. The Al film is then partially removed by liftoff using the photoresist film, thus forming the metallic electrodes  61  and  62  of the Al film into the predetermined patterns. Specifically, the metallic electrode  61 , which is electrically connected to the vibrator  21  through the semiconductor substrate  10  at the opening  55 , and the metallic electrodes  62 , which are electrically connected to the respective vibrators  22  at the openings  55 , are formed on the PSG films  52 . Changes in electrostatic capacity sensed between the vibrator  21  and vibrators  22  are outputted to the outside of the semiconductor device  1  through the metallic electrodes  61  and  62 . 
     (i) As shown in  FIGS. 12A to 12E , the bottom insulating films  42  exposed in the bottom surfaces of the side grooves  400  are removed by an RIE process. Because this RIE process is an anisotropic etching process, the side insulating films  40  are not etched and remain on the side surfaces of the side grooves  400 . 
     (j) Exposed upper portions of the upper surface of the semiconductor substrate  10  are removed by etching using the PSG films  52  and side insulating films  40  as a mask to expose bottom surfaces of the vibrators  21  and  22 . At the etching of the semiconductor substrate  10 , an isotropic etcher using xenon difluoride (XeF 2 ) or the like can be used. By release etching separating the semiconductor substrate  10  and the bottom surfaces of the vibrators  21  and  22 , the recess  100  is formed in the upper surface of the semiconductor substrate  10 . The vibrators  21  and  22  are thus arranged in the recess  100 . In such a manner, the semiconductor device  1  according to the first embodiment of the present invention is completed. 
     In the above description, as shown in  FIGS. 5A to 5E , the openings  55  are formed immediately after the step of forming the PSG films  52 . However, for example, the openings  55  may be formed after the step of forming the side insulating films  40  and insulating separation regions  30  shown in  FIGS. 9A to 9E . 
     The side surface grooves  400  and separation grooves  300  are not necessarily formed simultaneously. However, by simultaneously forming the side grooves  400  and separation grooves  300 , the side grooves  400  and separation grooves  300  can be positioned with higher accuracy. Moreover, the manufacturing process can be shortened. 
     As described above, the side insulating films  40  provided on the side surfaces of the vibrators  21  and  22  and the insulating separation regions  30  composed of the oxidized films filled in the separation grooves  300  are formed by thermal oxidation. Accordingly, the width w 2  of the separation grooves  300  is preferably not more than 1 μm. When the width w 2  is 1 μm, for example, the thickness of the side insulating films  40 , which are formed on the side surfaces of the semiconductor substrate  10  by thermal oxidation, is set to about 2 μm. When the width w 2  is 0.5 μm, the thickness of the side insulating films  40  may be about 1.5 μm. In the case of simultaneously forming the insulating separation regions  30  and side insulating films  40 , the side insulating films  40  are preferably formed to a thickness about 1.5 times the width w 2  in order to flatten the upper surfaces of the insulating separation regions  30 . Long time thermal oxidation roughens the upper surfaces of the insulating separation regions  30 . 
     By making the width w 2  minute as described above, the separation grooves  300  can be filled with the oxidized films by only the step of thermal oxidation. Moreover, by simultaneously forming the side grooves  400  and separation grooves  300  at the thermal oxidation step, the side insulating films  40  and insulating separation regions  30  can be positioned with higher accuracy, and the number of manufacturing steps can be reduced. 
     In the semiconductor device  1 , since the side surfaces of the vibrators  21  and  22  are covered with the side insulating films  40 , the semiconductor substrate  10  is horizontally etched at the position deeper than the side insulating films  40  to expose the bottom surfaces of the vibrators  21  and  22 . Accordingly, the thicknesses of the vibrator  21  and vibrators  22  are determined by the depth of the side grooves  400 . In other words, the thicknesses of the vibrator  21  and vibrators  22  depend on the performance of the D-RIE apparatus. By using a D-RIE apparatus with an aspect ratio of 60 to 100, for example, the thicknesses of the vibrators  21  and  22  can be set to 60 to 100 μm when the width w 2  is 1 μm. On the other hand, if the side insulating films  40  are formed by CVD, the depth to which the side insulating films  40  can be stably formed is 20 to 25 μm. 
     The bottom surfaces of the vibrators  21  and  22  are a little etched when the recess  100  is formed because the bottom surfaces of the vibrators  21  and  22  are not covered with insulating films. However, the bottom surfaces of the insulating separation regions  30  and vibrators  21  and  22  which are exposed in the recess  100  are substantially in a same plane. 
     As described above, in the method of manufacturing the semiconductor device  1  according to the first embodiment of the present invention, the formation of the side grooves  400  to expose the side surfaces of the vibrators  21  and  22  is simultaneously carried out with the formation of the separation grooves  300 . Accordingly, the 
     D-RIE apparatus is used at only one step, preventing an increase in the manufacturing process. Moreover, the side insulating films  40  on the side surfaces of the vibrators  21  and  22  and the insulating separation regions  30 , which are formed by filling the separation grooves  300 , are formed by thermal oxidation. Accordingly, unlike the method of forming the side insulating films  40  by CVD or the like, the side insulating films  40  can have uniform thickness and stable quality. According to the method of manufacturing a semiconductor device according to the first embodiment of the present invention, it is possible to provide a method of manufacturing a semiconductor device which can prevent an increase in the manufacturing process and can provide a semiconductor device with stable electrostatic capacity between the vibrators. 
     &lt;Modification&gt; 
       FIGS. 10A to 10E  show an example in which all of the SiN films  54  on the vibrators  21  and  22  are removed. In order to detect a vertical acceleration, however, the SiN films  54  on the vibrator  21  or vibrators  22  may be partially left. Herein, the vertical acceleration is acceleration in the thickness direction of the semiconductor substrate  10 .  FIG. 13  shows an example in which the SiN films  54  on the vibrators  22  are not removed.  FIG. 13  is a cross-sectional view of the same region as FIG.  10 B. 
     When the SiN films  54  on any of the vibrator  21  and vibrators  22  are left, because of the difference in stress therebetween, warpage of the vibrator  21  is different from warpage of the vibrators  22 . When the SiN films  54  on the vibrators  22  are left as shown in  FIG. 14 , the upper surfaces of the free ends of the vibrators  22  are raised higher than the upper surfaces of the free ends of the vibrator  21  as shown in  FIG. 15 . The vertical acceleration can be thus detected. 
     Second Embodiment 
     A method of manufacturing a semiconductor device according to a second embodiment is a method of manufacturing the semiconductor device  1  including beam-shaped vibrators  21  and  22  having fixed ends which are fixed to the semiconductor substrate  10  as shown in  FIG. 16 . Specifically, the manufacturing method includes: in the upper surface of the semiconductor substrate  10 , forming side grooves in which side insulating films  40  of the vibrators  21  and  22  are to be formed and forming separation grooves in which insulating separation regions  30  between the semiconductor substrate  10  and individual vibrators  22  are to be formed; thermally oxidizing surfaces of the side grooves and separation grooves to simultaneously form the side insulating films  40  composed of oxidized films filled in the side grooves and the insulating separation regions composed of oxidized films filled in the separation grooves; and etching the semiconductor substrate  10  using the side insulating films  40  and upper insulating films on the semiconductor substrate  10  as a mask to form the vibrators  21  and  22  arranged in the recess  100  formed in the surface of the semiconductor substrate  10 . 
     The vibrators  21  and  22  are beam-shaped vibrators. The semiconductor device  1  is a capacitive accelerometer detecting acceleration using fluctuations in electrostatic capacity which depends on distances between the vibrator  21  and the vibrators  22 . 
     The vibrator  21  is a fishbone-shaped beam vibrator including a central stripe section and a plurality of beam sections. The central stripe section has both ends fixed to the semiconductor substrate  10 . The beam sections have fixed ends fixed to the central stripe section and have free ends extended in the recess  100 . Each of the vibrators  22  is a beam vibrator having a fixed end which is fixed to the semiconductor substrate  10  and having a free end extending in the recess  100 . As shown in  FIG. 16 , the free ends of the vibrator  21  are interdigitated with the free ends of the vibrators  22 . 
       FIGS. 17A and 17B  show cross-sectional views along directions of IXVIIa-XVIIa and XVIIb-XVIIb of  FIG. 16 , respectively.  FIG. 17C  shows a perspective view of a region A of  FIG. 16 . 
     As shown in  FIG. 17A , side insulating films  40  are formed on the side surfaces of the vibrators  21  and  22 . On the upper surfaces of the vibrators  21  and  22 , upper oxidized films  50  are formed. Each of the upper insulating films  50  is composed of a single layer in  FIG. 17  but may be composed of a plurality of layers. For example, as shown in the first embodiment, each upper insulating film  50  may be a film stack of an upper oxidized film  51  and a PSG film  52 . The bottom surfaces of the vibrators  21  and  22  are exposed in the recess  100 . Herein, the upper insulating films  50  are not shown in  FIG. 16 . 
     As shown in  FIG. 17B , a part of the upper insulating film  50  on each vibrator  22  is removed to form an opening  55 . A metallic electrode  60 , which is electrically connected to the vibrator  22  in the opening  55 , is formed on the upper insulating film  50 . In  FIG. 16 , the openings  55  and metallic electrodes  60  are not shown. 
     The vibrators  21  and  22  are beam-shaped vibrators having the free ends extended in the recess  100 , and the free ends of the vibrators  21  and  22  change positions thereof according to external cause including external impact. Since the free ends of the vibrator  21  are interdigitated with the free ends of the vibrators  22 , changes in positions of the vibrators  21  and  22  change the electrostatic capacity between the vibrator  21  and vibrators  22 . The semiconductor device  1  detects acceleration based on the change in electrostatic capacity between the vibrator  21  and vibrators  22 . 
     The insulating separation regions  30 , which electrically separate the vibrators  22  and the semiconductor substrate  10 , are individually provided between the vibrators  22  and the semiconductor substrate  10 . The vibrator  21  and vibrators  22  thus serve as capacitor plates. Electrical signals from the vibrators  22  are outputted to the outside of the semiconductor device  1  through the metallic electrodes  60 . Electrical signals from the vibrator  21  are outputted to the outside of the semiconductor device  1  through the semiconductor substrate  10 . 
     With reference to  FIGS. 18A to 27 , a description is given of a method of manufacturing a semiconductor device according to the second embodiment of the present invention.  FIGS. 18A ,  19 A,  20 A,  21 A,  22 A,  23 A,  24 A,  25 A, and  26 A are cross-sectional views taken along a same direction as that of  FIG. 17A ;  FIGS. 18B ,  19 B,  20 B,  21 B,  22 B,  23 B,  24 B,  25 B, and  26 B are cross-sectional views taken along a same direction as that of  FIG. 17B ; and  FIGS. 19C ,  20 C,  21 C,  22 C, and  23 C are perspective views of a region same as the region A of  FIG. 16 . The below-described method of manufacturing a semiconductor device is an example, and it is obvious that the present invention can be implemented by other various manufacturing methods including modifications thereof. 
     (a) As shown in  FIGS. 18A and 18B , the upper surface of the semiconductor substrate  10 , which is a silicon substrate, is thermally oxidized to form an oxidized silicon film  101 . 
     (b) The oxidized silicon film  101  is partially removed by etching through a photoresist mask using a photolithography technique or the like to pattern the oxidized silicon film  101 . Specifically, portions of the oxidized silicon film  101  on regions of the side grooves  400  where the side insulating films  40  are to be formed and on regions of the separation grooves  300  where the insulating separation regions  30  are to be formed are removed. 
     Subsequently, the upper surface of the semiconductor substrate  10  is etched using the patterned oxidized silicon film  101  as a mask to form the side grooves  400  and separation grooves  300  as shown in  FIGS. 19A to 19C . Groove width W of the side grooves  400  and separation grooves  300  is about 0.5 to 1 μm, for example. Depth t of the side grooves  400  and separation grooves  300  is about 30 μm, for example. At the etching to form the side grooves  400  and separation grooves  300 , D-RIE or the like can be employed. Thereafter, the oxidized silicon film  101  is removed as shown in  FIGS. 20A and 20B . 
     (c) The surfaces of the side grooves  400  and separation grooves  300  are thermally oxidized to simultaneously form the side insulating films  40  composed of oxidized films filled in the side grooves  400  and the insulating separation films  30  composed of oxidized films filled in the separation grooves  300 . At this time, the upper surface of the semiconductor substrate  10  is thermally oxidized to form the upper insulating film  50  as shown in  FIGS. 21A and 21B . The upper insulating film  50  has a thickness d 0  of about 2 μm, for example. After the side grooves  400  and separation grooves  300  are filled with the oxidized films, the thermal oxidation process at the surfaces of the side grooves  400  and separation grooves  300  stops. 
     (d) As shown in  FIGS. 22A and 22B , the upper insulating films  50  on the vibrators  21  and  22  are etched back to a thickness d 1  of about 0.5 μm. For example, the surfaces of the upper insulating films  50  on the vibrators and  22  are etched back using a photolithography technique through a photoresist mask or the like. 
     (e) As shown in  FIGS. 23A to 23C , the upper insulating films  50  on regions of the semiconductor substrate  10  which are to be etched to form openings  53  for forming recesses. Simultaneously, an opening  55  for contact through which an electrical signal from each vibrator  22  is outputted is formed. The openings  53  and  55  can be formed using a photolithography technique through a photoresist mask or the like. 
     (f) As shown in  FIGS. 24A and 24B , a conductor layer  600  is formed on the entire upper surface of the semiconductor substrate  10  so as to fill the openings  55 . The conductor layer  600  can be composed of an aluminum (Al) or copper (Cu) film or the like. For example, an Al film with a thickness of about 1 to 3 μm is formed by sputtering. Thereafter, if necessary, the surface of the conductor layer  600  is flattened by chemical-mechanical polishing (CMP) or the like. For example, as shown in  FIGS. 25A and 25B , the surface of the conductor layer  600  is etched to expose the upper insulating films  50  for flattening. 
     (g) The conductor layer  600  is patterned using a photolithography technique or the like to form the metallic electrodes  60 . Specifically, as shown in  FIGS. 26A and 26B , the metallic electrode  60  electrically connected to each vibrator  22  in the opening  55  is formed on the upper insulating film  50 . Changes in electrostatic capacity sensed between the vibrator  21  and vibrators  22  are outputted through the metallic electrodes  60  to the outside of the vibrators  22 . Thereafter, the rear surface of the semiconductor substrate  10  is polished so that the semiconductor substrate  10  has a desired thickness if necessary. 
     (h) The surface of the semiconductor substrate  10  is etched using the side insulating films  40  and upper insulating films  50  as a mask to form the side surface portions of the vibrators  21  and  22  as shown in  FIG. 27 . In the case of etching the semiconductor substrate  10  by isotropic etching, etching to form the side surfaces of the vibrators  21  and  22  is performed, and then etching to form the bottom surfaces of the vibrators  21  and  22  is continuously performed. By such release etching of the semiconductor substrate  10 , the recess  100  is formed in the surface of the semiconductor substrate  10 , and the vibrators  21  and  22  are arranged in the recess  100 . At the release etching, an isotropic etcher using xenon difluoride (XeF 2 ) or the like can be used. In such a manner, the semiconductor device  1  according to the second embodiment of the present invention is completed. 
     As previously described, in the method of manufacturing the semiconductor device  1  according to the first embodiment, the side grooves  400  are formed in the region where the recess  100  is to be formed, or the region where spaces between the vibrators  21  and  22  are to be formed. On the other hand, in the method of manufacturing the semiconductor device  1  according to the second embodiment, the side grooves  400  are formed only in the region where the side insulating films  40  are to be formed. After the side insulating films  40  are formed, the portions of the semiconductor substrate  10  between the vibrators  21  and  22  are etched. 
     In the method of manufacturing the semiconductor device  1  according to the second embodiment, the side insulating films  40  composed of the oxidized films filled in the side grooves  400  and the insulating separation regions  30  composed of the oxide films filled in the separation grooves  300  are formed by thermal oxidation. Accordingly, groove width W of the side grooves  400  and separation grooves  300  is preferably not less than 1 μm. When the groove width W is 1 μm, the thickness d 0  of the upper insulating films  50  formed on the semiconductor substrate  10  by thermal oxidation is set to about 2 μm. When the groove width W is 0.5 μm, the thickness d 0  of the upper insulating films  50  is set to about 1.5 μm. 
     By making the groove width W minute as described above, the side grooves  400  and separation grooves  300  can be filled with the oxidized films by only the step of thermal oxidation. In this case, the surfaces of the upper insulating films  50  formed on the semiconductor substrate are flat, and there is no roughness above the side grooves  400  and separation grooves  300 . Accordingly, it is not necessary to perform a flattening step, thus shortening the manufacturing process of the semiconductor device  1 . 
     The side grooves  400  and separation grooves  300  are not necessarily formed simultaneously. However, by simultaneously forming the side grooves  400  and separation grooves  300 , the side insulating films  40  and insulating separation regions  30  can be positioned with higher accuracy. Moreover, the manufacturing process can be shortened. 
     The bottom surface of the vibrators  21  and  22  are not covered with insulating films and can be a little etched when the recess  100  is formed. However, the bottom surfaces of the insulating separation regions  30  and the vibrators and  22  which are exposed in the recess  100  are substantially in a same plane. 
     Similar to the semiconductor device  1  according to the first embodiment, in the semiconductor device  1  according to the second embodiment, the side surface portions of the vibrators  21  and  22  which are separated from the semiconductor substrate  10  by release etching are covered with the side insulating films  40 , and the thicknesses of the vibrators  21  and  22  are determined by the depth t of the side grooves  400 . Accordingly, the thicknesses of the vibrators  21  and  22  depend on the performance of the D-RIE apparatus. 
     The recess  100  is formed by isotropic etching instead of the D-RIE process. This can reduce the manufacturing cost of the semiconductor device  1  and increase the throughput thereof. 
     As described above, in the method of manufacturing the semiconductor device  1  according to the second embodiment of the present invention, the side insulating films  40  and insulating separation regions  30  are simultaneously formed by performing thermal oxidation to fill the side grooves  400  and separation grooves  300  with the oxidized films. Moreover, the recess  100  is formed by only isotropic etching. It is therefore possible to provide a method of manufacturing a semiconductor device  1  which can form the insulating separation regions  30  while preventing an increase in the manufacturing process. The other effects thereof are substantially the same as those of the first embodiment, and the redundant description thereof is omitted. 
     &lt;Modification&gt; 
       FIGS. 22A and 22B  show an example in which the upper insulating films  50  on the vibrators  21  and  22  are etched back to an equal thickness. However, in the case of detecting vertical acceleration, the semiconductor device  1  is manufactured with each of the upper insulating films  50  on the vibrators  21  or  22  partially etched back. Herein the vertical acceleration is acceleration in the thickness direction of the semiconductor substrate  10 .  FIG. 28  shows an example in which each of the upper insulating films  50 ( 50 A) on the vibrators  22  is partially etched back. 
     When there is a difference in thickness between the upper insulating films  50  on the vibrator  21  and those on the vibrators  22 , because of a difference in stress therebetween, warpage of the beams of the vibrator  21  is different from warpage of the vibrators  22 .  FIG. 29  shows an example in which the vibrators  22  are formed without etching back the upper insulating films  50 . When the upper insulating films  50  on the vibrators  22  are thicker than that on the vibrator  21  like this, as shown in  FIG. 30 , the upper surfaces of the free ends of the vibrators  22  are higher than the upper surfaces of the free ends of the vibrator  21 . The vertical acceleration can be therefore detected. 
     Other Embodiments 
     It should not be understood that the description and the drawings, which form a part of the disclosure of the above-described first and second embodiments, limit this invention. From this disclosure, a variety of alternative embodiments, examples and operation technologies will be obvious for those skilled in the art. 
     For example, as shown in  FIG. 31 , two pairs of the vibrators may be used to configure an accelerometer detecting accelerations in the X and Y directions. In the example shown in  FIG. 31 , the semiconductor device  1  serves as an accelerometer detecting the acceleration in the X direction while a semiconductor device  1 A serves as an accelerometer detecting the acceleration in the Y direction. As shown in  FIG. 31 , the accelerometer detecting acceleration in the X direction has the free ends of the vibrators extending in the direction perpendicular to the direction where the free ends of the vibrators of the accelerometer detecting acceleration in the Y direction extends. Alternatively, a pair of vibrators detecting vertical acceleration (in the Z direction) may be added to the accelerometer in  FIG. 31  to configure an accelerometer detecting three dimensional acceleration. 
     Moreover, although the semiconductor device  1  is an accelerometer in the aforementioned examples, the semiconductor device according to the embodiment of the present invention may be a SAW device or GBAR, for example, other than MEMS devices. The present invention is applicable to method of manufacturing semiconductor devices having vibrators other than the accelerometers, for example, such as gyrosensors. 
     As described above, it is obvious that the present invention includes various embodiments and the like not described above. Accordingly, the technical scope of the present invention is determined by only the invention elements according to claims appropriate from the viewpoint of the above explanation.