Abstract:
A method for manufacturing a MEMS device, includes: preparing a substrate provided with a first substrate in which a cavity is formed, and a second substrate that is bonded to a side of the first substrate on which the cavity is formed and includes a slit to delimit a movable portion in a position corresponding to the cavity, the second substrate, including a first surface thereof facing the first substrate, being provided with a thermally-oxidized film selectively formed on the first surface in a position corresponding to the movable portion; forming a first electrode layer on a second surface opposite to the first surface on which the thermally-oxidized film for the movable portion is formed; forming a sacrifice layer on the first electrode layer and the second substrate; forming a second electrode layer on the sacrifice layer; and removing the sacrifice layer and the thermally-oxidized film after the second electrode layer is formed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-056565, filed on Mar. 12, 2010, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are directed to a manufacturing method of a MEMS device, and a substrate used therefor. 
       BACKGROUND 
       [0003]    In recent years, devices having a micro structure and produced by a micro-machining technology, which is sometimes called “MEMS (Micro Electro Mechanical Systems) technology”, have been put into applications in a variety of fields. 
         [0004]    The MEMS devices include such types as a MEMS switch, a MEMS capacitor, a MEMS sensor, and so on for a high-frequency circuit. For example, the MEMS switch has an advantageous feature, as compared with a conventional semiconductor switch, such as a small loss, high insulating properties, and good distortion properties. 
         [0005]    As a conventional technology, Japanese Laid-open Patent Publication No. 2005-293918 proposes a MEMS switch in which a movable portion is formed on a substrate, and a contact provided to the movable portion makes contact with a contact electrode provided in a fixed manner relative to the substrate. 
         [0006]    In a MEMS device, the movable portion is fabricated by using, for example, an ordinary SOI wafer and applying a D-RIE process only to the active layer (device layer) thereof. Alternatively, the movable portion is sometimes fabricated by laminating Poly-Si, Poly-SiGe, or the like on the wafer as a device layer, and applying an etching process or removing a sacrifice layer. Depending on the MEMS device, there is also a method to fabricate the movable portion by bonding a layer to a base wafer, and applying a D-RIE process. Among these processes, the process of removing the sacrifice layer to make a structure laminated on lower and upper layers of the sacrifice layer movable is called a surface MEMS process. 
         [0007]      FIG. 13  is a plan view illustrating an example of a MEMS switch  80   j,  and  FIG. 14  is a cross sectional view of the MEMS switch  80   j  illustrated in  FIG. 13  taken along a line J-J. 
         [0008]    Referring to  FIGS. 13 and 14 , the MEMS switch  80   j  includes a substrate  81 , a lower contact electrode  82 , an upper contact electrode  83 , a lower driving electrode  84 , an upper driving electrode  85 , and so on, all of which are formed on the substrate  81 . The lower contact electrode  82  and the lower driving electrode  84  are integrally provided to a movable portion KBj that constitutes a cantilever. 
         [0009]    An SOI substrate is used as the substrate  81 . The movable portion KBj is formed by cutting off the active layer of the SOI substrate by a slit SL. The lower contact electrode  82  and the lower driving electrode  84  are formed on the active layer by plating. 
         [0010]    When a driving voltage is applied between the upper driving electrode  85  and the lower driving electrode  84 , an electrostatic attractive force is generated therebetween, with which the lower driving electrode  84  is attracted toward and moved to the upper driving electrode  85 . In this way, the movable portion KBj and the lower contact electrode  82  that are integrated with the lower driving electrode  84  move, and the lower contact electrode  82  touches the upper contact electrode  83  so that the contacts close. At this time, if the driving voltage is set at zero, the contacts return to the positions separated from each other due to the elasticity of the movable portion KBj. 
         [0011]    The MEMS switch  80   j  described above has a structure in which a cavity is present below the lower surface of the movable portion KBj, and only one end of the movable portion KBj is connected to and supported by the substrate  81 . The movable portion KBj is capable of bending upward and downward with the supported portion serving as a fulcrum point. 
         [0012]    During a process of manufacturing the MEMS switch  80   j,  when an electrode having a coefficient of thermal expansion larger than that of the base material is laminated on the upper surface of the movable portion KBj, and when the temperature goes down to a room temperature, a stress is generated to cause the movable portion KBj to warp upwardly. When a sacrifice layer such as SiO 2  is further laminated thereon, the laminated sacrifice layer generates a stress which causes the movable portion KBj to warp downwardly. Although the warpage of the movable portion KBj caused by the electrode is small, for example, about 0.3 μm, the downward warpage of the movable portion KBj caused by the sacrifice layer sometimes becomes, for example, about 1 μm of which the influence is great. 
         [0013]    In other words, during a process of manufacturing the MEMS switch  80   j,  a half etching of the sacrifice layer is performed to form the contact of the upper contact electrode  83 . However, if the movable portion KBj largely warps, the adjustment or the control of the etching depth can not be accurately performed. For this reason, the accuracy of the interelectrode gap between the contact of the upper contact electrode  83  and the lower contact electrode  82  after the sacrifice layer is removed is worsened. Accordingly, desired switching properties may not be obtained. 
         [0014]    In addition, if large downward warpage of the movable portion KBj is caused, there are sometimes cases where the upper surface portion of the slit SL may not be completely filled with the sacrifice layer. In such a case, the resist or polymer may infiltrate into a gap of the slit SL during a post-process, which makes it difficult to remove such a substance by cleaning, and reduces yields. 
       SUMMARY 
       [0015]    According to an aspect of the invention (embodiment), a method for manufacturing a MEMS device, includes: preparing a substrate provided with a first substrate in which a cavity is formed, and a second substrate that is bonded to a side of the first substrate on which the cavity is formed and includes a slit to delimit a movable portion in a position corresponding to the cavity, the second substrate, including a first surface thereof facing the first substrate, being provided with a thermally-oxidized film selectively formed on the first surface in a position corresponding to the movable portion; forming a first electrode layer on a second surface opposite to the first surface on which the thermally-oxidized film for the movable portion is formed; forming a sacrifice layer on the first electrode layer and the second substrate; forming a second electrode layer on the sacrifice layer; and removing the sacrifice layer and the thermally-oxidized film after the second electrode layer is formed. 
         [0016]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0017]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a plan view of a MEMS switch according to the present embodiment; 
           [0019]      FIGS. 2A and 2B  are cross sectional views of the MEMS switch illustrated in  FIG. 1 ; 
           [0020]      FIGS. 3A ,  3 B, and  3 C are diagrams illustrating a manufacturing process of the MEMS switch according to the present embodiment; 
           [0021]      FIGS. 4A ,  4 B, and  4 C are diagrams illustrating the manufacturing process of the MEMS switch according to the present embodiment; 
           [0022]      FIGS. 5A and 5B  are diagrams illustrating the manufacturing process of an SOI substrate; 
           [0023]      FIGS. 6A ,  6 B, and  6 C are diagrams illustrating a manufacturing process of the SOI substrate; 
           [0024]      FIGS. 7A and 7B  are diagrams illustrating the manufacturing process of the SOI substrate; 
           [0025]      FIGS. 8A ,  8 B, and  8 C are diagrams illustrating the manufacturing process of the SOI substrate; 
           [0026]      FIGS. 9A and 9B  are diagrams illustrating the manufacturing process of the SOI substrate; 
           [0027]      FIGS. 10A and 10B  are diagrams illustrating the manufacturing process of the SOI substrate; 
           [0028]      FIGS. 11A ,  11 B,  11 C, and  11 D are diagrams illustrating comparative examples of manufacturing processes of the MEMS switch; 
           [0029]      FIG. 12  is a diagram depicting an outline of the manufacturing method of the MEMS switch; 
           [0030]      FIG. 13  is a plan view illustrating an example of a MEMS switch; and 
           [0031]      FIG. 14  is a cross sectional view illustrating the MEMS switch illustrated in  FIG. 13  taken along a line J-J. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     MEMS Switch   
       [0032]    In this embodiment, a MEMS switch  1  is taken as an example of a MEMS device, and a description will be given thereof. Various structures may be employed as a MEMS switch other than those in the examples described hereinafter. Manufacturing methods described later can also be applied to various types of MEMS devices such as a MEMS capacitor other than a MEMS switch. 
         [0033]      FIG. 1  is a plan view of a MEMS switch  1  according to one embodiment.  FIG. 2A  is a cross sectional view taken along a line A-A in  FIG. 1 .  FIG. 2B  is a cross sectional view of the MEMS switch  1  illustrated in  FIG. 1  including a portion taken along a step-like line and partially taken along in a revolving manner. To be specific,  FIG. 2B  is a revolved sectional view, including a portion i) taken along a line starting from “A” indicated in the left side of  FIG. 1  and ending at a point at which a line A-A intersects with a line X-X, a portion ii) taken along a line staring from the point at which the line A-A intersects with the line X-X and ending at a point at which the line X-X intersects with a line C-C, and a portion iii) starting from the point at which the line X-X intersects with the line C-C and ending at a point where “C” is indicated in the right side of  FIG. 1 . However, the illustration of the portion ii) is partially omitted. It should be noted that  FIGS. 3A-3C ,  4 A- 4 C, and  11 A- 11 D of which descriptions will be given later are also illustrated in a manner similar to  FIG. 2B . 
         [0034]    Referring to  FIGS. 1 ,  2 A, and  2 B, the MEMS switch  1  includes an SOI substrate  11 , a movable contact electrode  12 , a fixed contact electrode  13 , a movable driving electrode  14 , a fixed driving electrode  15 , a wall portion  17 , a support portion  18 , and so on. 
         [0035]    The SOI substrate  11  is a three-layer SOI (Silicon On Insulator) substrate formed of a support substrate (handle layer)  11   a,  a BOX layer (intermediate oxide film layer)  11   b,  and an active layer (device layer)  11   c.  The support substrate  11   a  is made of silicon having a thickness of about 500 μm. The BOX layer  11   b  is an insulating layer made of SiO 2  having a thickness of about 4 μm. The active layer  11   c  is a silicon thin film having a thickness of about 15 μm. 
         [0036]    The active layer  11   c  is provided with a slit  16  having a horizontal U-shape in a front view (plan view). This means that the movable portion KB is delimited by the slit  16 . The support substrate  11   a  is provided with a cavity (space)  21  corresponding to a region including the movable portion KB. 
         [0037]    In other words, the cavity  21  is provided in a manner to extend to an inner surface of the active layer  11   c  (lower side of the active layer  11   c  in the illustration) in the support substrate  11   a.  Here, during the manufacturing process of the MEMS switch  1 , although an oxide film layer having been subjected to patterning is formed on a surface of the active layer  11   c  in the cavity  21 , the oxide film layer will be removed later. 
         [0038]    In addition, a layer similar to the BOX layer  11   b  may be formed continuously from the BOX layer on a surface (surrounding surface) other than that of the active layer  11   c  in the cavity  21 . The manufacturing process of the MEMS switch  1  will be described in detail later. 
         [0039]    The movable portion KB constitutes a cantilever with a portion in which the slit  16  is not provided serving as a fulcrum, warps with the fulcrum or the vicinity thereof serving as a center of warpage, and an end portion opposite to the fulcrum can move in upper and lower directions in  FIGS. 2A and 2B . Electrode portions  12   a  and  14   a,  which will be described later, are formed in intimate contact with the surface of the movable portion KB. 
         [0040]    The movable contact electrode  12  includes the electrode portion  12   a  that is thin and elongated and formed in intimate contact with the movable portion KB, and the anchor portion  12   b  formed on one end portion of the electrode portion  12   a.    
         [0041]    The fixed contact electrode  13  includes an electrode base portion  13   a  formed in intimate contact with the active layer  11   c,  and a fixed contact portion  13   b  provided continuously from the electrode base portion  13   a  in a manner to oppose thereto above the electrode portion  12   a.  The fixed contact portion  13   b  is provided with a contact portion ST. 
         [0042]    An openable and closable contact is formed between the electrode portion  12   a  and the contact portion ST of the fixed contact portion  13   b.  The contact closes when the movable portion KB warps upward to thereby cause the electrode portion  12   a  to make contact with the fixed contact portion  13   b.  A signal line SL is formed of the movable contact electrode  12  and the fixed contact electrode  13 . When the contact closes, the signal line SL passes a high-frequency signal therethrough. 
         [0043]    The movable driving electrode  14  includes an electrode portion  14   a  formed of an elongated portion formed in intimate contact with the movable portion KB and a rectangular portion formed continuously from a front end portion of the elongated portion, and an anchor portion  14   b  formed on one end portion of the electrode portion  14   a.    
         [0044]    The fixed driving electrode  15  is formed of electrode base portions  15   a  and  15   c  that are formed in intimate contact with the active layer  11   c,  and an electrode opposing portion  15   b  that is supported by the electrode base portions  15   a  and  15   c  and forms a bridge straddling the movable portion KB thereabove. The electrode opposing portion  15   b  faces the rectangular portion of the electrode portion  14   a  thereabove. 
         [0045]    The wall portion  17  is provided, on the SOI substrate  11 , in a rectangular frame shape so as to surround the movable contact electrode  12 , the fixed contact electrode  13 , the movable driving electrode  14 , the fixed driving electrode  15 , and so on. The height of the wall portion  17  is the same as or higher than the other electrodes. 
         [0046]    A metallic material, for example, gold is used as a material for the movable contact electrode  12 , the fixed contact electrode  13 , the movable driving electrode  14 , the fixed driving electrode  15 , and the wall portion  17 . 
         [0047]    Sometimes, a membrane material  20  is bonded onto the wall portion  17  to seal space including functional portion KN such as the movable contact electrode  12 , the fixed contact electrode  13 , the movable driving electrode  14 , the fixed driving electrode  15 , and the like, that is, the space surrounded by the wall portion  17 , against outside. 
       Manufacturing Method of MEMS Switch   
       [0048]    Next, a description will be given of a manufacturing method of the MEMS switch  1 . 
         [0049]    As illustrated in  FIG. 3A , the SOI substrate  11  is prepared. As described before, the SOI substrate  11  includes the support substrate  11   a,  the BOX layer  11   b,  and the active layer  11   c.  According to the SOI substrate  11  used in this embodiment, the cavity  21  is further provided to the support substrate  11   a,  and an oxide film layer  22  is formed on a surface in the cavity  21  on the side of the active layer  11   c.    
         [0050]    The cavity  21  and the oxide film layer  22  are formed during the course of the production of the SOI substrate  11 . Referring to  FIG. 6A , the cavity  21 , in plan view, has a shape including a region corresponding to the movable portion KB of the MEMS switch  1  and a region correspond to the slit  16 . The depth of the cavity  21  is, for example, about a few μm to a few dozens μm. 
         [0051]    Referring to  FIG. 5B , the oxide film layer  22 , in plan view, has the same shape as that of the movable portion KB of the MEMS switch  1 . Referring to  FIG. 7A , alternatively, the shape of the oxide film layer  22  in plan view may be arranged identical to the shape of the lower electrode layer formed on a side of an upper surface of the movable portion KB, that is a combination of a shape of the electrode portion  12   a  and a shape of the electrode portion  14   a.  Yet alternatively, the shape of the oxide film layer  22  in plan view may be arranged as a shape corresponding to the above-mentioned shape but not identical. The oxide film layer  22  is, for example, a thermally-oxidized film made of, for example, SiO 2  and having a thickness of about 0.1 μm to a few μm, e.g., about 0.1 μm to 2 μm. 
         [0052]    Concave portions  11   d  for positioning are provided on the lower side of the outer surface of the support substrate  11   a.    
         [0053]    Next, a metallic layer serving as the lower electrode layer is formed by performing sputtering or the like using a metallic material on the surface of the active layer  11   c  of the SOI substrate  11 . Then, as illustrated in  FIG. 3B , patterning is performed on the metallic layer thus formed through a process of RIE of the like to form the electrode portion  12   a,  the electrode portion  14   a,  and the like. 
         [0054]    Further, the slit  16  is formed along a pattern of the cantilever of the movable portion KB by performing photolithography, D-RIE, and the like on the active layer  11   c.  The width of the slit  16  is, for example, about 1 μm to 2 μm. 
         [0055]    When the slit  16  is formed, the slit  16  is connected to the cavity  21  to thereby form the movable portion KB which serves as a cantilever. In addition, space KK which is sufficient for the movable portion KB to be operated and deformed therein is formed by the cavity  21 . 
         [0056]    When the electrode portion  12   a  and the electrode portion  14   a  are formed in the movable portion KB, slight upward warpage is caused in the movable portion KB due to a difference between coefficients of thermal expansion of the metallic material and the material for the active layer  11   c,  and also changes in the temperature during the process. Specifically, when the temperature during the process goes down to a room temperature, a tensile stress of the metallic material having a larger coefficient of thermal expansion exceeds that of the active layer  11   c.  This generates a stress that causes warpage toward the side of the electrode portion  12   a,  that is, toward upper side in the drawing. 
         [0057]    Since the material used for the oxide film layer  22  has a coefficient of thermal expansion larger than that of the material used for the active layer  11   c,  the presence of the oxide film layer  22  causes an action of the warpage to become larger toward the upper side of the movable portion KB. However, such warpage can be figured out in terms of scale by managing the process. This makes it possible to perform control for correcting the warpage as required in the post-process. 
         [0058]    Next, as illustrated in  FIG. 3C , the sacrifice layer  31  is formed by lamination on the active layer  11 , the electrode portions  12   a  and  13   a,  and the like by using SiO 2  etc. The temperature during the formation of the sacrifice layer  31  is, for example, about 150° C. The thickness of the sacrifice layer  31  is about a few μm to a few dozens for example, about 5 μm. 
         [0059]    By forming the sacrifice layer  31 , a stress is generated to cause the movable portion KB to warp downwardly because of the difference in the coefficient of thermal expansion and the change in the temperature. However, since the oxide film layer  22  is formed on the lower surface of the movable portion KB, the stress causing the sacrifice layer  31  to warp downwardly is reduced or cancelled by the stress generated by the oxide film layer  22  which causes the upward warpage. 
         [0060]    To be specific, the combined stress resulted from the stress caused by the oxide film layer  22  and the stress caused by the electrode portions  12   a  and  14   a  etc. is the stress that acts on the movable portion KB and causes the upward warpage. On the other hand, the stress caused by the sacrifice layer  31  is the stress that acts on the movable portion KB and causes the downward warpage. Thus, the stress that causes the movable portion KB to warp downward is reduced or cancelled by the stress that causes the movable portion KB to warp upward. To put it differently, these stresses balance with each other to substantially maintain the horizontal condition of the movable portion KB. As a result, the warpage caused by the formation of the sacrifice layer  31  disappears or reduces. 
         [0061]    The presence of the oxide film layer  22  greatly influences the reduction of the warpage of the movable portion KB caused by the formation of the sacrifice layer  31 . Therefore, such an oxide film layer  22  that reduces or cancels the warpage of the movable portion KB caused by the formation of the sacrifice layer  31  is selectively formed in advance. 
         [0062]    Since the warpage of the movable portion KB caused by the formation of the sacrifice layer  31  is reduced, the sacrifice layer  31  can be continuously formed without interruptions on the upper portion of the slit  16 . For this reason, the resist or polymer does not infiltrate into the slit  16  contrary to the conventional case. Here, the sacrifice layer  31  does not come into the cavity  21 . 
         [0063]    Next, as illustrated in  FIG. 4A , half-etching is performed the required number of times, and subsequently patterning is performed on the sacrifice layer  31  to selectively reduce the film thickness of the sacrifice layer  31 . The depth of the half-etching performed on the sacrifice layer  31  is controlled to thereby adjust an interelectrode gap GP 2  between the electrode portion  12   a  and the contact portion ST of the fixed contact portion  13   b  which will be formed later. 
         [0064]    Next, as illustrated in  FIG. 4B , a seed layer is formed, as necessary, on the electrode portions  12   a  and  14   a,  the sacrifice layer  31 , and the like, and plating or the like is performed using a metallic material. Through this process, a metallic layer serving as an upper electrode layer such as for the fixed contact portion  13   b  and the electrode opposing portion  15   b,  and as a structural body such as for the anchor portion  14   b,  the wall portion  17 , or the support portion  18 . 
         [0065]    Subsequently, as illustrated in  FIG. 4C , the sacrifice layer  31  and the oxide film layer  22  are removed by etching using HF (hydrofluoric acid) vapor etc. Through this process, the functional portion KN of the MEMS switch  1  is completed and ready for operation as the MEMS switch  1 . 
         [0066]    The membrane material  20  is bonded onto the wall portion  17  as necessary. In the case where the SOI substrate  11  is a disc-shaped wafer, a plurality of pieces of MEMS switch  1  formed on the SOI substrate  11  are cut out into individual pieces of MEMS switch  1  by dicing along the wall portion  17 . 
         [0067]    In this way, by using the SOI substrate  11  having the support substrate  11   a  in which the cavity  21  is provided, and the oxide film layer  22  formed on a surface in the cavity  21  on the side of the active layer  11   c,  it is possible to reduce the warpage of the movable portion KB caused when the sacrifice layer  31  is formed as much as possible. 
         [0068]    Furthermore, since the warpage of the movable portion KB caused when the sacrifice layer  31  is formed is small, the half-etching of the sacrifice layer  31  can be accurately performed, and the size of the interelectrode gap GP 2  etc. between the electrode portion  12   a  and the contact portion ST of the fixed contact portion  13   b  can be accurately adjusted. 
         [0069]    For example, if the oxide film layer  22  is not provided on the inner surface of the cavity  21   j,  the downward warpage of the movable portion KBj caused when the sacrifice layer  31  is formed becomes larger, for example, as illustrated in  FIG. 11A . For example, there is sometimes a case where the movable portion KBj sags by about 1 μm from the surface of the active layer  11   c.  For this reason, there may be a case where the sacrifice layer  31  sinks in the upper portion of the slit  16  and breaks. The resist or polymer may infiltrate into such a portion. Instead, the thickness of the sacrifice layer  31  in the vicinity of the slit  16  may fluctuate. 
         [0070]    In addition, for example, as illustrated in  FIG. 11B , the depth of a hole STA for the contact portion STj of the fixed contact portion  13   b,  when the sacrifice layer  31  is half-etched, can not be accurately controlled. As a result, for example, as illustrated in  FIG. 11C , the accuracy of the interelectrode gap GP between the contact portion STj and the electrode portion  12   j  is worsened when the metallic layer is formed by plating. 
         [0071]    For example, as illustrated in  FIG. 11D , after the sacrifice layer  31  is released, the movable portion KBj may warp upwardly as a reaction of the downward warpage thereof. If this occurs, the electrode portion  12   j  may be constantly kept in contact with the contact portion STj. In such a case, the MEMS switch  1  is determined faulty, which reduces yields. 
       Manufacturing Method of SOI Substrate   
       [0072]    Referring to  FIGS. 5A-10B , a description will be given of the manufacturing method of the SOI substrate  11 . 
         [0073]    First, a description will be given of an upper substrate BK 1  and a lower substrate BK 2  that are components for manufacturing the SOI substrate  11 . 
         [0074]      FIGS. 5A and 5B  illustrate the upper substrate BK 1  to be used for producing the SOI substrate  11 .  FIG. 5A  is a sectional side view, and  FIG. 5B  is a bottom view.  FIGS. 6A-6C  illustrate the lower substrate BK 2  to be used for producing the SOI substrate  11 .  FIG. 6A  is a plan view, and  FIGS. 6B and 6C  are cross sectional views. 
         [0075]    Referring to  FIGS. 5A and 5B , the upper substrate BK 1  is resulted from forming a thermally-oxidized film  42  on a lower surface of a silicon plate  41 . The silicon plate  41  is a portion to be polished and serves as the active layer  11   c  later, and the thermally-oxidized film  42  is to serve as the BOX layer  11   b  later. 
         [0076]    As illustrated in  FIG. 5B , the portion of the thermally-oxidized film  42  which will serve as the movable portion KB later is patterned in a shape identical to that of the movable portion KB on which the oxide film layer  22  is formed. 
         [0077]    Referring to  FIGS. 6A and 6B , the lower substrate BK 2  is resulted from forming the cavity  21  in the upper surface of the silicon plate  43  by D-RIE, wet etching, or the like. The planar shape of the cavity  21  is a shape that corresponds to a region including a portion to be turned to the movable portion KB. The silicon plate  43  is a portion that turns to be the support substrate  11   a  later. 
         [0078]      FIG. 6C  illustrates a variation example of the lower substrate BK 2 B. As the lower substrate BK 2 B illustrated in  FIG. 6C , the oxide film layers  23  and  24  formed of SiO2 etc. may be formed on the entire upper and lower surfaces of the silicon plate  43 . The entire upper and lower surfaces of the silicon plate  43  including the wall surface of the cavity  21 B are covered with the insulating layer by the oxide film layers  23  and  24 . 
         [0079]    In the manufacturing process of the SOI substrate  11 , the upper substrate BK 1  and the lower substrate BK 2  are bonded together so that the surface of the oxide film layer  22  coincides with a surface of the silicon plate  43  in which the cavity  21  is provided. 
         [0080]    Alternatively, as illustrated in  FIGS. 7A and 7B , the shape of the oxide film layer  22  of the upper substrate BK 1  may be made identical with the shapes of the electrode portions  12   a  and  14   a  formed on the upper side of the movable portion KB. 
         [0081]      FIG. 7B  illustrates, in plan view, the shapes of the electrode portions  12   a  and  14   a  formed in the movable portion KB, and  FIG. 7A  illustrates, in bottom view, the patterning for the oxide film layer  22 B formed on the thermally-oxidized film  42  of the upper substrate BK 1 B. In these illustrations, the shapes of the electrode portions  12   a  and  14   b  and the shape of the oxide film layer  22 B are in a mirror image relationship. 
         [0082]    Next, the manufacturing process of the SOI substrate  11  will be described. 
         [0083]    As illustrated in  FIG. 8A , the cavity  21  is formed on one side of the silicon plate  43  which is to serve as the lower substrate BK 2 , and the concave portion (alignment marker)  43   d  for positioning is also formed. As illustrated in  FIG. 8B , another concave portion  43   d  is also formed on the other side of the silicon plate  43  to serve as the lower substrate BK 2 . 
         [0084]    As illustrated in  FIG. 8C , the oxide film layers  23  and  24  are individually formed on two sides of the silicon plate  43  entirely as necessary to thereby form the lower substrate BK 2 B. 
         [0085]    As illustrated in  FIG. 9A , the upper substrate BK 1  illustrated in  FIGS. 5A and 5B  or, alternatively, the upper substrate BK 1 B illustrated in  FIGS. 7A and 7B  is bonded to the upper surface of the lower substrate BK 2  illustrated in  FIG. 8B . In this bonding process, for example, hydrophilic processing is performed on the bonding surfaces, and two surfaces are placed together which are then subjected to an annealing treatment at a high temperature of about 1000° C. 
         [0086]    Next, as illustrated in  FIG. 9B , the surface of the silicon plate  41  is polished to a predetermined thickness required as the active layer  11   c.    
         [0087]    Through this process, the thermally-oxidized film  42  turns to be the BOX layer  11   b,  and the silicon plate  43  turns to be the support substrate  11   a.  The cavity  21  extends to the surface inside the active layer  11   c  in the support substrate  11   a  where the oxide film layer  22  which has been subjected to patterning is formed. 
         [0088]    Further, as illustrated in  FIG. 10A , the upper substrate BK 1  illustrated in  FIGS. 5A and 5B  or, alternatively, the upper substrate BK 1 B illustrated in  FIGS. 7A and 7B  is bonded to the upper surface of the lower substrate BK 2 B illustrated in  FIG. 8C . Next, as illustrated in  FIG. 10B , the surface of the silicon plate  41  is polished to a predetermined thickness required as the active layer  11   c.    
         [0089]    Through this process, the thermally-oxidized film  42  and the oxide film layer  23  turn to be the BOX layer  11   b,  and the silicon plate  43  turns to be the support substrate  11   a.  The cavity  21  extends to the surface inside the active layer  11   c  in the support substrate  11   a  where the oxide film layer  22 , which has been subjected to patterning, is formed. The oxide film layer  23  is formed in the other portion of the inner surface of the cavity  21 . 
         [0090]    As described above, the SOI substrate  11  is produced by bonding together the lower substrate BK 2  having the cavity  21  and the upper substrate BK 1  having the oxide film layer  22  that has undergone the patterning. During this process, an oxide film layer  22  is formed and subjected to patterning so that the oxide film layer  22  causes a stress of the same quality as and equivalent to a stress that will be caused when the sacrifice layer  31  is formed later. This arrangement makes it possible to reduce the warpage that will be caused otherwise after the movable portion KB is formed. 
         [0091]    Consequently, it is possible to suppress the warpage or depression of the movable portion KB during the manufacturing process of the MEMS switch  1  and perform accurate control of the dimensions during the formation of the electrode by applying half-etching to the sacrifice layer  31 . Therefore, it is possible to manufacture the MEMS switch  1  having the desired driving properties at a higher yield rate. 
         [0092]    In addition, since it is possible to adopt a process using a wafer of the SOI substrate  11  having the cavity  21 , it is easy to arrange it in a wafer level package (WLP) structure that has a low profile and is implementable. Specifically, a single membrane material  20  is bonded onto an entire area in which a plurality of MEMS switches  1  are formed on the SOI substrate  11 , and dicing is preformed thereafter. In this way, it is possible to manufacture individual MEMS switches  1  having a low profile in large quantity. 
         [0093]    Hereinafter, a description will be given of the outline procedure of the manufacturing process of the MEMS switch  1  using the SOI substrate  11  referring to a flowchart. 
         [0094]    Referring to  FIG. 12 , an SOI substrate  11  is prepared. In the SOI substrate  11 , the support substrate  11   a  is provided with a cavity  21 , and the oxide film layer  22  is formed on the surface of the active layer  11   c  in the cavity  21  (step # 11 ). Then, the slit  16  is arranged to form the movable portion KB (# 12 ). 
         [0095]    The lower electrodes such as the electrode portions  12   a  and  14   a  are formed on the movable portion KB (# 13 ), and the sacrifice layer  31  is provided thereon (# 14 ). Half-etching is performed on the sacrifice layer  31  to thereby perform patterning (# 15 ). An upper electrode such as the fixed contact portion  13   b  is formed on the sacrifice layer  31  (# 16 ). Then, the sacrifice layer  31  and the oxide film layer  22  are removed (# 17 ). 
         [0096]    According to the foregoing embodiment, during the manufacturing of the MEMS switch  1 , the SOI substrate  11  is used. The SOI substrate  11  includes the support substrate  11   a  to which the cavity  21  is provided, and the oxide film layer  22  that is patterned on the inner surface of the active layer  11   c.  However, it is also possible to manufacture the MEMS switch  1  without using the above-mentioned SOI substrate  11  but using a different type of SOI substrate. 
         [0097]    For example, it is possible to use an SOI substrate formed of the support substrate  11   a,  the BOX layer  11   b,  and the active layer  11   c  without having the cavity  21  formed therein. In this case, the cavity is produced from the rear side of the active layer  11   c  after the device structure is formed on the active layer  11   c.    
         [0098]    According to the foregoing embodiment, since the movable portion is fixed relative to the BOX layer when the side of the active layer is being processed, the movable portion KB is not caused to warp when the sacrifice layer  31  is formed. Therefore, it is possible to perform accurate control on the dimensions of the interelectrode gap GP 2  between the electrode portion  12   a  and the contact portion ST of the fixed contact portion  13   b.  Instead of the distance between the electrode portion  12   a  and the contact portion ST or a distance between electrodes that make contact with each other, a distance between two electrodes that do not make contact with each other may be taken as the interelectrode gap GP 2 . This means that it is also possible to perform accurate control on dimensions of an interelectrode gap between the electrodes that do not make contact with each other. 
         [0099]    In the foregoing embodiment, the overall configurations of the other portions such as the SOI substrate  11 , the electrode portions  12   a  and  14   a,  the fixed contact portion  13   b,  the contact portion ST, the slit  16 , the cavity  21 , the oxide film layer  22 , the sacrifice layer  31 , the movable portion KB, and the MEMS switch  1 , the configurations of various parts thereof, the structure, the shape, the material, the quantity, the layout, the temperature, the production method, and the like may be altered as required in accordance with the subject matter of the present invention. 
         [0100]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.