Patent Publication Number: US-2023138044-A1

Title: Optical device production method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for manufacturing an optical device. 
     BACKGROUND ART 
     As a micro electro mechanical systems (MEMS) device constituted by a silicon on insulator (SOI) substrate, there is known an optical device including a base, a movable unit, an elastic support portion connected between the base and the movable unit, and an optical function unit disposed on the movable unit (for example, refer to Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: US Unexamined Patent Publication No. 2008/0284078 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the above-described optical device, it is considered to provide a rib portion to the movable unit or the elastic support portion so as to suppress deformation of the movable unit or the elastic support portion during movement. In the optical device, it is required to form the rib portion with accuracy from the viewpoint of controllability of the movable unit or the like. 
     An object of an aspect of the present disclosure is to provide a method for manufacturing an optical device capable of forming the rib portion with accuracy. 
     Solution to Problem 
     According to an aspect of the present disclosure, there is provided a method for manufacturing an optical device, the optical device includes: a base, a movable unit, an elastic support portion connected between the base and the movable unit, and an optical function unit disposed on the movable unit, the base, the movable unit, and the elastic support portion being constituted by a semiconductor substrate including a first semiconductor layer, a second semiconductor layer, and an insulating layer disposed between the first semiconductor layer and the second semiconductor layer, the base being constituted by the first semiconductor layer, the second semiconductor layer, and the insulating layer, at least one of the movable unit and the elastic support portion including a rib portion constituted at least by the first semiconductor layer and disposed on the second semiconductor layer, and the first semiconductor layer constituting the rib portion being thinner than the first semiconductor layer constituting the base. The method includes: a first step of preparing the semiconductor substrate that includes a portion corresponding to the base, the movable unit, and the elastic support portion; a second step of forming a first resist layer in a region corresponding to the base on a surface of the first semiconductor layer which is opposite to the insulating layer after the first step; a third step of forming a depression in the first semiconductor layer by etching the first semiconductor layer up to an intermediate portion in a thickness direction using the first resist layer as a mask after the second step; a fourth step of forming a second resist layer in a region corresponding to the rib portion on a bottom surface of the depression, a side surface of the depression, and the surface of the first semiconductor layer which is opposite to the insulating layer after the third step; and a fifth step of forming the rib portion by etching the first semiconductor layer until reaching the insulating layer using the second resist layer as a mask after the fourth step. 
     In the optical device obtained by the method for manufacturing the optical device, the first semiconductor layer constituting the rib portion is thinner than the first semiconductor layer constituting the base. Accordingly, it is possible to realize protection of the rib portion by suppressing the rib portion from protruding from the base while suppressing deformation of the movable unit and/or the elastic support portion by the rib portion. On the other hand, in a typical manufacturing method, it is difficult to form the rib portion with accuracy. In contrast, in the method for manufacturing the optical device, the rib portion is formed by two-stage etching using the first resist layer and the second resist layer as a mask. Hence, it is possible to form the rib portion with accuracy. 
     In the first step, the semiconductor substrate in which the first semiconductor layer is thicker than the second semiconductor layer may be prepared. In this case, in the obtained optical device, it is possible to secure the thickness of the rib portion, and it is possible to more appropriately suppress deformation of the movable unit and/or the elastic support portion. 
     The method for manufacturing the optical device according to the aspect of the present disclosure may further include a sixth step of forming the optical function unit on a surface of the second semiconductor layer on a side of the insulating layer after the fifth step. In this case, it is possible to protect the optical function unit by the base and the rib portion, and for example, it is possible to suppress damage of the optical function unit due to direct contact, for example, during transportation. 
     The rib portion may include a plurality of portions having widths different from each other. According to the method for manufacturing the optical device of the present disclosure, even in a case where the rib portion includes a plurality of portions having widths different from each other, it is possible to form the rib portion with accuracy. 
     The elastic support portion may support the movable unit so that the movable unit is capable of moving along a direction that intersects a main surface of the base. According to the method for manufacturing the optical device of the present disclosure, in the case of manufacturing the optical device, it is possible to form the rib portion with accuracy. 
     The elastic support portion may support the movable unit so that the movable unit is capable of swinging around a predetermined axial line. According to the method for manufacturing the optical device of the present disclosure, in the case of manufacturing the optical device, it is possible to form the rib portion with accuracy. 
     The optical device may further include a fixed comb electrode provided to the base and including a plurality of fixed comb fingers, and a movable comb electrode provided to at least one of the movable unit and the elastic support portion and including a plurality of movable comb fingers disposed alternately with the plurality of fixed comb fingers. According to the method for manufacturing the optical device of the present disclosure, in the case of manufacturing the optical device, it is possible to form the rib portion with accuracy. 
     The optical device may further include a coil or a piezoelectric element provided ton the movable unit. According to the method for manufacturing the optical device of the present disclosure, in the case of manufacturing the optical device, it is possible to form the rib portion with accuracy. 
     The rib portion may be constituted by the first semiconductor layer and the insulating layer. According to the method for manufacturing the optical device of the present disclosure, in the case of manufacturing the optical device, it is possible to form the rib portion with accuracy. 
     Advantageous Effects of Invention 
     According to the aspect of the present disclosure, it is possible to provide a method for manufacturing an optical device which is capable of forming a rib portion with accuracy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view of an optical module of a first embodiment. 
         FIG.  2    is a plan view of a mirror unit illustrated in  FIG.  1   . 
         FIG.  3    is a cross-sectional view of the mirror unit taken along line III-III illustrated in  FIG.  2   . 
         FIG.  4    is a cross-sectional view of the mirror unit taken along line IV-IV illustrated in  FIG.  2   . 
         FIG.  5    is a schematic cross-sectional view of a mirror device taken along line V-V illustrated in  FIG.  2   . 
         FIG.  6    is a partially enlarged view of the mirror device illustrated in  FIG.  2   . 
         FIG.  7    is a plan view of an optical function member illustrated in  FIG.  2   . 
         FIG.  8    is a cross-sectional view of the optical module taken along line VIII-VIII illustrated in  FIG.  1   . 
         FIG.  9    is a cross-sectional view of the optical module taken along line IX-IX illustrated in  FIG.  1   . 
         FIG.  10    is a schematic cross-sectional view of the mirror unit and a beam splitter unit illustrated in  FIG.  1   . 
         FIG.  11    is a schematic cross-sectional view of the mirror device taken along line XI-XI illustrated in  FIG.  2   . 
         FIG.  12    is a schematic cross-sectional view of a mirror device of the first embodiment. 
         FIG.  13 A  and  FIG.  13 B  are views for describing a method for manufacturing the mirror device according to the first embodiment. 
         FIG.  14 A  and  FIG.  14 B  are views for describing the method for manufacturing the mirror device according to the first embodiment. 
         FIG.  15 A  and  FIG.  15 B  are views for describing the method for manufacturing the mirror device according to the first embodiment. 
         FIG.  16 A  and  FIG.  16 B  are views for describing a method for manufacturing a mirror device according to a comparative example. 
         FIG.  17 A  and  FIG.  17 B  are views for describing the method for manufacturing the mirror device according to the comparative example. 
         FIG.  18    is a view for describing the method for manufacturing the mirror device according to the comparative example. 
         FIG.  19 A  and  FIG.  19 B  are views for describing an operational effect of the method for manufacturing the mirror device according to the first embodiment. 
         FIG.  20 A  and  FIG.  20 B  are views for describing the operational effect of the method for manufacturing the mirror device according to the first embodiment. 
         FIG.  21 A  and  FIG.  21 B  are views for describing the operational effect of the method for manufacturing the mirror device according to the first embodiment. 
         FIG.  22    is a plan view of an optical device of a second embodiment. 
         FIG.  23    is a bottom view of the optical device illustrated in  FIG.  22   . 
         FIG.  24    is a cross-sectional view of the optical device along line XXIV-XXIV illustrated in  FIG.  22   . 
         FIG.  25    is a cross-sectional view of the optical device along line XXV-XXV illustrated in  FIG.  22   . 
         FIG.  26    is a plan view of an optical device of a third embodiment. 
         FIG.  27    is a bottom view of the optical device illustrated in  FIG.  26   . 
         FIG.  28    is a cross-sectional view of the optical device along line XXVIII-XXVIII illustrated in  FIG.  26   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. The same reference numeral will be given to the same or equivalent parts in the respective drawings, and redundant description thereof will be omitted. 
     First Embodiment 
     [Configuration of Optical Module] 
     As illustrated in  FIG.  1   , an optical module  1  includes a mirror unit  2 , a beam splitter unit  3 , a light incident unit  4 , a first light detector  6 , a second light source  7 , a second light detector  8 , a support  9 , a first support structure  11 , and a second support structure  12 . The mirror unit  2  is disposed on one side of the support  9  in a Z-axis direction (a first direction), and is attached to the support  9 , for example, by an adhesive. For example, the support  9  is formed of copper tungsten, and has a rectangular plate shape. The mirror unit  2  includes a movable mirror  22  that moves in the Z-axis direction, and a fixed mirror  16  of which a position is fixed (details thereof will be described later). For example, the Z-axis direction is a vertical direction, and the one side in the Z-axis direction is an upper side. 
     The beam splitter unit  3  is disposed on one side of the mirror unit  2  in the Z-axis direction, and is supported by the first support structure  11 . The first support structure  11  is attached to the support  9 , for example, by an adhesive. The light incident unit  4  is disposed on one side of the beam splitter unit  3  in an X-axis direction (a third direction perpendicular to the first direction), and is supported by the second support structure  12 . The first light detector  6 , the second light source  7 , and the second light detector  8  are disposed on the one side of the beam splitter unit  3  in the Z-axis direction, and are supported by the second support structure  12 . The second support structure  12  is attached to the support  9 , for example, by a bolt. 
     In the optical module  1 , an interference optical system is constituted by the beam splitter unit  3 , the movable mirror  22 , and the fixed mirror  16  with respect to each of measurement light L 0  and laser light L 10 . The interference optical system constituted with respect to each of the measurement light L 0  and the laser light L 10  is, for example, a Michelson interference optical system. 
     With regard to the measurement light L 0 , interference light L 1  of measurement light is detected as follows. That is, when the measurement light L 0  that is incident from a first light source (not illustrated) through a measurement target (not illustrated) or the measurement light L 0  that is generated from the measurement target (for example, light emitted from the measurement target itself, or the like) is incident to the beam splitter unit  3  from the light incident unit  4 , the measurement light L 0  is divided into a part and the remainder in the beam splitter unit  3 . The part of the measurement light L 0  is reflected by the movable mirror  22  that reciprocates in the Z-axis direction, and returns to the beam splitter unit  3 . On the other hand, the remainder of the measurement light L 0  is reflected by the fixed mirror  16  and returns to the beam splitter unit  3 . The part and the remainder of the measurement light L 0 , which return to the beam splitter unit  3 , are emitted from the beam splitter unit  3  as the interference light L 1 , and the interference light L 1  of the measurement light is detected by the first light detector  6 . 
     With regard to the laser light L 10 , interference light L 11  of laser light is detected as follows. That is, when the laser light L 10  emitted from the second light source  7  is incident to the beam splitter unit  3 , the laser light L 10  is divided into a part and the remainder in the beam splitter unit  3 . The part of the laser light L 10  is reflected by the movable mirror  22  that reciprocates in the Z-axis direction, and returns to the beam splitter unit  3 . On the other hand, the remainder of the laser light L 10  is reflected by the fixed mirror  16  and returns to the beam splitter unit  3 . The part and the remainder of the laser light L 10 , which return to the beam splitter unit  3 , are emitted from the beam splitter unit  3  as the interference light L 11 , and the interference light L 11  of the laser light is detected by the second light detector  8 . 
     According to the optical module  1 , measurement of a position of the movable mirror  22  in the Z-axis direction can be measured based on a detection result of the interference light L 11  of the laser light, and spectral analysis with respect to the measurement target can be performed based on a measurement result of the position, and a detection result of the interference light L 1  of the measurement light. 
     [Configuration of Mirror Unit] 
     As illustrated in  FIG.  2   ,  FIG.  3   , and  FIG.  4   , the mirror unit  2  includes a mirror device (optical device)  20 , an optical function member  13 , the fixed mirror  16 , and a stress mitigation substrate  17 . The mirror device  20  includes a base  21 , the movable mirror  22 , and a drive unit  23 . 
     The base  21  includes a first surface  21   a  (surface on the one side in the Z-axis direction) and a second surface  21   b  opposite to the first surface  21   a . Each of the first surface  21   a  and the second surface  21   b  is a main surface of the base  21 . For example, the base  21  has a rectangular plate shape, and a size of approximately 10 mm×15 mm×0.35 mm (thickness). The movable mirror  22  includes a mirror surface (optical function member)  22   a , and a movable unit  22   b  in which the mirror surface  22   a  is disposed. The movable mirror  22  (movable unit  22   b ) is supported in the base  21  so that the movable mirror  22  can move in the Z-axis direction perpendicular to the first surface  21   a  (a first direction perpendicular to the first surface). The drive unit  23  moves the movable mirror  22  in the Z-axis direction. 
     A pair of light passage openings  24  and  25  are provided in the mirror device  20 . The pair of light passage openings  24  and  25  are respectively disposed on both sides of the movable mirror  22  in the X-axis direction. The light passage opening (first light passage portion)  24  constitutes a first portion of an optical path between the beam splitter unit  3  and the fixed mirror  16 . In this embodiment, the light passage opening  25  does not function as a light passage opening. 
     Here, a configuration of the mirror device  20  will be described in detail with reference to  FIG.  2   ,  FIG.  5   , and  FIG.  6   .  FIG.  5    is a schematic cross-sectional view of the mirror device  20  illustrated in  FIG.  3   , and  FIG.  5    schematically illustrates the mirror device  20 , for example, in a state in which dimensions in the Z-axis direction are enlarged in comparison to actual dimensions. 
     The base  21 , the movable unit  22   b  of the movable mirror  22 , and the drive unit  23  are constituted by a silicon on insulator (SOI) substrate (semiconductor substrate)  100 . That is, the mirror device  20  is constituted by the SOI substrate  100 . For example, the mirror device  20  is formed in a rectangular plate shape. The SOI substrate  100  includes a support layer  101 , a device layer  102 , and an intermediate layer  103 . The support layer  101  is a first silicon layer (a first semiconductor layer). The device layer  102  is a second silicon layer (a second semiconductor layer). The intermediate layer  103  is an insulating layer disposed between the support layer  101  and the device layer  102 . The SOI substrate  100  includes the support layer  101 , the intermediate layer  103 , and the device layer  102  in this order from the one side in the Z-axis direction. 
     The base  21  is constituted by a part of the support layer  101 , the device layer  102 , and the intermediate layer  103 . The first surface  21   a  of the base  21  is a surface of the support layer  101  which is opposite to the intermediate layer  103 . The second surface  21   b  of the base  21  is a surface of the device layer  102  which is opposite to the intermediate layer  103 . The support layer  101  that constitutes the base  21  is thicker than the device layer  102  that constitutes the base  21 . For example, the thickness of the support layer  101  that constitutes the base  21  is approximately four times the thickness of the device layer  102  that constitutes the base  21 . As will be described later, in the mirror unit  2 , the second surface  21   b  of the base  21  and a third surface  13   a  of the optical function member  13  are jointed to each other (refer to  FIG.  3    and  FIG.  4   ). 
     The movable mirror  22  is disposed in a state in which an intersection between an axial line R 1  and an axial line R 2  is set as the central position (gravity center position). The axial line R 1  is a straight line that extends in the X-axis direction. The axial line R 2  is a straight line that extends in a Y-axis direction (a second direction perpendicular to the first direction and the third direction). When viewed from the Z-axis direction, in the mirror device  20 , a portion other than a portion that overlaps a sixth surface  21   d  of the base  21  to be described later has a shape linearly symmetric to each of the axial line R 1  and the axial line R 2 . 
     The movable mirror  22  (movable unit  22   b ) includes an arrangement portion  221 , a frame portion  222 , a pair of connection portions  223 , and a rib portion  224 . The arrangement portion  221 , the frame portion  222 , and the pair of connection portions  223  are constituted by a part of the device layer  102 . The arrangement portion  221  has a circular shape when viewed from the Z-axis direction. The arrangement portion  221  includes a central portion  221   a  and an edge portion  221   b . For example, the mirror surface  22   a  is provided on a surface  221   as  of the central portion  221   a  on the one side in the Z-axis direction by forming a metal film (metal layer) thereon. The mirror surface  22   a  extends perpendicular to the Z-axis direction, and has a circular shape. The surface  221   as  of the central portion  221   a  is a surface on the intermediate layer  103  side in the device layer  102 . The mirror surface  22   a  is located on the other side in the Z-axis direction in comparison to the first surface  21   a  of the base  21 . In other words, the first surface  21   a  is located on the one side in the Z-axis direction in comparison to the mirror surface  22   a . The edge portion  221   b  surrounds the central portion  221   a  when viewed from the Z-axis direction. 
     The frame portion  222  extends in an annular shape to surround the arrangement portion  221  with a predetermined gap from the arrangement portion  221  when viewed from the Z-axis direction. For example, the frame portion  222  has a circular ring shape when viewed from the Z-axis direction. Each of the pair of connection portions  223  connects the arrangement portion  221  and the frame portion  222  to each other. The pair of connection portions  223  are respectively disposed on both sides of the arrangement portion  221  in the Y-axis direction. 
     The rib portion  224  is constituted by the support layer  101  and the intermediate layer  103  disposed on the device layer  102 . The rib portion  224  is disposed at the periphery of the mirror surface  22   a . The rib portion  224  includes an inner rib portion  224   a , an outer rib portion  224   b , and a pair of connection rib portions  224   c . The inner rib portion  224   a  is disposed on a surface of the edge portion  221   b  on the one side in the Z-axis direction. The inner rib portion  224   a  surrounds the mirror surface  22   a  when viewed from the Z-axis direction. For example, an outer edge of the inner rib portion  224   a  extends along an outer edge of the arrangement portion  221  with a predetermined gap from the outer edge of the arrangement portion  221  when viewed from the Z-axis direction. An inner edge of the inner rib portion  224   a  extends along an outer edge of the mirror surface  22   a  with a predetermined gap from the outer edge of the mirror surface  22   a  when viewed from the Z-axis direction. An end surface  224   as  of the inner rib portion  224   a  on the one side in the Z-axis direction is located on the one side in the Z-axis direction in comparison to the mirror surface  22   a.    
     The outer rib portion  224   b  is disposed on a surface of the frame portion  222  on the one side in the Z-axis direction. The outer rib portion  224   b  surrounds the inner rib portion  224   a  and the mirror surface  22   a  when viewed from the Z-axis direction. For example, an outer edge of the outer rib portion  224   b  extends along an outer edge of the frame portion  222  with a predetermined gap from the outer edge of the frame portion  222  when viewed from the Z-axis direction. An inner edge of the outer rib portion  224   b  extends along an inner edge of the frame portion  222  with a predetermined gap from the inner edge of the frame portion  222  when viewed from the Z-axis direction. An end surface  224   bs  of the outer rib portion  224   b  on the one side in the Z-axis direction is located on the one side in the Z-axis direction in comparison to the mirror surface  22   a.    
     The pair of connection rib portions  224   c  are respectively disposed on surfaces of the pair of connection portions  223  on the one side in the Z-axis direction. The connection rib portions  224   c  connect the inner rib portion  224   a  and the outer rib portion  224   b  to each other. End surfaces  224   cs  of the connection rib portions  224   c  on the one side in the Z-axis direction are located on the one side in the Z-axis direction in comparison to the mirror surface  22   a.    
     The thickness of the inner rib portion  224   a , the thickness of the outer rib portion  224   b , and the thickness of the respective connection rib portions  224   c  in the Z-axis direction are the same as each other. That is, the thickness of the support layer  101  that constitutes the inner rib portion  224   a , the outer rib portion  224   b , and the respective connection rib portions  224   c  is the same in each case. The end surface  224   as  of the inner rib portion  224   a , the end surface  224   bs  of the outer rib portion  224   b , and the end surfaces  224   cs  of the respective connection rib portions  224   c  are located on the same plane perpendicular to the Z-axis direction. The support layer  101  that constitutes the inner rib portion  224   a , the outer rib portion  224   b , and the respective connection rib portions  224   c  is thinner than the support layer  101  that constitutes the base  21 . Accordingly, the end surfaces  224   as ,  224   bs , and  224   cs  are located on the other side in the Z-axis direction in comparison to the first surface  21   a  of the base  21 . In other words, the first surface  21   a  is located on the one side in the Z-axis direction in comparison to the end surfaces  224   as ,  224   bs , and  224   cs.    
     When viewed from the Z-axis direction, a width of the outer rib portion  224   b  is wider than a width of the inner rib portion  224   a . The width of the inner rib portion  224   a  when viewed from the Z-axis direction is a length of the inner rib portion  224   a  in a direction perpendicular to the extending direction of the inner rib portion  224   a , and is a length of the inner rib portion  224   a  in a radial direction of the inner rib portion  224   a  in this embodiment. This is also true of a width of the outer rib portion  224   b  when viewed from the Z-axis direction. A width of each of the connection rib portions  224   c  is larger than the width of each of the inner rib portion  224   a  and the outer rib portion  224   b . The width of each of the connection rib portion  224   c  is a length of each of the connection rib portion  224   c  along the extending direction of the inner rib portion  224   a.    
     The drive unit  23  includes a first elastic support unit  26 , a second elastic support unit  27 , and an actuator unit  28 . The first elastic support unit  26 , the second elastic support unit  27 , and the actuator unit  28  are constituted by a part of the device layer  102 . 
     Each of the first elastic support unit  26  and the second elastic support unit  27  is connected between the base  21  and the movable mirror  22 . The first elastic support unit  26  and the second elastic support unit  27  support the movable mirror  22  so that the movable mirror  22  (movable unit  22   b ) can move in the Z-axis direction. 
     The first elastic support unit  26  includes a pair of levers  261 , a first link member  262 , a second link member  263 , a pair of beam members  264 , an intermediate member  265 , a pair of first torsion bars (first torsion support portions)  266 , a pair of second torsion bars (second torsion support portions)  267 , a pair of non-linearity mitigation springs  268 , and a plurality of electrode support portions  269 . 
     The pair of levers  261  are respectively disposed on both sides of the light passage opening  24  in the Y-axis direction, and face each other in the Y-axis direction. Each of the levers  261  has a plate shape that extends along a plane perpendicular to the Z-axis direction. The lever  261  includes a first portion  261   a , a second portion  261   b  disposed on a side opposite to the movable mirror  22  with respect to the first portion  261   a , and a third portion  261   c  connected to the first portion  261   a  and the second portion  261   b . The first portion  261   a  and the second portion  261   b  extend in the X-axis direction. A length of the first portion  261   a  in the X-axis direction is shorter than a length of the second portion  261   b  in the X-axis direction. The third portions  261   c  of the pair of levers  261  obliquely extend to be spaced away from each other as going away from the movable mirror  22 . 
     The first link member  262  bridges first ends  261   d  of the pair of levers  261  on a side opposite to the movable mirror  22 . The first link member  262  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. The second link member  263  bridges second ends  261   e  of the pair of levers  261  on the movable mirror  22  side. The second link member  263  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. A width of the second link member  263  in the X-axis direction is narrower than a width of the first link member  262  in the X-axis direction. A length of the second link member  263  in the Y-axis direction is shorter than a length of the first link member  262  in the Y-axis direction. 
     The pair of beam members  264  respectively bridge the second portions  261   b  of the pair of levers  261  and the first link member  262 . The respective beam members  264  have a plate shape that extends along a plane perpendicular to the Z-axis direction. The pair of beam members  264  obliquely extend to approach each other as going away from the movable mirror  22 . The pair of levers  261 , the first link member  262 , the second link member  263 , and the pair of beam members  264  define the light passage opening  24 . The light passage opening  24  has a polygonal shape when viewed from the Z-axis direction. For example, the light passage opening  24  is a cavity (hole). Alternatively, a material having optical transparency with respect to the measurement light L 0  and the laser light L 10  may be disposed in the light passage opening  24 . 
     The intermediate member  265  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. The intermediate member  265  is disposed between the movable mirror  22  and the second link member  263  (in other words, between the movable mirror  22  and the light passage opening  24 ). The intermediate member  265  is connected to the movable mirror  22  through the non-linearity mitigation springs  268  as to be described later. 
     The pair of first torsion bars  266  respectively bridge the first end  261   d  of one lever  261  and the base  21 , and the first end  261   d  of the other lever  261  and the base  21 . That is, the pair of first torsion bars  266  are respectively connected between the pair of levers  261  and the base  21 . The first torsion bars  266  extend in the Y-axis direction. The pair of first torsion bars  266  are disposed on the same central line parallel to the Y-axis direction. In this embodiment, the central line of the first torsion bars  266  and the central line of the first link member  262  are located on the same straight line. A protrusion  261   f  that protrudes outward in the Y-axis direction is provided in each of the first ends  261   d  of the levers  261 , and each of the first torsion bars  266  is connected to the protrusion  261   f.    
     The pair of second torsion bars  267  respectively bridge the second end  261   e  of one lever  261  and one end of the intermediate member  265 , and the second end  261   e  of the other lever  261  and the other end of the intermediate member  265 . That is, the pair of second torsion bars  267  are respectively connected between the pair of levers  261  and the movable mirror  22 . The respective second torsion bars  267  extend in the Y-axis direction. The pair of second torsion bars  267  are disposed on the same central line parallel to the Y-axis direction. 
     The pair of non-linearity mitigation springs  268  are connected between the movable mirror  22  and the intermediate member  265 . That is, the pair of non-linearity mitigation springs  268  are connected between the movable mirror  22  and the second torsion bar  267 . Each of the non-linearity mitigation springs  268  includes a meandering portion  268   a  that extends in a meandering manner when viewed from the Z-axis direction. The meandering portion  268   a  includes a plurality of straight portions  268   b  which extend in the Y-axis direction and are aligned in the X-axis direction, and a plurality of folded portions  268   c  which alternately connect both ends of the plurality of straight portions  268   b . One end of the meandering portion  268   a  is connected to the intermediate member  265 , and the other end of the meandering portion  268   a  is connected to the frame portion  222 . In the meandering portion  268   a , a portion on the frame portion  222  side has a shape along the outer edge of the frame portion  222 . 
     The non-linearity mitigation spring  268  is constituted as follows. In a state in which the movable mirror  22  has moved in the Z-axis direction, the amount of deformation of the non-linearity mitigation spring  268  around the Y-axis direction becomes smaller than the amount of deformation of each of the first torsion bar  266  and the second torsion bar  267  around the Y-axis direction, and the amount of deformation of the non-linearity mitigation spring  268  in the X-axis direction becomes larger than the amount of deformation of each of the first torsion bar  266  and the second torsion bar  267  in the X-axis direction. Accordingly, it is possible to suppress occurrence of non-linearity in twist deformation of the first torsion bar  266  and the second torsion bar  267 , and it is possible to suppress deterioration of control characteristics of the movable mirror  22  due to the non-linearity. The amount of deformation of the first torsion bar  266 , the second torsion bar  267 , and the non-linearity mitigation spring  268  around the Y-axis direction represents, for example, an absolute value of a twist amount (twist angle). The amount of deformation of the first torsion bar  266 , the second torsion bar  267 , and the non-linearity mitigation spring  268  in the X-axis direction represents, for example, an absolute value of a deflection amount. The amount of deformation of a member around the Y-axis direction represents the amount of deformation of the member in a peripheral direction of a circle of which the center is set to an axial line that passes through the center of the member and is parallel to the Y-axis. This is also true of first torsion bars  276 , second torsion bars  277 , and a non-linearity mitigation spring  278  to be described later. 
     The plurality of electrode support portions  269  include a pair of first electrode support portions  269   a , a pair of second electrode support portions  269   b , and a pair of third electrode support portions  269   c . Each of the electrode support portions  269   a ,  269   b , and  269   c  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. Each of the electrode support portions  269   a ,  269   b , and  269   c  extends from the second portion  261   b  of the lever  261  toward a side opposite to the light passage opening  24 . The pair of first electrode support portions  269   a  are disposed on the same central line parallel to the Y-axis direction. The pair of second electrode support portions  269   b  are disposed on the same central line parallel to the Y-axis direction. The pair of third electrode support portions  269   c  are disposed on the same central line parallel to the Y-axis direction. In the X-axis direction, the first electrode support portions  269   a , the second electrode support portions  269   b , and the third electrode support portions  269   c  are aligned in this order from the movable mirror  22  side. 
     The second elastic support unit  27  includes a pair of levers  271 , a first link member  272 , a second link member  273 , a pair of beam members  274 , an intermediate member  275 , a pair of first torsion bars (first torsion support portions)  276 , a pair of second torsion bars (second torsion support portions)  277 , a pair of non-linearity mitigation springs  278 , and a plurality of electrode support portions  279 . 
     The pair of levers  271  are respectively disposed on both sides of the light passage opening  25  in the Y-axis direction, and face each other in the Y-axis direction. Each of the levers  271  has a plate shape that extends along a plane perpendicular to the Z-axis direction. The lever  271  includes a first portion  271   a , a second portion  271   b  disposed on a side opposite to the movable mirror  22  with respect to the first portion  271   a , and a third portion  271   c  connected to the first portion  271   a  and the second portion  271   b . The first portion  271   a  and the second portion  271   b  extend in the X-axis direction. A length of the first portion  271   a  in the X-axis direction is shorter than a length of the second portion  271   b  in the X-axis direction. The third portions  271   c  of the pair of levers  271  obliquely extend to be spaced away from each other as going away from the movable mirror  22 . 
     The first link member  272  bridges first ends  271   d  of the pair of levers  271  on a side opposite to the movable mirror  22 . The first link member  272  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. The second link member  273  bridges second ends  271   e  of the pair of levers  271  on the movable mirror  22  side. The second link member  273  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. A width of the second link member  273  in the X-axis direction is narrower than a width of the first link member  272  in the X-axis direction. A length of the second link member  273  in the Y-axis direction is shorter than a length of the first link member  272  in the Y-axis direction. 
     The pair of beam members  274  respectively bridge the second portions  271   b  of the pair of levers  271  and the first link member  272 . The respective beam members  274  have a plate shape that extends along a plane perpendicular to the Z-axis direction. The pair of beam members  274  obliquely extend to approach each other as going away from the movable mirror  22 . The pair of levers  271 , the first link member  272 , the second link member  273 , and the pair of beam members  274  define the light passage opening  25 . The light passage opening  25  has a polygonal shape when viewed from the Z-axis direction. For example, the light passage opening  25  is a cavity (hole). Alternatively, a material having optical transparency with respect to the measurement light L 0  and the laser light L 10  may be disposed in the light passage opening  25 . 
     The intermediate member  275  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. The intermediate member  275  is disposed between the movable mirror  22  and the second link member  273  (in other words, between the movable mirror  22  and the light passage opening  25 ). The intermediate member  275  is connected to the movable mirror  22  through the non-linearity mitigation springs  278  as to be described later. 
     The pair of first torsion bars  276  respectively bridge the first end  271   d  of one lever  271  and the base  21 , and the first end  271   d  of the other lever  271  and the base  21 . That is, the pair of first torsion bars  276  are respectively connected between the pair of levers  271  and the base  21 . The first torsion bars  276  extend in the Y-axis direction. The pair of first torsion bars  276  are disposed on the same central line parallel to the Y-axis direction. In this embodiment, the central line of the first torsion bars  276  and the central line of the first link member  272  are located on the same straight line. A protrusion  271   f  that protrudes outward in the Y-axis direction is provided in each of the first ends  271   d  of the levers  271 , and each of the first torsion bars  276  is connected to the protrusion  271   f.    
     The pair of second torsion bars  277  respectively bridge the second end  271   e  of one lever  271  and one end of the intermediate member  275 , and the second end  271   e  of the other lever  271  and the other end of the intermediate member  275 . That is, the pair of second torsion bars  277  are respectively connected between the pair of levers  271  and the movable mirror  22 . The respective second torsion bars  277  extend in the Y-axis direction. The pair of second torsion bars  277  are disposed on the same central line parallel to the Y-axis direction. 
     The pair of non-linearity mitigation springs  278  are connected between the movable mirror  22  and the intermediate member  275 . That is, the pair of non-linearity mitigation springs  278  are connected between the movable mirror  22  and the second torsion bar  277 . Each of the non-linearity mitigation springs  278  includes a meandering portion  278   a  that extends in a meandering manner when viewed from the Z-axis direction. The meandering portion  278   a  includes a plurality of straight portions  278   b  which extend in the Y-axis direction and are aligned in the X-axis direction, and a plurality of folded portions  278   c  which alternately connect both ends of the plurality of straight portions  278   b . One end of the meandering portion  278   a  is connected to the intermediate member  275 , and the other end of the meandering portion  278   a  is connected to the frame portion  222 . In the meandering portion  278   a , a portion on the frame portion  222  side has a shape along the outer edge of the frame portion  222 . 
     The non-linearity mitigation spring  278  is constituted as follows. In a state in which the movable mirror  22  has moved in the Z-axis direction, the amount of deformation of the non-linearity mitigation spring  278  around the Y-axis direction becomes smaller than the amount of deformation of each of the first torsion bar  276  and the second torsion bar  277  around the Y-axis direction, and the amount of deformation of the non-linearity mitigation spring  278  in the X-axis direction becomes larger than the amount of deformation of each of the first torsion bar  276  and the second torsion bar  277  in the X-axis direction. Accordingly, it is possible to suppress occurrence of non-linearity in twist deformation of the first torsion bar  276  and the second torsion bar  277 , and it is possible to suppress deterioration of control characteristics of the movable mirror  22  due to the non-linearity. 
     The plurality of electrode support portions  279  includes a pair of first electrode support portions  279   a , a pair of second electrode support portions  279   b , and a pair of third electrode support portions  279   c . Each of the electrode support portions  279   a ,  279   b , and  279   c  has a plate shape that extends along a plane perpendicular to the Z-axis direction, and extends in the Y-axis direction. Each of the electrode support portions  279   a ,  279   b , and  279   c  extends from the second portion  271   b  of the lever  271  toward a side opposite to the light passage opening  25 . The pair of first electrode support portions  279   a  are disposed on the same central line parallel to the Y-axis direction. The pair of second electrode support portions  279   b  are disposed on the same central line parallel to the Y-axis direction. The pair of third electrode support portions  279   c  are disposed on the same central line parallel to the Y-axis direction. In the X-axis direction, the first electrode support portions  279   a , the second electrode support portions  279   b , and the third electrode support portions  279   c  are aligned in this order from the movable mirror  22  side. 
     The actuator unit  28  moves the movable mirror  22  in the Z-axis direction. The actuator unit  28  includes a fixed comb electrode  281 , a movable comb electrode  282 , a fixed comb electrode  283 , and a movable comb electrode  284 . Positions of the fixed comb electrodes  281  and  283  are fixed. The movable comb electrodes  282  and  284  move in accordance with movement of the movable mirror  22 . 
     The fixed comb electrode  281  is provided on a part of a surface, which faces the electrode support portions  269 , of the device layer  102  of the base  21 . The fixed comb electrode  281  includes a plurality of fixed comb fingers  281   a  which extend along a plane perpendicular to the Y-axis direction. The fixed comb fingers  281   a  are aligned in the Y-axis direction with a predetermined gap therebetween. 
     The movable comb electrode  282  is provided on a surface of each of the first electrode support portions  269   a  on the movable mirror  22  side, on surfaces of each of the second electrode support portions  269   b  on both sides in the X-axis direction, and on a surface of each of the third electrode support portion  269   c  on the movable mirror  22  side. The movable comb electrode  282  includes a plurality of movable comb fingers  282   a  which extend along a plane perpendicular to the Y-axis direction. The movable comb fingers  282   a  are aligned in the Y-axis direction with a predetermined gap therebetween. 
     In the fixed comb electrode  281  and the movable comb electrode  282 , the plurality of fixed comb fingers  281   a  and the plurality of movable comb fingers  282   a  are alternately arranged. That is, each of the fixed comb fingers  281   a  of the fixed comb electrode  281  is located between the movable comb fingers  282   a  of the movable comb electrode  282 . The fixed comb fingers  281   a  and the movable comb fingers  282   a , which are adjacent to each other, face each other in the Y-axis direction. A distance between the fixed comb finger  281   a  and the movable comb finger  282   a , which are adjacent to each other, is approximately several μm. 
     The fixed comb electrode  283  is provided on a part of a surface, which faces the electrode support portions  279 , of the device layer  102  of the base  21 . The fixed comb electrode  283  includes a plurality of fixed comb fingers  283   a  which extend along a plane perpendicular to the Y-axis direction. The fixed comb fingers  283   a  are aligned in the Y-axis direction with a predetermined gap therebetween. 
     The movable comb electrode  284  is provided on a surface of each of the first electrode support portion  279   a  on the movable mirror  22  side, on surfaces of each of the second electrode support portions  279   b  on both sides in the X-axis direction, and on a surface of each of the third electrode support portion  279   c  on the movable mirror  22  side. The movable comb electrode  284  includes a plurality of movable comb fingers  284   a  which extend along a plane perpendicular to the Y-axis direction. The movable comb fingers  284   a  are aligned in the Y-axis direction with a predetermined gap therebetween. 
     In the fixed comb electrode  283  and the movable comb electrode  284 , the plurality of fixed comb fingers  283   a  and the plurality of movable comb fingers  284   a  are alternately arranged. That is, each of the fixed comb fingers  283   a  of the fixed comb electrode  283  is located between the movable comb fingers  284   a  of the movable comb electrode  284 . The fixed comb fingers  283   a  and the movable comb fingers  284   a , which are adjacent to each other, face each other in the Y-axis direction. For example, a distance between the fixed comb finger  283   a  and the movable comb finger  284   a , which are adjacent to each other, is approximately several μm. 
     A plurality of electrode pads  211  are provided in the base  21 . The electrode pads  211  are disposed on a surface of the device layer  102  in an opening  213  formed in the first surface  21   a  of the base  21  to reach the device layer  102 . Some of the plurality of electrode pads  211  are electrically connected to the fixed comb electrode  281  or the fixed comb electrode  283  via the device layer  102 . Several other electrode pads  211  among the plurality of electrode pads  211  are electrically connected to the movable comb electrode  282  or the movable comb electrode  284  via the first elastic support unit  26  or the second elastic support unit  27 . In addition, a pair of electrode pads  212  which can be used as ground electrodes are provided in the base  21 . The pair of electrode pads  212  are disposed on the first surface  21   a  to be located on both sides of the movable mirror  22  in the Y-axis direction. 
     A configuration of the periphery of the electrode pads  211  will be described with reference to  FIG.  11   . Hereinafter, description will be made with reference to one electrode pad  211 , but other electrode pads  211  also have the same configuration. As illustrated in  FIG.  11   , each of the electrode pads  211  is disposed on a surface  102   a  of the device layer  102  on one side in the Z-axis direction in an opening  213  formed in a surface  101   a  of the support layer  101  on one side in the Z-axis direction to reach the device layer  102 . 
     The opening  213  includes a bottom surface  214  constituted by the surface  102   a , and a lateral surface  215  constituted by the support layer  101  and the intermediate layer  103 . For example, the bottom surface  214  has a rectangular shape. The lateral surface  215  includes a first surface  215   a  that extends continuously from the bottom surface  214  and approximately vertically to the bottom surface  214 , a stepped surface  215   b  that extends continuously from the first surface  215   a  and in approximately parallel to the bottom surface  214 , and a second surface  215   c  that extends continuously from the stepped surface  215   b  and approximately vertically to the bottom surface  214 . The stepped surface  215   b  extends in an annular shape along an edge of the opening  213  when viewed from the Z-axis direction. 
     The electrode pad  211  is disposed along an entire surface of the bottom surface  214 . In addition, the electrode pad  211  extends along the bottom surface  214  and the lateral surface  215 . More specifically, the electrode pad  211  is formed so that the electrode pad  211  reaches the first surface  215   a  of the lateral surface  215  and does not reach the stepped surface  215   b  of the lateral surface  215 . For example, the electrode pad  211  is constituted by a metal film (metal layer). For example, the metal film is formed by sputtering using a hard mask. The metal film that constitutes the electrode pad  211  is thicker than a metal film that constitutes the mirror surface  22   a.    
     The base  21  includes a groove  216  formed in the surface  101   a  of the support layer  101  to reach the device layer  102 . The groove  216  extends in an annular shape to surround the opening  213  when viewed from the Z-axis direction. For example, the groove  216  has a rectangular shape when viewed from the Z-axis direction. Because the groove  216  is formed, it is possible to reliably electrically insulate the electrode pads  211  from each other. That is, as in this embodiment, in a case where the metal film that constitutes the electrode pad  211  is formed to reach the lateral surface  215 , and the electrode pad  211  is in contact with the support layer  101 , there is a concern that the electrode pads  211  may be electrically connected to each other through the support layer  101 . In contrast, in the mirror device  20 , because the groove  216  is provided, even in the above-described case, it is possible to reliably electrically insulate the electrode pads  211  from each other. 
     In the mirror device  20  configured as described above, an electric signal for moving the movable mirror  22  in the Z-axis direction, is input to the drive unit  23  through a lead pin  113  to be described later and a wire (not illustrated). Accordingly, for example, an electrostatic force is generated between the fixed comb electrode  281  and the movable comb electrode  282  which face each other, and the fixed comb electrode  283  and the movable comb electrode  284  which face each other so that the movable mirror  22  moves to one side in the Z-axis direction. At this time, the first torsion bars  266  and  276  and the second torsion bars  267  and  277  in the first elastic support unit  26  and the second elastic support unit  27  are twisted, and an elastic force is generated in the first elastic support unit  26  and the second elastic support unit  27 . In the mirror device  20 , when a periodic electric signal is applied to the drive unit  23 , it is possible to reciprocate the movable mirror  22  in the Z-axis direction at a resonance frequency level. In this manner, the drive unit  23  functions as an electrostatic actuator. 
     [Another Configuration of Mirror Unit] 
     As illustrated in  FIG.  2   ,  FIG.  3   ,  FIG.  4   , and  FIG.  7   , the optical function member  13  includes the third surface  13   a  (a surface on the one side in the Z-axis direction) that faces the second surface  21   b  of the base  21 , and a fourth surface  13   b  opposite to the third surface  13   a . The optical function member  13  is disposed on the other side in the Z-axis direction with respect to the mirror device  20 . When viewed from the Z-axis direction, an outer edge  13   c  of the optical function member  13  is located outside of an outer edge  21   c  of the base  21 . That is, when viewed from the Z-axis direction, the outer edge  13   c  of the optical function member  13  surrounds the outer edge  21   c  of the base  21 . The optical function member  13  is integrally formed by a material having transparency with respect to the measurement light L 0  and the laser light L 10 . For example, the optical function member  13  is formed in a rectangular plate shape by glass, and has a size of approximately 15 mm×20 mm×4 mm (thickness). For example, the material of the optical function member  13  is selected in accordance with a sensitivity wavelength of the optical module  1 . For example, the material is set to glass in a case where the sensitivity wavelength of the optical module  1  is a near infrared region, and the material is set to silicon in a case where the sensitivity wavelength of the optical module  1  is an intermediate infrared region. 
     A pair of light transmitting portions  14  and  15  are provided in the optical function member  13 . The light transmitting portion  14  is a portion, which faces the light passage opening  24  of the mirror device  20  in the Z-axis direction, in the optical function member  13 . The light transmitting portion  15  is a portion, which faces the light passage opening  25  of the mirror device  20  in the Z-axis direction, in the optical function member  13 . A surface  14   a  of the light transmitting portion  14  on the mirror device  20  side, and a surface  15   a  of the light transmitting portion  15  on the mirror device  20  side are located on the same plane as the third surface  13   a . The light transmitting portion (second light passage portion)  14  constitutes a second portion (partial portion) of an optical path between the beam splitter unit  3  and the fixed mirror  16 . The light transmitting portion  14  is a portion that corrects an optical path difference that occurs between an optical path between the beam splitter unit  3  and the movable mirror  22 , and an optical path between the beam splitter unit  3  and the fixed mirror  16 . In this embodiment, the light transmitting portion  15  does not function as a light transmitting portion. 
     The optical function member  13  includes a fifth surface  13   d  that faces the movable mirror  22  and the drive unit  23  of the mirror device  20 . The fifth surface  13   d  is located on the fourth surface  13   b  side in comparison to the third surface  13   a . The fifth surface  13   d  extends to the outer edge  13   c  of the optical function member  13  when viewed from the Z-axis direction. In this embodiment, the fifth surface  13   d  extends to a pair of opposite sides which extend in the Y-axis direction in the outer edge  13   c  of the optical function member  13  while surrounding ends of the respective light transmitting portions  14  and  15  on the mirror device  20  side. 
     The third surface  13   a  of the optical function member  13  is joined to the second surface  21   b  of the base  21  by direct bonding (for example, plasma activation bonding, surface-activated room-temperature bonding (SAB), atomic diffusion bonding (ADB), anodic bonding, fusion bonding, hydrophilic bonding, and the like). In this embodiment, the third surface  13   a  extends to face a plurality of the electrode pads  211  and  212  provided in the base  21  on both sides of the fifth surface  13   d  in the Y-axis direction. Here, the fifth surface  13   d  is located on the fourth surface  13   b  side in comparison to the third surface  13   a , and thus the fifth surface  13   d  is separated from the mirror device  20  in a region where the fifth surface  13   d  faces the movable mirror  22  and the drive unit  23 . In addition, the surface  14   a  of the light transmitting portion  14  and the surface  15   a  of the light transmitting portion  15  respectively face the light passage openings  24  and  25  of the mirror device  20 . Accordingly, in the mirror unit  2 , when the movable mirror  22  reciprocates in the Z-axis direction, the movable mirror  22  and the drive unit  23  are prevented from coming into contact with the optical function member  13 . 
     A sixth surface  21   d , which is separated from the optical function member  13  in a state in which the third surface  13   a  of the optical function member  13  and the second surface  21   b  of the base  21  are joined to each other, is provided in the base  21  of the mirror device  20 . The sixth surface  21   d  is separated from the optical function member  13  in a region that includes at least a part of an outer edge of the base  21  when viewed from the Z-axis direction. In this embodiment, the sixth surface  21   d  is formed by removing the device layer  102  and the intermediate layer  103  along one side, which extends in the Y-axis direction, in the outer edge of the base  21  by etching. In addition, a plurality of reference holes  13   e  are formed in the third surface  13   a  of the optical function member  13 . In this embodiment, the plurality of reference holes  13   e  are formed in the third surface  13   a  to correspond to a plurality of corners of the base  21 . When the third surface  13   a  of the optical function member  13  and the second surface  21   b  of the base  21  are joined to each other, handling of the mirror device  20  is performed in a state in which a portion of the base  21  which corresponds to the sixth surface  21   d  is gripped, and thus a position of the mirror device  20  in the X-axis direction and the Y-axis direction, and an angle of the mirror device  20  in a plane perpendicular to the Z-axis direction are adjusted based on of the plurality of reference holes  13   e  formed in the third surface  13   a.    
     As illustrated in  FIG.  3    and  FIG.  4   , the fixed mirror  16  is disposed on the other side (side opposite to the mirror device  20 ) in the Z-axis direction with respect to the optical function member  13 , and a position of the mirror device  20  with respect to the base  21  is fixed. For example, the fixed mirror  16  is formed on the fourth surface  13   b  of the optical function member  13  by vapor deposition. The fixed mirror  16  includes a mirror surface  16   a  perpendicular to the Z-axis direction. In this embodiment, the mirror surface  22   a  of the movable mirror  22 , and the mirror surface  16   a  of the fixed mirror  16  face one side (beam splitter unit  3  side) in the Z-axis direction. The fixed mirror  16  is formed continuously with the fourth surface  13   b  of the optical function member  13  to reflect light that is transmitted through the respective light transmitting portions  14  and  15  of the optical function member  13 . However, a fixed mirror that reflects light transmitted through the light transmitting portion  14 , and a fixed mirror that reflects light transmitted through the light transmitting portion  15  may be provided, respectively. 
     The stress mitigation substrate  17  is attached to the fourth surface  13   b  of the optical function member  13  via the fixed mirror  16 . For example, the stress mitigation substrate  17  is attached to the fixed mirror  16 , for example, by an adhesive. When viewed from the Z-axis direction, an outer edge of the stress mitigation substrate  17  is located outside of the outer edge  13   c  of the optical function member  13 . That is, when viewed from the Z-axis direction, the outer edge of the stress mitigation substrate  17  surrounds the outer edge  13   c  of the optical function member  13 . A thermal expansion coefficient of the stress mitigation substrate  17  is closer to a thermal expansion coefficient of the base  21  of the mirror device  20  (more specifically, a thermal expansion coefficient of the support layer  101 ) in comparison to a thermal expansion coefficient of the optical function member  13 . In addition, the thickness of the stress mitigation substrate  17  is closer to the thickness of the base  21  of the mirror device  20  in comparison to the thickness of the optical function member  13 . For example, the stress mitigation substrate  17  is formed in a rectangular plate shape by silicon, and has a size of approximately 16 mm×21 mm×0.65 mm (thickness). 
     As illustrated in  FIG.  1   , the mirror unit  2  configured as described above is attached to the support  9  by fixing a surface of the stress mitigation substrate  17  on a side opposite to the optical function member  13  to a surface  9   a  of the support  9  (surface on the one side in the Z-axis direction), for example, by an adhesive. When the mirror unit  2  is attached to the support  9 , as illustrated in  FIG.  8   , a position of the mirror device  20  in the X-axis direction and the Y-axis direction and an angle of the mirror device  20  in a plane perpendicular to the Z-axis direction are adjusted based on a reference hole  9   b  formed in the support  9 . In  FIG.  8   , the second support structure  12  is not illustrated. 
     [Configuration of First Support Structure and Beam Splitter Unit] 
     As illustrated in  FIG.  1    and  FIG.  8   , the first support structure  11  includes a frame body  111 , a light transmitting member  112 , and a plurality of lead pins  113 . The frame body  111  is formed so as to surround the mirror unit  2  when viewed from the Z-axis direction, and is attached to the surface  9   a  of the support  9 , for example, by an adhesive such as silver solder. For example, the frame body  111  is formed of ceramic, and has a rectangular frame shape. An end surface  111   a  of the frame body  111  on a side opposite to the support  9  is located on a side opposite to the support  9  in comparison to the first surface  21   a  of the base  21  of the mirror device  20 . 
     The light transmitting member  112  is formed so as to close an opening of the frame body  111 , and is attached to the end surface  111   a  of the frame body  111 , for example, with an adhesive. The light transmitting member  112  is formed of a material having transparency with respect to the measurement light L 0  and the laser light L 10 , and has a rectangular plate shape for example. Here, the end surface  111   a  of the frame body  111  is located on a side opposite to the support  9  in comparison to the first surface  21   a  of the base  21  of the mirror device  20 , and thus the light transmitting member  112  is separated from the mirror device  20 . Accordingly, in the optical module  1 , when the movable mirror  22  reciprocates in the Z-axis direction, the movable mirror  22  and the drive unit  23  are prevented from coming into contact with the light transmitting member  112 . In the optical module  1 , the support  9 , the frame body  111 , and the light transmitting member  112  constitute a package that accommodates the mirror unit  2 . 
     The respective lead pins  113  are provided in the frame body  111  in such a manner that one end  113   a  is located inside of the frame body  111 , and the other end (not illustrated) is located outside of the frame body  111 . The one ends  113   a  of the lead pins  113  are electrically connected to the electrode pads  211  and  212  corresponding to the lead pins  113  in the mirror device  20  by wires (not illustrated). In the optical module  1 , an electric signal for moving the movable mirror  22  in the Z-axis direction is input to the drive unit  23  through the plurality of lead pins  113 . In this embodiment, a stepped surface  111   b  that extends in the X-axis direction on both sides of the optical function member  13  in the Y-axis direction is formed in the frame body  111 , and one end  113   a  of each of the lead pins  113  is disposed on the stepped surface  111   b . The lead pin  113  extends in the Z-axis direction on both sides of the support  9  in the Y-axis direction, and the other end of the lead pin  113  is located on the other side in the Z-axis direction in comparison to the support  9 . 
     As illustrated in  FIG.  10   , the beam splitter unit  3  is attached to a surface  112   a  of the light transmitting member  112  on a side opposite to the mirror device  20 , for example, by an optical adhesive that also functions as a refractive index matching agent. The beam splitter unit  3  includes a first mirror surface  31 , a second mirror surface  32 , and a plurality of optical surfaces  33   a ,  33   b ,  33   c , and  33   d . The beam splitter unit  3  is constituted by joining a plurality of optical blocks  34  and  35 . The respective optical blocks  34  and  35  are formed of a material having a refractive index that is the same as or similar to that of the optical function member  13 .  FIG.  10    is a schematic cross-sectional view of the mirror unit  2  and the beam splitter unit  3  illustrated in  FIG.  1   , and in  FIG.  10   , the mirror device  20  is schematically illustrated, for example, in a state in which dimensions in the Z-axis direction are enlarged in comparison to actual dimensions. 
     The first mirror surface  31  is a mirror surface (for example, a half mirror surface) inclined with respect to the Z-axis direction, and is formed between the optical block  34  and the optical block  35 . In this embodiment, the first mirror surface  31  is a surface that is parallel to the Y-axis direction, has an angle of 45° with respect to the Z-axis direction, and is inclined to be spaced away from the light incident unit  4  as it approaches the mirror device  20 . The first mirror surface  31  has a function of reflecting a part of the measurement light L 0  and allowing the remainder of the measurement light L 0  to be transmitted therethrough, and a function of reflecting a part of the laser light L 10  and allowing the remainder of the laser light L 10  to be transmitted therethrough. For example, the first mirror surface  31  is formed of a dielectric multi-layer film. The first mirror surface  31  overlaps the light passage opening  24  of the mirror device  20 , the light transmitting portion  14  of the optical function member  13 , and the mirror surface  16   a  of the fixed mirror  16  when viewed from the Z-axis direction, and overlaps the light incident unit  4  when viewed form the X-axis direction (refer to  FIG.  1   ). That is, the first mirror surface  31  faces the fixed mirror  16  in the Z-axis direction, and faces the light incident unit  4  in the X-axis direction. 
     The second mirror surface  32  is a mirror surface (for example, a total reflection mirror surface) that is parallel to the first mirror surface  31 , and is formed in the optical block  35  to be located on a side opposite to the light incident unit  4  with respect to the first mirror surface  31 . The second mirror surface  32  has a function of reflecting the measurement light L 0  and a function of reflecting the laser light L 10 . For example, the second mirror surface  32  is formed of a metal film. The second mirror surface  32  overlaps the mirror surface  22   a  of the movable mirror  22  of the mirror device  20  when viewed from the Z-axis direction, and overlaps the first mirror surface  31  when viewed from the X-axis direction. That is, the second mirror surface  32  faces the movable mirror  22  in the Z-axis direction, and faces the first mirror surface  31  in the X-axis direction. 
     The optical surface  33   a  is a surface perpendicular to the Z-axis direction, and is formed in the optical block  35  to be located on a side opposite to the mirror device  20  with respect to the first mirror surface  31 . The optical surface  33   b  is a surface perpendicular to the Z-axis direction, and is formed in the optical block  35  to be located on the mirror device  20  side with respect to the second mirror surface  32 . The optical surface  33   c  is a surface perpendicular to the Z-axis direction, and is formed in the optical block  34  to be located on the mirror device  20  side with respect to the first mirror surface  31 . The optical surface  33   b  and the optical surface  33   c  are located on the same plane. The optical surface  33   d  is a surface perpendicular to the X-axis direction, and is formed in the optical block  34  to be located on the light incident unit  4  side with respect to the first mirror surface  31 . The respective optical surfaces  33   a ,  33   b ,  33   c , and  33   d  have a function of allowing the measurement light L 0  to be transmitted therethrough, and a function of allowing the laser light L 10  to be transmitted therethrough. 
     The beam splitter unit  3  configured as described above is attached to the light transmitting member  112  by fixing the optical surface  33   b  and the optical surface  33   c  which are located on the same plane to the surface  112   a  of the light transmitting member  112 , for example, by an optical adhesive. When the beam splitter unit  3  is attached to the light transmitting member  112 , as illustrated in  FIG.  9   , a position of the beam splitter unit  3  in the X-axis direction and the Y-axis direction, and an angle of the beam splitter unit  3  in a plane perpendicular to the Z-axis direction are adjusted based on the reference hole  9   b  formed in the support  9 . In  FIG.  9   , the second support structure  12  is not illustrated. 
     Here, the optical path of the measurement light L 0  and the optical path of the laser light L 10  in the mirror unit  2  and the beam splitter unit  3  will be described in detail with reference to  FIG.  10   . 
     As illustrated in  FIG.  10   , when the measurement light L 0  is incident to the beam splitter unit  3  in the X-axis direction through the optical surface  33   d , a part of the measurement light L 0  is transmitted through the first mirror surface  31 , is reflected by the second mirror surface  32 , and reaches the mirror surface  22   a  of the movable mirror  22  through the optical surface  33   b  and the light transmitting member  112 . The part of the measurement light L 0  is reflected by the mirror surface  22   a  of the movable mirror  22 , and proceeds on the same optical path P 1  in an opposite direction, and is reflected by the first mirror surface  31 . The remainder of the measurement light L 0  is reflected by the first mirror surface  31 , and reaches the mirror surface  16   a  of the fixed mirror  16  through the optical surface  33   c , the light transmitting member  112 , the light passage opening  24  of the mirror device  20 , and the light transmitting portion  14  of the optical function member  13 . The remainder of the measurement light L 0  is reflected by the mirror surface  16   a  of the fixed mirror  16 , proceeds on the same optical path P 2  in an opposite direction, and is transmitted through the first mirror surface  31 . The part of the measurement light L 0  reflected by the first mirror surface  31 , and the remainder of the measurement light L 0  transmitted through the first mirror surface  31  become interference light L 1 , and the interference light L 1  of the measurement light is emitted from the beam splitter unit  3  through the optical surface  33   a  along the Z-axis direction. 
     On the other hand, when the laser light L 10  is incident to the beam splitter unit  3  in the Z-axis direction through the optical surface  33   a , a part of the laser light L 10  is reflected by the first mirror surface  31  and the second mirror surface  32 , and reaches the mirror surface  22   a  of the movable mirror  22  through the optical surface  33   b  and the light transmitting member  112 . The part of the laser light L 10  is reflected by the mirror surface  22   a  of the movable mirror  22 , proceeds on the same optical path P 3  in an opposite direction, and is reflected by the first mirror surface  31 . The remainder of the laser light L 10  is transmitted through the first mirror surface  31 , and reaches the mirror surface  16   a  of the fixed mirror  16  through the optical surface  33   c , the light transmitting member  112 , the light passage opening  24  of the mirror device  20 , and the light transmitting portion  14  of the optical function member  13 . The remainder of the laser light L 10  is reflected by the mirror surface  16   a  of the fixed mirror  16 , proceeds on the same optical path P 4  in an opposite direction, and is transmitted through the first mirror surface  31 . The part of the laser light L 10  reflected by the first mirror surface  31 , and the remainder of the laser light L 10  transmitted through the first mirror surface  31  become interference light L 11 , and the interference light L 11  of the laser light is emitted from the beam splitter unit  3  through the optical surface  33   a  along the Z-axis direction. 
     As described above, the light passage opening  24  of the mirror device  20  constitutes a first portion P 2   a  of the optical path P 2  of the measurement light L 0  and a first portion P 4   a  of the optical path P 4  of the laser light L 10  in an optical path between the beam splitter unit  3  and the fixed mirror  16 . In addition, the light transmitting portion  14  of the optical function member  13  constitutes a second portion P 2   b  of the optical path P 2  of the measurement light L 0  and a second portion P 4   b  of the optical path P 4  of the laser light L 10  in the optical path between the beam splitter unit  3  and the fixed mirror  16 . 
     The second portion P 2   b  of the optical path P 2  of the measurement light L 0  is constituted by the light transmitting portion  14 , thus an optical path difference between both the optical paths P 1  and P 2  is corrected so that a difference between an optical path length (optical path length in consideration of a refractive index of respective media through which the optical path passes) of the optical path P 1  of the measurement light L 0 , and an optical path length of the optical path P 2  of the measurement light L 0  decreases. Similarly, the second portion P 4   b  of the optical path P 4  of the laser light L 10  is constituted by the light transmitting portion  14 , thus an optical path difference between both the optical paths P 3  and P 4  is corrected so that a difference between an optical path length of the optical path P 3  of the laser light L 10  and an optical path length of the optical path P 4  of the laser light L 10  decreases. In this embodiment, a refractive index of the light transmitting portion  14  is the same as a refractive index of the respective optical blocks which constitute the beam splitter unit  3 , and a distance between the first mirror surface  31  and the second mirror surface  32  in the X-axis direction is the same as the thickness of the light transmitting portion  14  in the Z-axis direction (that is, a distance between the surface  14   a  of the light transmitting portion  14  and the fourth surface  13   b  of the optical function member  13  in the Z-axis direction). 
     [Configuration of Second Support Structure, Light Incident Unit, and the Like] 
     As illustrated in  FIG.  1   , the second support structure  12  includes a connection unit  120 . The connection unit  120  includes a main body portion  121 , a frame body  122 , and a fixing plate  123 . The main body portion  121  includes a pair of side wall portions  124  and  125 , and a ceiling wall portion  126 . The pair of side wall portions  124  and  125  face each other in the X-axis direction. An opening  124   a  is formed in the side wall portion  124  on one side in the X-axis direction. The ceiling wall portion  126  faces the support  9  in the Z-axis direction. An opening  126   a  is formed in the ceiling wall portion  126 . For example, the main body portion  121  is integrally formed of a metal. A plurality of positioning pins  121   a  are provided in the main body portion  121 . The main body portion  121  is positioned with respect to the support  9  by inserting the positioning pins  121   a  into the reference hole  9   b  and the hole  9   c  which are formed in the support  9 , and is attached to the support  9  in this state, for example, by a bolt. 
     The frame body  122  is disposed on a surface of the side wall portion  124  on a side opposite to the beam splitter unit  3 . An opening of the frame body  122  faces the beam splitter unit  3  through the opening  124   a  of the side wall portion  124 . The light incident unit  4  is disposed in the frame body  122 . The fixing plate  123  is a member that fixes the light incident unit  4  disposed in the frame body  122  to the main body portion  121  (details will be described later). 
     The second support structure  12  further includes a holding unit  130 . The holding unit  130  includes a main body portion  131 , a frame body  132  and a fixing plate  133 . The main body portion  131  is attached to a surface of the ceiling wall portion  126  which is opposite to the support  9 . The main body portion  131  is positioned with respect to the main body portion  121  of the connection unit  120  by a plurality of positioning pins  131   a , and is attached to the ceiling wall portion  126  in this state, for example, by a bolt. A depression  134  is formed in a surface of the main body portion  131  which is opposite to the support  9 . A first light passage hole  135 , a second light passage hole  136 , and a third light passage hole  137  are formed in a bottom surface of the depression  134 . The first light passage hole  135  is formed at a position that faces the first mirror surface  31  of the beam splitter unit  3  in the Z-axis direction. The second light passage hole  136  is formed on the other side of the first light passage hole  135  in the X-axis direction (that is, on a side opposite to the light incident unit  4 ). The third light passage hole  137  is formed on the other side of the second light passage hole  136  in the X-axis direction. 
     The frame body  132  is disposed on the bottom surface of the depression  134 . An opening of the frame body  132  faces the third light passage hole  137 . The second light source  7  is disposed in the frame body  132 . The first light detector  6  is disposed on the bottom surface of the depression  134  in a state of facing the first light passage hole  135 . The second light detector  8  is disposed on the bottom surface of the depression  134  in a state of facing the second light passage hole  136 . The fixing plate  133  is a member that fixes the first light detector  6  and the second light detector  8  which are disposed on the bottom surface of the depression  134 , and the second light source  7  disposed in the frame body  132  to the main body portion  131  (details will be described later). 
     The light incident unit  4  includes a holder  41  and a collimator lens  42 . The holder  41  holds the collimator lens  42 , and is configured so that an optical fiber (not illustrated) that guides the measurement light L 0  can be connected to the holder  41 . The collimator lens  42  collimates the measurement light L 0  emitted from the optical fiber. When the optical fiber is connected to the holder  41 , an optical axis of the optical fiber matches an optical axis of the collimator lens  42 . 
     A flange portion  41   a  is provided in the holder  41 . The flange portion  41   a  is disposed between the frame body  122  and the fixing plate  123 . In this state, fixing plate  123  is attached to the side wall portion  124 , for example, by a bolt, and the light incident unit  4  disposed in the frame body  122  is fixed to the main body portion  121 . In this manner, the light incident unit  4  is disposed on one side of the beam splitter unit  3  in the X-axis direction, and is supported by the second support structure  12 . The light incident unit  4  allows measurement light L 0  that is incident from the first light source through a measurement target or measurement light L 0  that is generated from the measurement target (in this embodiment, the measurement light L 0  guided by the optical fiber) to be incident to the beam splitter unit  3 . 
     A filter  54  is attached to the frame body  122 . The filter  54  has a function of cutting off the laser light L 10 . The filter  54  is disposed in the opening  124   a  of the side wall portion  124  in a state of being inclined with respect to an optical axis of the light incident unit  4 . The filter  54  closes the opening of the frame body  122  when viewed form the X-axis direction. In this manner, the filter  54  is disposed between the light incident unit  4  and the beam splitter unit  3 , and is supported by the second support structure  12  in a state of being inclined with respect to an optical axis of the light incident unit  4 . In this embodiment, an optical surface of the filter  54  is a surface that is parallel to the Z-axis direction and has an angle of 10° to 20° with respect to the Y-axis direction. The optical axis of the light incident unit  4  is parallel to the X-axis direction. 
     Accordingly, even when light in the same wavelength range as the laser light L 10  is included in the measurement light L 0 , the light is prevented from being incident to the beam splitter unit  3 , and thus it is possible to obtain a position of the movable mirror  22  in the Z-axis direction with accuracy based on a detection result of the interference light L 11  of the laser light. In addition, because the filter  54  is inclined with respect to the optical axis of the light incident unit  4 , light in the same wavelength range as the laser light L 10  is reflected to the outside of an interference optical system, and thus it is possible to reliably prevent the light from being stray light. In this embodiment, light in the same wavelength range as the laser light L 10  emitted from the beam splitter unit  3  in the X-axis direction is reflected by the filter  54 , and is emitted to the outside of the interference optical system from between the pair of side wall portions  124  and  125  in the main body portion  121  of the second support structure  12 . Accordingly, it is possible to reliably prevent the light from being stray light. 
     The first light detector  6  includes a holder  61 , a light detection element  62 , and a condensing lens  63 . The holder  61  holds the light detection element  62  and the condensing lens  63 . The light detection element  62  detects the interference light L 1  of the measurement light. For example, the light detection element  62  is an InGaAs photodiode. The condensing lens  63  condenses the interference light L 1  of the measurement light incident to the light detection element  62  to the light detection element  62 . In the holder  61 , an optical axis of the light detection element  62  and an optical axis of the condensing lens  63  match each other. 
     A flange portion  61   a  is provided in the holder  61 . The flange portion  61   a  is disposed between the bottom surface of the depression  134  of the main body portion  131 , and the fixing plate  133 . In this state, the fixing plate  133  is attached to the main body portion  131 , for example, by a bolt, and thus the first light detector  6  disposed on the bottom surface of the depression  134  is fixed to the main body portion  131 . In this manner, the first light detector  6  is disposed on one side of the beam splitter unit  3  in the Z-axis direction, and is supported by the second support structure  12 . The first light detector  6  faces the first mirror surface  31  of the beam splitter unit  3  in the Z-axis direction. The first light detector  6  detects the interference light L 1  of the measurement light emitted from the beam splitter unit  3 . 
     The second light detector  8  includes a holder  81 , a light detection element  82 , and a condensing lens  83 . The holder  81  holds the light detection element  82  and the condensing lens  83 . The light detection element  82  detects the interference light L 11  of the laser light. For example, the light detection element  82  is a Si photodiode. The condensing lens  83  condenses the interference light L 11  of the laser light incident to the light detection element  82  to the light detection element  82 . In the holder  81 , an optical axis of the light detection element  82  and an optical axis of the condensing lens  83  match each other. 
     A flange portion  81   a  is provided in the holder  81 . The flange portion  81   a  is disposed between the bottom surface of the depression  134  of the main body portion  131 , and the fixing plate  133 . In this state, the fixing plate  133  is attached to the main body portion  131 , for example, by a bolt, and thus the second light detector  8  disposed on the bottom surface of the depression  134  is fixed to the main body portion  131 . In this manner, the second light detector  8  is disposed on one side of the beam splitter unit  3  in the Z-axis direction, and is supported by the second support structure  12 . The second light detector  8  detects the interference light L 11  of the laser light emitted from the beam splitter unit  3 . 
     The second light source  7  includes a holder  71 , a light-emitting element  72 , and a collimator lens  73 . The holder  71  holds the light-emitting element  72  and the collimator lens  73 . The light-emitting element  72  emits the laser light L 10 . For example, the light-emitting element  72  is a semiconductor laser such as a VCSEL. The collimator lens  73  collimates the laser light L 10  emitted from the light-emitting element  72 . In the holder  71 , an optical axis of the light-emitting element  72  and an optical axis of the collimator lens  73  match each other. 
     A flange portion  71   a  is provided in the holder  71 . The flange portion  71   a  is disposed between the frame body  132  and the fixing plate  133 . In this state, the fixing plate  133  is attached to the main body portion  131 , for example, by a bolt, and thus the second light source  7  disposed in the frame body  132  is fixed to the main body portion  131 . In this manner, the second light source  7  is disposed on one side of the beam splitter unit  3  in the Z-axis direction, and is supported by the second support structure  12 . The second light source  7  emits the laser light L 10  that is to be incident to the beam splitter unit  3 . 
     As described above, the holding unit  130  holds the first light detector  6 , the second light detector  8 , and the second light source  7  so that the first light detector (first optical device)  6 , the second light detector (second optical device)  8 , and the second light source (third optical device)  7  face the same side, and the first light detector  6 , the second light detector  8 , and the second light source  7  are aligned in this order. In this embodiment, on one side of the beam splitter unit  3  in the Z-axis direction, the holding unit  130  holds the first light detector  6 , the second light detector  8 , and the second light source  7  so that the first light detector  6 , the second light detector  8 , and the second light source  7  face the other side in the Z-axis direction (that is, the beam splitter unit  3  side). In addition, the holding unit  130  holds the first light detector  6 , the second light detector  8 , and the second light source  7  so that the first light detector  6 , the second light detector  8 , and the second light source  7  are aligned in this order from one side (that is, the light incident unit  4  side) in the X-axis direction. 
     A first mirror  51 , a second mirror  52 , and a third mirror  53  are attached to the main body portion  131  of the holding unit  130 . The first mirror  51  is held by the holding unit  130  to be located on a side opposite to the first light detector  6  with respect to the first light passage hole  135 . The second mirror  52  is held by the holding unit  130  to be located on a side opposite to the second light detector  8  with respect to the second light passage hole  136 . The third mirror  53  is held by the holding unit  130  to be located on a side opposite to the second light source  7  with respect to the third light passage hole  137 . 
     The first mirror  51  is a dichroic mirror that has a function of allowing the measurement light L 0  to be transmitted therethrough and of reflecting the laser light L 10 , and is inclined with respect to the optical axis of the first light detector  6 . The first mirror  51  is disposed between the beam splitter unit  3  and the first light detector  6 . That is, the first mirror  51  is disposed to face the beam splitter unit  3  and the first light detector  6 . In this embodiment, an optical surface of the first mirror  51  is a surface that is parallel to the Y-axis direction and has an angle of 45° with respect to the Z-axis direction. The optical axis of the first light detector  6  is parallel to the Z-axis direction. 
     The second mirror  52  is a mirror (for example, a half mirror) that has a function of reflecting a part of the laser light L 10  and allowing the remainder of the laser light L 10  to be transmitted therethrough, and is parallel to the first mirror  51 . The second mirror  52  is disposed to overlap the first mirror  51  when viewed from the X-axis direction, and to overlap the second light detector  8  when viewed from the Z-axis direction. That is, the second mirror  52  is disposed to face the first mirror  51  and the second light detector  8 . In this embodiment, an optical surface of the second mirror  52  is a surface that is parallel to the Y-axis direction, and has an angle of 45° with respect to the Z-axis direction. 
     The third mirror  53  is a mirror (for example, a total reflection mirror) that has a function of reflecting the laser light L 10  and is parallel to the second mirror  52 . The third mirror  53  is disposed to overlap the second mirror  52  when viewed from the X-axis direction, and overlap the second light source  7  when viewed from the Z-axis direction. That is, the third mirror  53  is disposed to face the second mirror  52  and the second light source  7 . In this embodiment, an optical surface of the third mirror  53  is a surface that is parallel to the Y-axis direction, and has an angle of 45° with respect to the Z-axis direction. 
     An aperture  55  is attached to the main body portion  131  of the holding unit  130 . The aperture  55  is held by the holding unit  130  to be located between the first mirror  51  and the first light detector  6 . The aperture  55  is a member in which an opening having a circular shape is formed when viewed from the Z-axis direction, and is disposed in the first light passage hole  135 . 
     The interference light L 1  of the measurement light, which is emitted from the beam splitter unit  3  in the Z-axis direction, is transmitted through the first mirror  51 , is incident to the first light detector  6  through the aperture  55 , and is detected by the first light detector  6 . On the other hand, the laser light L 10  emitted from the second light source  7  is reflected by the third mirror  53 , is transmitted through the second mirror  52 , is reflected by the first mirror  51 , and is incident to the beam splitter unit  3  in the Z-axis direction. The interference light L 11  of the laser light, which is emitted from the beam splitter unit  3  in the Z-axis direction, is reflected by the first mirror  51  and the second mirror  52 , is incident to the second light detector  8 , and is detected by the second light detector  8 . 
     [Method for Manufacturing Mirror Device] 
     A method for manufacturing the mirror device  20  according to the first embodiment will be described with reference to  FIG.  12    to  FIG.  17 B . In  FIG.  12    to  FIG.  17 B , respective configurations of the mirror device  20  are schematically illustrated. In the mirror device  20  illustrated in  FIG.  12   , the rib portion  224  includes a first rib portion  224   d  provided in the movable unit  22   b , and a second rib portion  224   e  provided in the first elastic support unit  26  and the second elastic support unit  27 . For example, the first rib portion  224   d  and the second rib portion  224   e  has the same configuration as in the inner rib portion  224   a , the outer rib portion  224   b , and the connection rib portions  224   c . For example, the second rib portion  224   e  may be disposed on each of the levers  261  and  271  illustrated in  FIG.  6   , or instead of or in addition to this, the second rib portion  224   e  may be disposed on each of the electrode support portions  269  and  279  illustrated in  FIG.  6   . The rib portion  224  may include only one of the first rib portion  224   d  and the second rib portion  224   e . In  FIG.  12   , illustration of the second rib portion  224   e  provided in the second elastic support portion  27  is omitted. 
     In the manufacturing method according to the first embodiment, first, the SOI substrate  100  including portions corresponding to the base  21 , the movable unit  22   b , and the drive unit  23  is prepared (first step). In the first step, the SOI substrate  100  in which the support layer  101  is thicker than the device layer  102  is prepared. Note that, for example, the “portions corresponding to the base  21 , the movable unit  22   b , and the drive unit  23 ” represents portions which become the base  21 , the movable unit  22   b , and the drive unit  23  after processing. 
     Next, a first resist layer  104  is formed in a region corresponding to the base  21  on the surface  101   a  of the support layer  101  which is opposite to the intermediate layer  103  (second step,  FIG.  13 A ). The first resist layer  104  is not formed in a region corresponding to a region, on which each of the electrode pads  211  is to be formed, on the surface  101   a . Note that, a “region corresponding to an element on an surface” represents, for example, a region that overlaps the element on the surface when viewed from the Z-axis direction. For example, a region corresponding to the base  21  on the surface  101   a  represents a region that overlaps the base  21  on the surface  101   a  when viewed from the Z-axis direction. 
     Next, the support layer  101  is etched up to an intermediate portion in the thickness direction (Z-axis direction) using the first resist layer  104  as a mask to form a depression  105  in the support layer  101  (third step). Next, the first resist layer  104  is peeled off ( FIG.  13 B ). In other words, in the third step, the support layer  101  is etched so as not to reach the intermediate layer  103 . The depth of the depression  105  is determined in accordance with the thickness of the rib portion  224 . In the case of forming the mirror device  20 , a plurality of the depressions  105  are formed in the third step. One of the depressions  105  is formed at a position corresponding to the movable unit  22   b  and the drive unit  23 . The other depressions  105  are formed at positions corresponding to positions where the electrode pads  211  are to be formed. 
     Next, a second resist layer  106  is formed in a region corresponding to the rib portion  224  in a bottom surface  105   a  of each of the depressions  105 , on a side surface  105   b  of the depression  105 , and on the surface  101   a  of the support layer  101  (forth step,  FIG.  14 A ). The second resist layer  106  is formed on an entire surface of the side surface  105   b  and on an entire surface of the surface  101   a . As illustrated in  FIG.  14 A , the second resist layer  106  is also formed in a region along a boundary with the side surface  105   b  in the bottom surface  105   a . The reason for this is because a part of the second resist layer  106  remains along the boundary with the side surface  105   b  in the bottom surface  105   a  when patterning the second resist layer  106 . For example, the second resist layer  106  is formed by a spray coating method, but may be formed by a dip coating method. 
     Next, the support layer  101  is etched until reaching the intermediate layer  103  using the second resist layer  106  as a mask to form the rib portion  224  (fifth step). Next, after peeling off the second resist layer  106 , the exposed intermediate layer  103  is peeled off (FIG.  14 B). Since the second resist layer  106  is formed to reach the bottom surface  105   a  from the side surface  105   b  of the depression  105 , in the fifth step, a stepped surface  107  is formed in the base  21 . The stepped surface  107  extends along an edge of each of the depressions  105  when viewed from the Z-axis direction. In  FIG.  5    and  FIG.  6   , the stepped surface  107  is illustrated. In addition, in the second step to the fifth step, the opening  213  and the groove  216  ( FIG.  11   ) described above are formed in parallel with the formation of the rib portion  224 . The stepped surface  107  corresponds to the above-described stepped surface  215   b.    
     Next, a third resist layer  108  is formed in a region corresponding to the base  21 , the movable unit  22   b , and the drive unit  23  on a surface  102   b  of the device layer  102  which is opposite to the intermediate layer  103  ( FIG.  15 A ). Next, the device layer  102  is etched through using the third resist layer  108  as a mask. Accordingly, the base  21 , the movable unit  22   b , and the drive unit  23  are separated from each other. Next, the third resist layer  108  is peeled off ( FIG.  15 B ). 
     Next, the mirror surface  22   a  is formed on the surface  102   a  of the device layer  102  on the intermediate layer  103  side (sixth step). For example, the mirror surface  22   a  is formed through sputtering using a hard mask. Next, the electrode pads  211  are formed on the surface  102   a  of the device layer  102 , and the electrode pads  212  are formed on the surface  101   a  of the support layer  101  ( FIG.  12   ). For example, the electrode pads  211  and  212  are formed through sputtering using a hard mask. Through the above-described processes, the mirror device  20  illustrated in  FIG.  12    is obtained. 
     Note that, the order of the processes is not limited to the above-described example. For example, formation of the third resist layer  108  and etching of the device layer  102  may be carried out before the second step or before the fourth step. Formation of the electrode pads  211  and electrode pads  212  may be carried out before forming the mirror surface  22   a . Formation of the electrode pads  211  and  212  may be carried out between the fifth step and the sixth step. In the above description, it is explained with focus given to the one mirror device  20 , but each of processing steps may be performed with respect to a wafer (semiconductor substrate) including a plurality of the portions corresponding to the SOI substrate  100 , and mirror devices  20  which have been processed may be diced along a dicing line set to a boundary between the portions corresponding to the SOI substrate  100  to collectively form a plurality of the mirror devices  20 . In this case, for example, the wafer is cut off in a state in which the wafer is fixed by adhering the surface  101   a  of the support layer  101  to a dicing tape. 
     [Function and Effect of First Embodiment] 
     A method for manufacturing a mirror device according to a comparative example will be described with reference to  FIG.  16 A  to  FIG.  18   . The mirror device manufactured by the manufacturing method of the comparative example has a similar configuration as in the mirror device  20  of the first embodiment. That is, the mirror device of the comparative example includes a base  21 A, a movable unit  22   b A, and a drive unit  23 A ( FIG.  18   ). The movable unit  22   b A includes a rib portion  224 A. In the manufacturing method of the comparative example, first, an SOI substrate  100 A including a support layer  101 A, a device layer  102 A, and an intermediate layer  103 A is prepared. The SOI substrate  100 A includes portions corresponding to the base  21 A, the movable unit  22   b A, and the drive unit  23 A. Next, an oxide film  109   a  is formed on a surface  101   a A of the support layer  101 A which is opposite to the intermediate layer  103 A, and an oxide film  109   b  is formed on a surface  102   b A of the device layer  102 A which is opposite to the intermediate layer  103 A. 
     Next, a resist layer  110   a  is formed in regions corresponding to the base  21 A and the rib portion  224 A on a surface of the oxide film  109   a  ( FIG.  16 A ). Next, after the oxide film  109   a  is etched using the resist layer  110   a  as a mask, the resist layer  110   a  is peeled off ( FIG.  16 B ). Next, a resist layer  110   b  is formed in a region corresponding to the base  21  on the surface of the oxide film  109   a  ( FIG.  17 A ). Next, when the support layer  101 A is etched up to the intermediate layer  103 A using the resist layer  110   b  as a mask, the rib portion  224 A is formed ( FIG.  17 B ). The reason for this is as follows. At the time of the etching, the exposed oxide film  109   a  is simultaneously etched, and thus in the region in which the oxide film  109   a  is formed, an etching rate is slower in comparison to a region in which the oxide film  109   a  is not formed. Next, after the resist layer  110   b , the oxide film  109   a , and the oxide film  109   b  are peeled off, the exposed intermediate layer  103 A is removed ( FIG.  18   ). 
     It is also possible to obtain a mirror device including the rib portion  224 A by the manufacturing method of the comparative example, but in the manufacturing method of the comparative example, it is difficult to form the rib portion  224 A with accuracy. The reason for this will be described with reference to  FIG.  19 A  to  FIG.  21 B .  FIG.  19 A ,  FIG.  20 A , and  FIG.  21 A  schematically illustrate a state in the middle of the etching for forming the rib portion  224 A, and  FIG.  19 B ,  FIG.  20 B , and  FIG.  21 B  schematically illustrate a state after the etching. 
     As illustrated in  FIG.  19 A  and  FIG.  19 B , in the manufacturing method of the comparative example (silicon deep digging etching, Bosch process), the rib portion  224 A is likely to be formed in an inverted taper shape in which a width of a tip end is wider than a width of a base end. Accordingly, it is difficult to control the width of the rib portion  224 A to a desired width. Depending on a type of an applied etching process (for example, in a case where a non-Bosch process is used), the rib portion  224 A may be likely to be formed in a forward taper shape in which the width of the tip end is narrower than the width of the base end, but it is also difficult to control the width of the rib portion  224 A. 
     In addition, as illustrated in  FIG.  20 A  and  FIG.  20 B , in the manufacturing method of the comparative example, in a case where the rib portion  224 A includes portions  224 Aa and  224 Ab having widths different from each other, there is a concern that a difference occurs in an etching rate between the portions  224 Aa and  224 Ab, and thus a height of the portion  224 Aa and a height of the portion  224 Ab may be different from each other. A reference numeral DP indicates a deposited layer formed on a side surface of the rib portion  224 A during the etching. The difference in the etching rate between the portions  224 Aa and  224 Ab occurs due to a microloading effect in which the etching rate decreases as an aspect ratio (a ratio between pattern dimensions and a depth) of a groove portion surrounding the rib porting  224 A increases. In a case where the height of the rib portion  224 A is same, as the width of the groove portion is narrower, the aspect ratio increases, and thus etching is slower. 
     In addition, as illustrated in  FIG.  21 A  and  FIG.  21 B , in the manufacturing method of the comparative example, the oxide film  109   a  is completely removed in the middle of the etching for forming the rib portion  224 A, but in a case where the etching is continuously progressed, a residue RD is likely to occur in an edge portion of the rib portion  224 A. The reason for this is considered as follows. When the aspect ratio of the groove portion surrounding the rib portion  224 A is high, an etching rate is slow, and the rib portion  224 A is likely to be formed in a forward taper shape. In the case of the forward taper shape, particularly, as the etching is progressed, removal of the deposited layer DP on a top surface of the rib portion  224 A is difficult, and the deposited layer DP is likely to remain near a peripheral edge of the top surface. The deposited layer DP that remains near the peripheral edge serves as a mask, and the support layer  101 A is not etched. As a result, the residue RD occurs. 
     As described above, in the manufacturing method of the comparative example, it is difficult to form the rib portion  224 A with accuracy. In contrast, in the method for manufacturing the mirror device  20  according to the first embodiment, as described above, the rib portion  224  is formed by two-stage etching using the first resist layer  104  and the second resist layer  106  as a mask. Accordingly, it is possible to form the rib portion  224  with accuracy. That is, in the manufacturing method of the first embodiment, it is easy to control the width of the rib portion  224  in comparison to the manufacturing method of the comparative example. In addition, in a case where the rib portion  224  includes portions having widths different from each other, it is easy to form the portions in the same height. In addition, it is possible to suppress occurrence of a residue at an edge portion of the rib portion  224 . 
     In addition, in the mirror device  20 , the support layer  101  constituting the rib portion  224  is thinner than the support layer  101  constituting the base  21 . Accordingly, the rib portion  224  is suppressed from protruding from the base  21 , and thus it is possible to realize protection of the rib portion  224 . More specifically, at least during stoppage of the movable unit  22   b , the rib portion  224  does not protrude from the base  21 , and thus it is possible to realize protection of the rib portion  224  during stoppage of the movable unit  22   b . In addition, since the rib portion  224  does not protrude from the base  21  during movement of the movable unit  22   b , it is possible to realize protection of the rib portion  224  during movement of the movable unit  22   b . In addition, since the rib portion  224  is suppressed from protruding from the base  21 , it is possible to increase a movement amount of the movable unit  22   b  in the Z-axis direction. In addition, in a case where the thickness of the support layer  101  constituting the rib portion  224  is the same as the thickness of the support layer  101  constituting the base  21 , when dicing the SOI substrate  100 , the rib portion  224  is adhered to the dicing tape. Accordingly, there is a concern that the rib portion  224  may be broken during peeling-off. However, in the mirror device  20 , it is possible to avoid the problem. 
     In the first step of the manufacturing method according to the first embodiment, the SOI substrate  100  in which the support layer  101  is thicker than the device layer  102  is prepared. In the mirror device  20  obtained, it is possible to secure the thickness of the rib portion  224 , and it is possible to more appropriately suppress deformation of the movable unit  22   b.    
     In the sixth step of the manufacturing method according to the first embodiment, the mirror surface  22   a  is formed on the surface  102   a  of the device layer  102  on the intermediate layer  103  side. Accordingly, it is possible to protect mirror surface  22   a  by the base  21  and the rib portion  224 , and it is possible to suppress damage of the mirror surface  22   a  due to direct contact, for example, during transportation or the like. 
     In the mirror device  20 , the rib portion  224  includes a plurality of portions (the inner rib portion  224   a , the outer rib portion  224   b , and the connection rib portion  224   c , or the first rib portion  224   d  and the second rib portion  224   e ) having widths different from each other. In the manufacturing method of the first embodiment, even in a case where the rib portion  224  includes a plurality of portions having widths different from each other, it is possible to form the rib portion  224  with accuracy. 
     In the mirror device  20 , the first elastic support unit  26  and the second elastic support unit  27  support the movable unit  22   b  so that the movable unit  22   b  can move along the Z-axis direction. In the manufacturing method of the first embodiment, in the case of manufacturing the mirror device  20 , it is possible to form the rib portion  224  with accuracy. 
     The mirror device  20  includes the fixed comb electrodes  281  and  283  and the movable comb electrodes  282  and  284 . In the manufacturing method of the first embodiment, in the case of manufacturing the mirror device  20 , it is possible to form the rib portion  224  with accuracy. 
     Modification Example of First Embodiment 
     In the first embodiment, materials and shapes of respective configurations are not limited to the above-described materials and shapes, and various materials and shapes can be employed. For example, each of the arrangement portion  221  and the mirror surface  22   a  may have any shape such as a rectangular shape and an octagonal shape when viewed from the Z-axis direction. The frame portion  222  may have any ring shape such as a rectangular ring shape and an octagonal ring shape when viewed from the Z-axis direction. Each of the light passage opening  24  and the light passage opening  25  may have any shape such as a circular shape and an octagonal shape when viewed from the Z-axis direction. The mirror device  20  may include a hole or a notch formed in the base  21  as a first light passage portion instead of the light passage opening  24  or the light passage opening  25 . The semiconductor substrate that constitutes the mirror device  20  may not be an SOI substrate, and may be a substrate including a first semiconductor layer, a second semiconductor layer, and an insulating layer disposed between the first semiconductor layer and the second semiconductor layer. 
     Each of the inner rib portion  224   a , the outer rib portion  224   b , and the connection rib portion  224   c  may be formed in any shape. For example, the rib portions may extend obliquely with respect to X-axis direction or the Y-axis direction, or may extend in a zigzag shape. The arrangement, the number, the length, the width, and the thickness of the rib portions may be arbitrarily set. For example, the thicknesses of the inner rib portion  224   a , the outer rib portion  224   b , and the connection rib portion  224   c  may be different from each other. At least one of the rib portions may be omitted. The rib portion  224  may not surround the mirror surface  22   a  when viewed from the Z-axis direction. The shape of the first torsion bars  266  and  276 , and the second torsion bars  267  and  277  are not limited, and may be any shape such as a rod shape. The mirror surface  22   a  may be disposed on a surface, which is opposite to the intermediate layer  103 , in the device layer  102  that constitutes the movable unit  22   b . In this case, for example, the surface, which is opposite to the intermediate layer  103 , in the support layer  101  that constitutes the base  21  is joined to the third surface  13   a  of the optical function member  13 . The second surface  21   b  of the base  21  and the third surface  13   a  of the optical function member  13  may be joined to each other with means (for example, an adhesive such as a UV-curable resin) other than the direct bonding. In a case where the fixed mirror  16  is disposed on a side opposite to the mirror device  20  with respect to the optical function member  13 , the fixed mirror  16  may be separated from the fourth surface  13   b  of the optical function member  13 . 
     In the first embodiment, the rib portion  224  is constituted by the support layer  101  and the intermediate layer  103 , but the rib portion  224  may be constituted by at least the support layer  101 . For example, the rib portion  224  may be constituted by only the support layer  101 . For example, the intermediate layer  103  may be disposed over the device layer  102  that constitutes the movable unit  22   b  and the drive unit  23 . In this case, the mirror surface  22   a  is provided on the intermediate layer  103 . When manufacturing the mirror device  20 , after forming the rib portion  224  in the fifth step, the intermediate layer  103  is not removed. According to the method for manufacturing the mirror device  20 , as in the first embodiment, it is possible to form the rib portion  224  with accuracy. Here, the intermediate layer  103  functions as an etching stopper layer, and thus a variation is likely to occur in the thickness of the intermediate layer  103 . In the first embodiment, since the exposed intermediate layer  103  is removed, it is possible to suppress the variation of the thickness. 
     The optical device of the present disclosure is not limited to the mirror device, and may be an optical device in which another optical function unit other than the mirror surface  22   a  is disposed on the movable unit  22   b . Examples of the other optical function unit include a lens and the like. The drive unit  23  of the mirror device  20  may include three or more elastic support portions which elastically support the movable mirror  22 . In the mirror device  20 , the movement direction of the movable unit  22   b  is a direction perpendicular to the first surface  21   a  of the base  21 , but the movement direction of the movable unit  22   b  may be a direction that intersects the first surface  21   a . The actuator unit  28  is not limited to the electrostatic actuator, and may be, for example, a piezoelectric type actuator, an electromagnetic type actuator, or the like. The mirror device  20  is not limited to a device that constitutes the FTIR, and may constitute another optical system. 
     Second Embodiment 
     An optical device  301  according to a second embodiment will be described with reference to  FIG.  22    to  FIG.  25   . The optical device  301  includes a base  305 , a movable unit  307 , a first torsion bar (elastic support portion)  311 , and a second torsion bar (elastic support portion)  312 . The first and second torsion bars  311  and  312  supports the movable unit  307  so that the movable unit  307  can swing around an axial line L 301  to be described later. The optical device  301  is constituted as an MEMS device by a silicon on insulator (SOI) substrate (semiconductor substrate)  309 . For example, the optical device  301  has a rectangular plate shape. For example, the optical device  301  has a size of approximately 9 mm×7 mm×0.4 mm. 
     The base  305  is formed by parts of a handle layer (first semiconductor layer)  391 , a device layer (second semiconductor layer)  392 , and an intermediate layer  393  which constitute the SOI substrate  309 . The handle layer  391  is a first silicon layer. The device layer  392  is a second silicon layer. The intermediate layer  393  is an insulating layer disposed between the handle layer  391  and the device layer  392 . 
     The movable unit  307  is disposed in a state in which an intersection between the axial line L 301  and an axial line L 302  is set as a central position (gravity center position). The axial line L 301  is a straight line that extends in the X-axis direction (a direction parallel to the X-axis, a first direction). The axial line L 302  is a straight line that extends in the Y-axis direction (a direction parallel to the Y-axis, a second direction perpendicular to the first direction). When viewed from a Z-axis direction (a direction parallel to the Z-axis, a third direction perpendicular to the first direction and the second direction), the movable unit  307  has a shape linearly symmetric to the axial line L 301  and is linearly symmetric to the axial line L 302 . 
     The movable unit  307  includes an optical function unit  371 , a fifth main body portion  372 , a frame  373 , a plurality of fifth connection portions  374 , a main body rib portion  375 , a frame rib portion  376 , and a plurality of connection rib portions  377 . The optical function unit  371  is provided in the fifth main body portion  372 . The optical function unit  371  is a mirror formed on a surface  392   a , which is opposite to the handle layer  391 , in the device layer  392  that constitutes the fifth main body portion  372 . For example, the mirror is obtained by forming a metallic film on the surface  392   a  of the device layer  392  that constitutes the fifth main body portion  372  through vapor deposition. 
     The fifth main body portion  372  is formed by a part of the device layer  392 . For example, the fifth main body portion  372  has a circular shape when viewed from the Z-axis direction. The frame  373  surrounds the fifth main body portion  372  when viewed from the z-axis direction. The frame  373  is formed by a part of the device layer  392 . For example, the frame  373  has an octagonal ring shape when viewed from the Z-axis direction. The plurality of fifth connection portions  374  are respectively disposed on both sides of the fifth main body portion  372  on the axial line L 301 , and both sides of the fifth main body portion  372  on the axial line L 2 . Specifically, the plurality of fifth connection portions  374  are disposed at a position corresponding to a first end  307   a  (position between the first end  307   a  and a central position of the movable unit  307 ), a position corresponding to a second end  307   b  (position between the second end  307   b  and a central position of the movable unit  307 ), a position on an extending line of the first torsion bar  311 , and a position on an extending line of the second torsion bar  312 . Each of the fifth connection portions  374  is connected to the fifth main body portion  372  and the frame  373 . Each of the fifth connection portions  374  is bridged between the fifth main body portion  372  and the frame  373 . The fifth connection portion  374  is formed by a part of the device layer  392 . 
     The main body rib portion  375  extends along an outer edge of the fifth main body portion  372 . The main body rib portion  375  is formed by parts of the handle layer  391  and the intermediate layer  393 . The main body rib portion  375  is formed on a surface  392   b  on the handle layer  391  side in the device layer  392  that constitutes the fifth main body portion  372 . For example, the main body rib portion  375  has a circular ring shape when viewed from the Z-axis direction. The frame rib portion  376  extends along the frame  373 . The frame rib portion  376  is formed by parts of the handle layer  391  and the intermediate layer  393 . The frame rib portion  376  is formed on the surface  392   b  of the device layer  392  that constitutes the frame  373 . For example, the frame rib portion  376  has an octagonal ring shape when viewed from the Z-axis direction. The plurality of connection rib portions  377  are respectively disposed in the plurality of fifth connection portions  374 . Each of the connection rib portion  377  is connected to the main body rib portion  375  and the frame rib portion  376 . Each of the connection rib portions  377  is bridged between the main body rib portion  375  and the frame rib portion  376 . Each of the connection rib portions  377  is formed by parts of the handle layer  391  and the intermediate layer  393 . Each of the connection rib portions  377  is formed on the surface  392   b  of the device layer  392  that constitutes the fifth connection portion  374 . The handle layer  391  that constitutes the main body rib portion  375 , the frame rib portion  376 , and each of the connection rib portions  377  is thinner than the handle layer  391  that constitutes the base  305 . 
     The first torsion bar  311  is disposed on one side of the movable unit  307  in the X-axis direction. The first torsion bar  311  extends on the axial line L 301  along the X-axis direction. The first torsion bar  311  is formed by a part of the device layer  392 . The first torsion bar  311  is connected to the base  305  and the movable unit  307 . The first torsion bar  311  is bridged between the base  305  and the movable unit  307  (the frame  373  in the optical device  301 ). The first torsion bar  311  is connected to the movable unit  307  in such a manner that curvatures of an outer edge of the first torsion bar  311  and an outer edge of the movable unit  307  (an outer edge of the frame  373  in the optical device  301 ) are continuous when viewed from the Z-axis direction. Specifically, in the first torsion bar  311 , a portion connected to the movable unit  307  has a shape in which both side surfaces of the portion are curved in a concave shape so that a width of the portion in the Y-axis direction increases as it approaches the movable unit  307 . Similarly, in the first torsion bar  311 , a portion connected to the base  305  has a shape in which both sides surfaces of the portion are curved in a concave shape so that a width of the portion in the Y-axis direction increases as it approaches the base  305 . 
     The second torsion bar  312  is disposed on the other side of the movable unit  307  in the X-axis direction. The second torsion bar  312  extends on the axial line L 301  along the X-axis direction. The second torsion bar  312  is formed by a part of the device layer  392 . The second torsion bar  312  is connected to the base  305  and the movable unit  307 . The second torsion bar  312  is bridged between the base  305  and the movable unit  307  (the frame  373  in the optical device  301 ). The second torsion bar  312  is connected to the movable unit  307  in such a manner that curvatures of an outer edge of the second torsion bar  312  and an outer edge of the movable unit  307  (the outer edge of the frame  373  in the optical device  301 ) are continuous when viewed from the Z-axis direction. Specifically, in the second torsion bar  312 , a portion connected to the movable unit  307  has a shape in which both side surfaces of the portion are curved in a concave shape so that a width of the portion in the Y-axis direction increases as it approaches the movable unit  307 . Similarly, in the second torsion bar  312 , a portion connected to the base  305  has a shape in which both sides surfaces of the portion are curved in a concave shape so that a width of the portion in the Y-axis direction increases as it approaches the base  305 . 
     The optical device  301  further includes a first support portion  321 , a second support portion  322 , a third support portion  323 , and a fourth support portion  324 . The first support portion  321  is disposed on one side of the first torsion bar  311  in the Y-axis direction, and is connected to the movable unit  307 . The second support portion  322  is disposed on the other side of the first torsion bar  311  in the Y-axis direction, and is connected to the movable unit  307 . The third support portion  323  is disposed on one side of the second torsion bar  312  in the Y-axis direction, and is connected to the movable unit  307 . The fourth support portion  324  is disposed on the other side of the second torsion bar  312  in the Y-axis direction, and the fourth support portion  324  is connected to the movable unit  307 . 
     The first support portion  321  includes a first main body portion  321   a , a first connection portion  321   b , and a first rib portion  321   c . The first main body portion  321   a  extends along the X-axis direction in a state in which a gap is formed between the first main body portion  321   a  and the first torsion bar  311 . The first main body portion  321   a  is formed by a part of the device layer  392 . The first connection portion  321   b  is connected to the first main body portion  321   a  and the movable unit  307 . The first connection portion  321   b  is bridged between the first main body portion  321   a  and the movable unit  307  (the frame  373  in the optical device  301 ). The first connection portion  321   b  is formed by a part of the device layer  392 . The first connection portion  321   b  has a shape bent to be spaced apart from the portion of the first torsion bar  311  which is connected to the movable unit  307 . The first rib portion  321   c  is formed in the first main body portion  321   a  and the first connection portion  321   b  so that the thickness of the first support portion  321  in the Z-axis direction becomes larger than the thickness of the first torsion bar  311  in the Z-axis direction. The first rib portion  321   c  extends over the first main body portion  321   a  and the first connection portion  321   b , and is connected to the frame rib portion  376 . The first rib portion  321   c  is formed by a part of the handle layer  391  and the intermediate layer  393 . A width of the first rib portion  321   c  in the Y-axis direction is smaller than a width of the first main body portion  321   a  in the Y-axis direction. The first rib portion  321   c  is formed on the surface  392   b  of the device layer  392  that constitutes the first main body portion  321   a  and the first connection portion  321   b . In the optical device  301 , the first rib portion  321   c  is a portion that protrudes from the surface  392   b  of the device layer  392  that constitutes the first and second torsion bars  311  and  312  in the Z-axis direction. The handle layer  391  that constitutes the first rib portion  321   c  is thinner than the handle layer  391  that constitutes the base  305 . 
     The second support portion  322  includes a second main body portion  322   a , a second connection portion  322   b , and a second rib portion  322   c . The second main body portion  322   a  extends along the X-axis direction in a state in which a gap is formed between the second main body portion  322   a  and the first torsion bar  311 . The second main body portion  322   a  is formed by a part of the device layer  392 . The second connection portion  322   b  is connected to the second main body portion  322   a  and the movable unit  307 . The second connection portion  322   b  is bridged between the second main body portion  322   a  and the movable unit  307  (the frame  373  in the optical device  301 ). The second connection portion  322   b  is formed by a part of the device layer  392 . The second connection portion  322   b  has a shape bent to be spaced apart from the portion of the first torsion bar  311  which is connected to the movable unit  307 . The second rib portion  322   c  is formed in the second main body portion  322   a  and the second connection portion  322   b  so that the thickness of the second support portion  322  in the Z-axis direction becomes larger than the thickness of the first torsion bar  311  in the Z-axis direction. The second rib portion  322   c  extends over the second main body portion  322   a  and the second connection portion  322   b , and is connected to the frame rib portion  376 . The second rib portion  322   c  is formed by a part of the handle layer  391  and the intermediate layer  393 . A width of the second rib portion  322   c  in the Y-axis direction is smaller than a width of the second main body portion  322   a  in the Y-axis direction. The second rib portion  322   c  is formed on the surface  392   b  of the device layer  392  that constitutes the second main body portion  322   a  and the second connection portion  322   b . In the optical device  301 , the second rib portion  322   c  is a portion that protrudes from the surface  392   b  of the device layer  392  that constitutes the first and second torsion bars  311  and  312  in the Z-axis direction. The handle layer  391  that constitutes the second rib portion  322   c  is thinner than the handle layer  391  that constitutes the base  305 . The third support portion  323  includes a third main body portion  323   a , a third connection portion  323   b , and a third rib portion  323   c.    
     The third main body portion  323   a  extends along the X-axis direction in a state in which a gap is formed between the third main body portion  323   a  and the second torsion bar  312 . The third main body portion  323   a  is formed by a part of the device layer  392 . The third connection portion  323   b  is connected to the third main body portion  323   a  and the movable unit  307 . The third connection portion  323   b  is bridged between the third main body portion  323   a  and the movable unit  307  (the frame  373  in the optical device  301 ). The third connection portion  323   b  is formed by a part of the device layer  392 . The third connection portion  323   b  has a shape bent to be spaced apart from the portion of the second torsion bar  312  which is connected to the movable unit  307 . The third rib portion  323   c  is formed in the third main body portion  323   a  and the third connection portion  323   b  so that the thickness of the third support portion  323  in the Z-axis direction becomes larger than the thickness of the second torsion bar  312  in the Z-axis direction. The third rib portion  323   c  extends over the third main body portion  323   a  and the third connection portion  323   b , and is connected to the frame rib portion  376 . The third rib portion  323   c  is formed by a part of the handle layer  391  and the intermediate layer  393 . A width of the third rib portion  323   c  in the Y-axis direction is smaller than a width of the third main body portion  323   a  in the Y-axis direction. The third rib portion  323   c  is formed on the surface  392   b  of the device layer  392  that constitutes the third main body portion  323   a  and the third connection portion  323   b . In the optical device  301 , the third rib portion  323   c  is a portion that protrudes from the surface  392   b  of the device layer  392  that constitutes the first and second torsion bars  311  and  312  in the Z-axis direction. The handle layer  391  that constitutes the third rib portion  323   c  is thinner than the handle layer  391  that constitutes the base  305 . 
     The fourth support portion  324  includes a fourth main body portion  324   a , a fourth connection portion  324   b , and a fourth rib portion  324   c . The fourth main body portion  324   a  extends along the X-axis direction in a state in which a gap is formed between the fourth main body portion  324   a  and the second torsion bar  312 . The fourth main body portion  324   a  is formed by a part of the device layer  392 . The fourth connection portion  324   b  is connected to the fourth main body portion  324   a  and the movable unit  307 . The fourth connection portion  324   b  is bridged between the fourth main body portion  324   a  and the movable unit  307  (the frame  373  in the optical device  301 ). The fourth connection portion  324   b  is formed by a part of the device layer  392 . The fourth connection portion  324   b  has a shape bent to be spaced apart from the portion of the second torsion bar  312  which is connected to the movable unit  307 . The fourth rib portion  324   c  is formed in the fourth main body portion  324   a  and the fourth connection portion  324   b  so that the thickness of the fourth support portion  324  in the Z-axis direction becomes larger than the thickness of the second torsion bar  312  in the Z-axis direction. The fourth rib portion  324   c  extends over the fourth main body portion  324   a  and the fourth connection portion  324   b , and is connected to the frame rib portion  376 . The fourth rib portion  324   c  is formed by a part of the handle layer  391  and the intermediate layer  393 . A width of the fourth rib portion  324   c  in the Y-axis direction is smaller than a width of the fourth main body portion  324   a  in the Y-axis direction. The fourth rib portion  324   c  is formed on the surface  392   b  of the device layer  392  that constitutes the fourth main body portion  324   a  and the fourth connection portion  324   b . In the optical device  301 , the fourth rib portion  324   c  is a portion that protrudes from the surface  392   b  of the device layer  392  that constitutes the first and second torsion bars  311  and  312  in the Z-axis direction. The handle layer  391  that constitutes the fourth rib portion  324   c  is thinner than the handle layer  391  that constitutes the base  305 . 
     The optical device  301  further includes a first movable comb electrode  331 , a second movable comb electrode  332 , a third movable comb electrode  333 , a fourth movable comb electrode  334 , a fifth movable comb electrode  335 , and a sixth movable comb electrode  336 . The first movable comb electrode  331  is provided in the first main body portion  321   a  of the first support portion  321 . The second movable comb electrode  332  is provided in the second main body portion  322   a  of the second support portion  322 . The third movable comb electrode  333  is provided in the third main body portion  323   a  of the third support portion  323 . The fourth movable comb electrode  334  is provided in the fourth main body portion  324   a  of the fourth support portion  324 . The fifth movable comb electrode  335  is provided a portion including the first end  307   a  in movable unit  307 . The first end  307   a  is an end on one side of the movable unit  307  in the Y-axis direction. In the optical device  301 , the fifth movable comb electrode  335  is provided in a portion located between the first connection portion  321   b  of the first support portion  321  and the third connection portion  323   b  of the third support portion  323  in the frame  373 , and includes the first end  307   a . The sixth movable comb electrode  336  is provided in a portion including the second end  307   b  in the movable unit  307 . The second end  307   b  is an end on the other side of the movable unit  307  in the Y-axis direction. In the optical device  301 , the sixth movable comb electrode  336  is provided in a portion located between the second connection portion  322   b  of the second support portion  322  and the fourth connection portion  324   b  of the fourth support portion  324 , and includes the second end  307   b  in the frame  373 . 
     The first movable comb electrode  331  is formed by a part of the device layer  392 . The first movable comb electrode  331  is disposed between the first main body portion  321   a  of the first support portion  321  and the first end  307   a  of the movable unit  307  when viewed from the X-axis direction. The first movable comb electrode  331  includes a plurality of first movable comb fingers  331   a . Each of first movable comb fingers  331   a  is provided on a side surface, which is opposite to the first torsion bar  311 , in the first main body portion  321   a  of the first support portion  321 . Each of the first movable comb fingers  331   a  extends along a plane perpendicular to the X-axis direction. The plurality of first movable comb fingers  331   a  are arranged in such a manner that an interval between the first movable comb fingers  331   a  adjacent to each other in the X-axis direction becomes constant. 
     The second movable comb electrode  332  is formed by a part of the device layer  392 . The second movable comb electrode  332  is disposed between the second main body portion  322   a  of the second support portion  322  and the second end  307   b  of the movable unit  307  when viewed from the X-axis direction. The second movable comb electrode  332  includes a plurality of second movable comb fingers  332   a . Each of the second movable comb fingers  332   a  is provided on a side surface, which is opposite to the first torsion bar  311 , in the second main body portion  322   a  of the second support portion  322 . Each of the second movable comb fingers  332   a  extends long a plane perpendicular to the X-axis direction. The plurality of second movable comb fingers  332   a  are arranged in such a manner that an interval between the second movable comb fingers  332   a  adjacent to each other in the X-axis direction becomes constant. 
     The third movable comb electrode  333  is formed by a part of the device layer  392 . The third movable comb electrode  333  is disposed between the third main body portion  323   a  of the third support portion  323  and the first end  307   a  of the movable unit  307  when viewed from the X-axis direction. The third movable comb electrode  333  includes a plurality of third movable comb fingers  333   a . Each of the third movable comb fingers  333   a  is provided on a side surface, which is opposite to the second torsion bar  312 , in the third main body portion  323   a  of the third support portion  323 . Each of the third movable comb fingers  333   a  extends along a plane perpendicular to the X-axis direction. The plurality of third movable comb fingers  333   a  are arranged in such a manner that an interval between the third movable comb fingers  333   a  adjacent to each other in the X-axis direction becomes constant. 
     The fourth movable comb electrode  334  is formed by a part of the device layer  392 . The fourth movable comb electrode  334  is disposed between the fourth main body portion  324   a  of the fourth support portion  324  and the second end  307   b  of the movable unit  307  when viewed from the X-axis direction. The fourth movable comb electrode  334  includes a plurality of fourth movable comb fingers  334   a . Each of the fourth movable comb fingers  334   a  is provided on a side surface, which is opposite to the second torsion bar  312 , in the fourth main body portion  324   a  of the fourth support portion  324 . Each of the fourth movable comb fingers  334   a  extends along a plane perpendicular to the X-axis direction. The plurality of fourth movable comb fingers  334   a  are arranged in such a manner that an interval between the fourth movable comb fingers  334   a  adjacent to each other in the X-axis direction becomes constant. 
     The fifth movable comb electrode  335  is formed by a part of the device layer  392 . The fifth movable comb electrode  335  includes a plurality of fifth movable comb fingers  335   a . Each of the fifth movable comb fingers  335   a  is provided on a side surface, which is opposite to the fifth main body portion  372 , in a portion including the first end  307   a  in the frame  373 . Each of the fifth movable comb fingers  335   a  extends along a plane perpendicular to the X-axis direction. The plurality of fifth movable comb fingers  335   a  are arranged in such a manner that an interval between the fifth movable comb fingers  335   a  adjacent to each other in the X-axis direction becomes constant. 
     The sixth movable comb electrode  336  is formed by a part of the device layer  392 . The sixth movable comb electrode  336  includes a plurality of sixth movable comb fingers  336   a . Each of the sixth movable comb fingers  336   a  is provided on a side surface, which is opposite to the fifth main body portion  372 , in a portion including the second end  307   b  in the frame  373 . Each of the sixth movable comb fingers  336   a  extends along a plane perpendicular to the X-axis direction. The plurality of the sixth movable comb fingers  336   a  are arranged in such a manner that an interval between the sixth movable comb fingers  336   a  adjacent to each other in the X-axis direction becomes constant. 
     The optical device  301  further includes a first fixed comb electrode  341 , a second fixed comb electrode  342 , a third fixed comb electrode  343 , a fourth fixed comb electrode  344 , a fifth fixed comb electrode  345 , and a sixth fixed comb electrode  346 . The first fixed comb electrode  341 , the second fixed comb electrode  342 , the third fixed comb electrode  343 , the fourth fixed comb electrode  344 , the fifth fixed comb electrode  345 , and the sixth fixed comb electrode  346  are provided in the base  305 . 
     The first fixed comb electrode  341  is formed by a part of the device layer  392 . The first fixed comb electrode  341  includes a plurality of first fixed comb fingers  341   a . Each of the first fixed comb fingers  341   a  is provided on a side surface of the base  305  which faces the side surface of the first main body portion  321   a  on which the plurality of first movable comb fingers  331   a  are provided. Each of the first fixed comb fingers  341   a  extends along a plane perpendicular to the X-axis direction. The plurality of first fixed comb fingers  341   a  are arranged in such a manner that an interval between the first fixed comb fingers  341   a  adjacent to each other in the X-axis direction becomes constant, and are disposed alternately with the plurality of first movable comb fingers  331   a . The first movable comb finger  331   a  and the first fixed comb finger  341   a  adjacent to each other face each other in the X-axis direction. For example, an interval between the first movable comb finger  331   a  and the first fixed comb finger  341   a  adjacent to each other is approximately several μm. 
     The second fixed comb electrode  342  is formed by a part of the device layer  392 . The second fixed comb electrode  342  includes a plurality of second fixed comb fingers  342   a . Each of the second fixed comb fingers  342   a  is provided on a side surface of the base  305  which faces the side surface of the second main body portion  322   a  on which the plurality of second movable comb fingers  332   a  are provided. Each of the second fixed comb fingers  342   a  extends along a plane perpendicular to the X-axis direction. The plurality of second fixed comb fingers  342   a  are arranged in such a manner that an interval between the second fixed comb fingers  342   a  adjacent to each other in the X-axis direction becomes constant, and are deposed alternately with the plurality of second movable comb fingers  332   a . The second movable comb finger  332   a  and the second fixed comb finger  342   a  which are adjacent face each other in the X-axis direction. For example, an interval between the second movable comb finger  332   a  and the second fixed comb finger  342   a  which are adjacent to each other is approximately several μm. 
     The third fixed comb electrode  343  is formed by a part of the device layer  392 . The third fixed comb electrode  343  includes a plurality of third fixed comb fingers  343   a . Each of the third fixed comb fingers  343   a  is provided on a side surface of the base  305  which faces the side surface of the third main body portion  323   a  on which the plurality of third movable comb fingers  333   a  are provided. Each of the third fixed comb fingers  343   a  extends along a plane perpendicular to the X-axis direction. The plurality of third fixed comb fingers  343   a  are arranged in such a manner that an interval between the third fixed comb fingers  343   a  adjacent to each other in the X-axis direction becomes constant, and are disposed alternately with the plurality of third movable comb fingers  333   a . The third movable comb finger  333   a  and the third fixed comb finger  343   a  adjacent to each other face each other in the X-axis direction. For example, an interval between the third movable comb finger  333   a  and the third fixed comb finger  343   a  adjacent to each other is approximately several μm. 
     The fourth fixed comb electrode  344  is formed by a part of the device layer  392 . The fourth fixed comb electrode  344  includes a plurality of fourth fixed comb fingers  344   a . Each of the fourth fixed comb fingers  344   a  is provided on a side surface of the base  305  which faces the side surface of the fourth main body portion  324   a  on which the plurality of fourth movable comb fingers  334   a  are provided. Each of the fourth fixed comb fingers  344   a  extends along a plane perpendicular to the X-axis direction. The plurality of fourth fixed comb fingers  344   a  are arranged in such a manner that an interval between the fourth fixed comb fingers  344   a  adjacent to each other in the X-axis direction becomes constant, and are disposed alternately with the plurality of fourth movable comb fingers  334   a . The fourth movable comb finger  334   a  and the fourth fixed comb finger  344   a  adjacent to each other face each other in the X-axis direction. For example, an interval between the fourth movable comb finger  334   a  and the fourth fixed comb finger  344   a  adjacent to each other is approximately several μm. 
     The fifth fixed comb electrode  345  is formed by a part of the device layer  392 . The fifth fixed comb electrode  345  includes a plurality of fifth fixed comb fingers  345   a . Each of the fifth fixed comb fingers  345   a  is provided on a side surface of the base  305  which faces the side surface of the frame  373  on which the plurality of fifth movable comb fingers  335   a  are provided. Each of the fifth fixed comb fingers  345   a  extends along a plane perpendicular to the X-axis direction. The plurality of fifth fixed comb fingers  345   a  are arranged in such a manner that an interval between the fifth fixed comb fingers  345   a  adjacent to each other in the X-axis direction becomes constant, and are disposed alternately with the plurality of fifth movable comb fingers  335   a . The fifth movable comb finger  335   a  and the fifth fixed comb finger  345   a  adjacent to each other face each other in the X-axis direction. For example, an interval between the fifth movable comb finger  335   a  and the fifth fixed comb finger  345   a  adjacent to each other is approximately several μm. 
     The sixth fixed comb electrode  346  is formed by a part of the device layer  392 . The sixth fixed comb electrode  346  includes a plurality of sixth fixed comb fingers  346   a . Each of the sixth fixed comb fingers  346   a  is provided on a side surface of the base  305  which faces the side surface of the frame  373  on which the plurality of sixth movable comb fingers  336   a  are provided. Each of the sixth fixed comb fingers  346   a  extends along a plane perpendicular to the X-axis direction. The plurality of sixth fixed comb fingers  346   a  are arranged in such a manner that an interval between the sixth fixed comb fingers  346   a  adjacent to each other in the X-axis direction becomes constant, and are disposed alternately with the plurality of sixth movable comb fingers  336   a . The sixth movable comb finger  336   a  and the sixth fixed comb finger  346   a  which are adjacent to each other face each other in the X-axis direction. For example, an interval between the sixth movable comb finger  336   a  and the sixth fixed comb finger  346   a  adjacent to each other is approximately several μm. 
     A plurality of electrode pads  302 ,  303 , and  304  are provided on the surface  392   a  of the device layer  392  that constitutes the base  305 . A plurality of wiring portions  351 ,  352 , and  353  are formed in the device layer  392  that constitutes the base  305  by defining parts of the device layer  392  with grooves. The electrode pads  302  are electrically connected to the first movable comb electrode  331 , the second movable comb electrode  332 , the third movable comb electrode  333 , the fourth movable comb electrode  334 , the fifth movable comb electrode  335 , and the sixth movable comb electrode  336  through the wiring portions  351 . The electrode pad  303  located near the fifth fixed comb electrode  345  is electrically connected to the fifth fixed comb electrode  345  through the wiring portion  352  located near the fifth fixed comb electrode  345 . The electrode pad  303  located near the sixth fixed comb electrode  346  is electrically connected to the sixth fixed comb electrode  346  through the wiring portion  352  located near the sixth fixed comb electrode  346 . The electrode pad  304  located near the first fixed comb electrode  341  is electrically connected to the first fixed comb electrode  341  through the wiring portion  353  located near the first fixed comb electrode  341 . The electrode pad  304  located near the second fixed comb electrode  342  is electrically connected to the second fixed comb electrode  342  through the wiring portion  353  located near the second fixed comb electrode  342 . The electrode pad  304  located near the third fixed comb electrode  343  is electrically connected to the third fixed comb electrode  343  through the wiring portion  353  located near the third fixed comb electrode  343 . The electrode pad  304  located near the fourth fixed comb electrode  344  is electrically connected to the fourth fixed comb electrode  344  through the wiring portion  353  located near the fourth fixed comb electrode  344 . Note that the plurality of electrode pads  302 ,  303 , and  304  are not illustrated in  FIG.  24    and  FIG.  25   . 
     The fifth movable comb electrode  335  and the fifth fixed comb electrode  345 , and the sixth movable comb electrode  336  and the sixth fixed comb electrode  346  are used as electrodes for driving. Specifically, a voltage is periodically applied to between the fifth movable comb electrode  335  and the fifth fixed comb electrode  345 , and between the sixth movable comb electrode  336  and the sixth fixed comb electrode  346  through the plurality of electrode pads  302  and  303 , respectively. Accordingly, an electrostatic force is generated between the fifth movable comb electrode  335  and the fifth fixed comb electrode  345  and between the sixth movable comb electrode  336  and the sixth fixed comb electrode  346 , and an elastic force is generated in the first torsion bar  311  and the second torsion bar  312 . In the optical device  301 , it is possible to cause the movable unit  307  to swing at an oscillation frequency level with the axial line L 301  set as a central line by applying a periodic electric signal to the plurality of electrode pads  302  and  303  (that is, the optical function unit  371  swings). 
     The first movable comb electrode  331  and the first fixed comb electrode  341 , the second movable comb electrode  332  and the second fixed comb electrode  342 , the third movable comb electrode  333  and the third fixed comb electrode  343 , and the fourth movable comb electrode  334  and the fourth fixed comb electrode  344  are used as electrode for monitoring. Specifically, electrostatic capacitance between the first movable comb electrode  331  and the first fixed comb electrode  341 , between the second movable comb electrode  332  and the second fixed comb electrode  342 , between the third movable comb electrode  333  and the third fixed comb electrode  343 , and between the fourth movable comb electrode  334  and the fourth fixed comb electrode  344  is detected through the plurality of electrode pads  302  and  304 . The electrostatic capacitance varies in correspondence with a swing angle of the movable unit  307  (that is, a swing angle of the optical function unit  371 ). Accordingly, it is possible to feedback-control the swing angle of the movable unit  307  (that is, the swing angle of the optical function unit  371 ) by adjusting the drive signal (magnitude, a cycle, and the like of an application voltage) in correspondence with detected electrostatic capacitance. 
     In the optical device  301 , a portion except for the optical function unit  371  and the plurality of electrode pads  303  and  304  is integrally formed in the SOI substrate  309  with an MEMS technology (patterning and etching). In the optical device  301 , at least, the portion integrally formed in the SOI substrate  309  has a shape linearly symmetric to the axial line L 301  and is linearly symmetric to the axial line L 302  when viewed from the z-axis direction. 
     The optical device  301  of the second embodiment can be manufactured by a manufacturing method similar to the method of manufacturing the mirror device  20  according to the first embodiment. That is, the rib portion (main body rib portions  375 , the frame rib portion  376 , the connection rib portion  377 , and the first, second, third, and fourth rib portions  321   c ,  322   c ,  323   c , and  324   c ) can be formed by two-stage etching using the first and second resist layers as a mask. In this case, as in the first embodiment, it is possible to form the rib portions with accuracy. Note that, a stepped surface  3107  corresponding to the above-described stepped surface  107  is illustrated in  FIG.  23    to  FIG.  25   . 
     Third Embodiment 
     An optical device  401  according to a third embodiment will be described with reference to  FIG.  26    to  FIG.  28   . The optical device  401  includes a support portion (base)  402 , a first movable unit  403 , a second movable unit  404 , a pair of first torsion bars (elastic support portions)  405  and  406 , a pair of second torsion bars (elastic support portions)  407  and  408 , and a magnetic field generation portion  409 . 
     In the optical device  401 , the first movable unit  403  on which a mirror surface (optical function unit)  410  is disposed is caused to swing around each of an X-axis (first axial line) and a Y-axis (second axial line perpendicular to the first axial line) which are perpendicular to each other. For example, the optical device  401  is used in an optical switch for optical communication, an optical scanner, and the like. 
     For example, the support portion  402 , the first movable unit  403 , the second movable unit  404 , the pair of first torsion bars  405  and  406 , and the pair of second torsion bars  407  and  408  are integrally formed by a silicon on insulator (SOI) substrate (semiconductor substrate)  460 . That is, the optical device  401  is constituted as an MEMS device. The SOI substrate  460  includes a support layer  461 , a device layer  462 , and an intermediate layer  463 . The support layer  461  is a first silicon layer (a first semiconductor layer). The device layer  462  is a second silicon layer (a second semiconductor layer). The intermediate layer  463  is an insulating layer disposed between the support layer  461  and the device layer  462 . 
     For example, the magnetic field generation portion  409  is constituted by a permanent magnet having a Halbach array, or the like. For example, the magnetic field generation portion  409  generates a magnetic field in a direction D inclined by 45° with respect to each of the X-axis and the Y-axis in plan view, and causes the magnetic field to act on the coil  414 . Note that, “in plan view” represents “when viewed from a direction perpendicular to the mirror surface  410 ”, in other words, “when viewed from a direction perpendicular to the X-axis and the Y-axis”. The direction D of the magnetic field generated by the magnetic field generation portion  409  may be inclined by an angle other than 45° with respect to the X-axis and the Y-axis in plan view. 
     For example, the support portion  402  has a square external shape in plan view, and is formed in a frame shape. The support portion  402  is disposed on one side in a direction perpendicular to the X-axis and the Y-axis with respect to the magnetic field generation portion  409 . The support portion  402  supports the first movable unit  403 , the second movable unit  404 , and the like. The support portion  402  is constituted by parts of the support layer  461 , the device layer  462 , and the intermediate layer  463 . 
     The first movable unit  403  is disposed on an inner side of the support portion  402  in a state of being spaced apart from the magnetic field generation portion  409 . The first movable unit  403  has a shape symmetric with respect to the X-axis and the Y-axis in plan view. The first movable unit  403  includes a main body portion  403   a , a ring-shaped portion  403   b , and a rib portion  403   c . The main body portion  403   a  and the ring-shaped portion  403   b  are constituted by parts of the device layer  462 . 
     The main body portion  403   a  has a circular shape in plan view, but may be formed in any shape such as an elliptical shape, a square shape, and a rhomboidal shape. In plan view, a center P of the main body portion  403   a  matches an intersection between the X-axis and the Y-axis. For example, a circular mirror surface  410  is provided by a metal film formed from aluminum on a surface of the main body portion  403   a  which is opposite to the magnetic field generation portion  409  (on a surface opposite to the intermediate layer  463  in the device layer  462  that constitutes the main body portion  403   a ). The mirror surface  410  is provided on an approximately entire surface of the surface, but may be provided on a part of the surface. The ring-shaped portion  403   b  is formed in a ring shape to surround the main body portion  403   a  in plan view. The ring-shaped portion  403   b  has an octagonal external shape in plan view, but may have any external shape such as a circular shape, an elliptical shape, a square shape, and a rhomboidal shape. The ring-shaped portion  403   b  is connected to the main body portion  403   a  on both sides of the Y-axis direction parallel to the Y-axis. 
     The rib portion  403   c  is constituted by the support layer  461  and the intermediate layer  463  which are disposed on the device layer  462 . The rib portion  403   c  includes a central portion  403   ca , and a plurality of (eight in this example) extending portions  403   cb . The central portion  403   ca  is disposed at the center P of the main body portion  403   a  and has an approximately circular shape in plan view. The plurality of extending portions  403   cb  straightly extend from the central portion  403   ca  in a radial direction. The support layer  461  that constitutes the central portion  403   ca  and the extending portion  403   cb  is thinner than the support layer  461  that constitutes the base  402 . 
     The second movable unit  404  is formed in a frame shape, and is disposed on an inner side of the support portion  402  to surround the first movable unit  403  in a state of being spaced apart from the magnetic field generation portion  409 . The second movable unit  404  includes a pair of first connection portions  441 A and  441 B, a pair of second connection portions  442 A and  442 B, a pair of first linear portions  443 A and  443 B, a pair of second linear portions  444 A and  444 B, a pair of third linear portions  445 A and  445 B, a pair of fourth linear portions  446 A and  446 B, and a rib portion  447 . The connection portions  441 A to  442 B and the linear portions  443 A to  446 B are constituted by a part of the device layer  462 . The second movable unit  404  has a symmetric shape with respect to each of the X-axis and the Y-axis in plan view. In the following description, symmetry with respect to the X-axis or the Y-axis represents symmetry in plan view. 
     The first connection portions  441 A and  441 B are located on both sides of the first movable unit  403  in the X-axis direction parallel to the X-axis. The first connection portions  441 A and  441 B extend along the Y-axis direction. The second connection portions  442 A and  442 B are located on both sides of the first movable unit  403  in the Y-axis direction parallel to the Y-axis. The second connection portions  442 A and  442 B extend along the X-axis direction. An inner edge of each of the second connection portions  442 A and  442 B in plan view includes a depression  451  recessed in the Y-axis direction, and an outer edge of each of the second connection portions  442 A and  442 B in plan view includes a protrusion  452  that protrudes in the Y-axis direction. The depression  451  and the protrusion  452  are located on the Y-axis in plan view. 
     The first linear portions  443 A and  443 B are located on both sides of the second connection portion  442 A in the X-axis direction, and are connected to the second connection portion  442 A. The first linear portions  443 A and  443 B extend along the X-axis direction. The first linear portions  443 A and  443 B are disposed to be symmetric to each other with respect to the Y-axis. The second linear portions  444 A and  444 B are located on both sides of the second connection portion  442 B in the X-axis direction, and is connected to the second connection portion  442 B. The second linear portions  444 A and  444 B extend along the X-axis direction. The second linear portions  444 A and  444 B are disposed to be symmetric to each other with respect to the Y-axis. 
     The third linear portions  445 A and  445 B are located on a side opposite to the second connection portion  442 A with respect to the first linear portions  443 A and  443 B, and are connected to the first linear portions  443 A and  443 B, and the first connection portions  441 A and  441 B. The third linear portion  445 A extends in a direction inclined by 45° with respect to each of the X-axis and the Y-axis in plan view. The third linear portion  445 B extends to be symmetric to the third linear portion  445 A with respect to the Y-axis. 
     The fourth linear portions  446 A and  446 B are located on a side opposite to the second connection portion  442 B with respect to the second linear portions  444 A and  444 B, and are connected to the second linear portions  444 A and  444 B and the first connection portions  441 A and  441 B. The fourth linear portion  446 A extends to be symmetric to the third linear portion  445 A with respect to the X-axis. The fourth linear portion  446 B extends to be symmetric to the fourth linear portion  446 A with respect to the Y-axis, and extends to be symmetric to the third linear portion  445 B with respect to the X-axis. 
     The rib portion  447  is constituted by the support layer  461  and the intermediate layer  463  which are disposed on the device layer  462 . The rib portion  447  is disposed on the connection portions  441 A to  442 B and the linear portions  443 A to  446 B, and extends in an annular shape to surround the first movable unit  403  in plan view. A width of the rib portion  447  in the first connection portions  441 A and  441 B and the linear portions  445 A to  446 B is narrower than a width of the rib portion  447  in the second connection portions  442 A and  442 B and the linear portions  443 A to  444 B. The support layer  461  that constitutes the rib portion  447  is thinner than the support layer  461  that constitutes the base  402 . 
     The first torsion bars  405  and  406  are disposed on both sides of the first movable unit  403  on the X-axis. The first torsion bars  405  and  406  connect the first movable unit  403  (ring-shaped portion  403   b ) and the second movable unit  404  to each other on the X-axis so that the first movable unit  403  can swing around the X-axis (with the X-axis set as a central line). As to be described later, the first torsion bars  405  and  406  are connected to the support portion  402  through the second movable unit  404  and the second torsion bars  407  and  408 . That is, it may be regarded that the first torsion bars  405  and  406  connect the first movable unit  403  and the support portion  402  so that the first movable unit  403  can swing around the X-axis. The first torsion bars  405  and  406  are connected to the second movable unit  404  in the first connection portions  441 A and  441 B. Each of the first torsion bars  405  and  406  is twisted and deformed when the first movable unit  403  swings around the X-axis. The first torsion bars  405  and  406  have a plate shape that extends along a plane parallel to the mirror surface  410 . The entirety of the first torsion bars  405  and  406  are located on the X-axis. The first torsion bar  406  is disposed to be symmetric to the first torsion bar  405  with respect to the Y-axis. The first torsion bars  405  and  406  are constituted by a part of the device layer  462 . 
     The second torsion bars  407  and  408  are disposed on both sides of the second movable unit  404  on the Y-axis. The second torsion bars  407  and  408  connect the second movable unit  404  and the support portion  402  on the Y-axis so that the second movable unit  404  can swing around the Y-axis (with the Y-axis set as a central line). The second torsion bars  407  and  408  are connected to the second movable unit  404  in the second connection portions  442 A and  442 B. The second torsion bars  407  and  408  are twisted and deformed when the second movable unit  404  swings around the Y-axis. The second torsion bars  407  and  408  extend in a zigzag manner in plan view. Each of the second torsion bars  407  and  408  includes a plurality of linear portions  411  and a plurality of folded portions  412 . The plurality of linear portions  411  extend along the Y-axis direction and are aligned in the X-axis direction. The plurality of folded portions  412  alternately connect both ends of the linear portions  411  which are adjacent to each other. The second torsion bars  407  and  408  are constituted by a part of the device layer  462 . 
     The optical device  401  further includes a pair of coils  414  and  415 , a first wiring  421 , a second wiring  422 , a third wiring  423 , a fourth wiring  424 , a first external terminal  425 , a second external terminal  426 , a third external terminal  427 , and a fourth external terminal  428 . Each of the coils  414  and  415  is provided in the second movable unit  404  to surround the first movable unit  403 , and has a spiral shape in plan view. Each of the coils  414  and  415  is disposed along a plane including the X-axis and the Y-axis. Each of the coils  414  and  415  wound around the first movable unit  403  a plurality of times. The pair of coils  414  and  415  are disposed to be alternately aligned in a width direction of the second movable unit  404  in plan view. In  FIG.  26   , a disposition region R 401  in which the coils  414  and  415  are disposed is indicated by hatching. For example, the coils  414  and  415  are damascene wiring embedded in the second movable unit  404 . 
     Each of the external terminals  425  to  428  is, for example, an electrode pad provided in the support portion  402 . The first wiring  421  is electrically connected to an inner end of the coil  414  and the first external terminal  425 . The first wiring  421  extends from the inner end of the coil  414  up to the first external terminal  425  through the second torsion bar  407 . The second wiring  422  is electrically connected to an outer end of the coil  414  and the second external terminal  426 . For example, the second wiring  422  is connected to the outer end of the coil  414  on the Y-axis. The second wiring  422  extends from the outer end of the coil  414  up to the second external terminal  426  through the second torsion bar  408 . 
     The third wiring  423  is electrically connected to an inner end of the coil  415  and the third external terminal  427 . The third wiring  423  extends from the inner end of the coil  415  up to the third external terminal  427  through the second torsion bar  407 . The fourth wiring  424  is electrically connected to an outer end of the coil  415  and the fourth external terminal  428 . For example, the fourth wiring  424  is connected to an outer end of the coil  415  on the Y-axis. The fourth wiring  424  extends from the outer end of the coil  415  up to the fourth external terminal  428  through the second torsion bar  408 . 
     In the optical device  401  configured as described above, when a drive signal for a linear operation is input to the coil  414  through each of the external terminals  425  and  426  and each of the wiring  421  and  422 , a Lorenz force acts on the coil  414  by a mutual interaction with a magnetic field generated by the magnetic field generation portion  409 . It is possible to cause the mirror surface  410  (first movable unit  403 ) to linearly operate around the Y-axis together with the second movable unit  404  by using balance between the Lorenz force and the elastic force of the second torsion bars  407  and  408 . 
     On the other hand, when a drive signal for an oscillation operation is input to the coil  415  through each of the external terminals  427  and  428  and each of the wirings  423  and  424 , a Lorenz force acts on the coil  415  by a mutual interaction with the magnetic field generated by the magnetic field generation portion  409 . It is possible to cause the mirror surface  410  (first movable unit  403 ) to oscillate around the X-axis at an oscillation frequency by using oscillation of the first movable unit  403  in addition to the Lorenz force. Specifically, when a drive signal of the same frequency as the oscillation frequency of the first movable unit  403  around the X-axis is input to the coil  415 , the second movable unit  404  slightly vibrates around the X-axis at the frequency. When the vibration is transmitted to the first movable unit  403  through the first torsion bars  405  and  406 , it is possible to cause the first movable unit  403  to swing around the X-axis at the frequency. 
     The optical device  401  of the third embodiment can be manufactured by a manufacturing method similar to the method for manufacturing the mirror device  20  according to the first embodiment. That is, it is possible to form the rib portions (rib portion  403   c  and the rib portion  447 ) by two-stage etching using the first and second resist layers as a mask. In this case, as in the first embodiment, it is possible to form the rib portions with accuracy. Note that, a stepped surface  4107  corresponding to the above-described stepped surface  107  is illustrated in  FIG.  27    and  FIG.  28   . 
     In the optical device  401 , driving of the first movable unit  403  is performed by an electromagnetic force, but the driving of the first movable unit  403  may be performed by a piezoelectric element. In this case, for example, in the second movable unit  404 , a first piezoelectric film for causing the first movable unit  403  to swing around the X-axis is provided in the second movable unit  404  instead of the coils  414  and  415 . For example, the first piezoelectric film is disposed on the first connection portions  441 A and  441 B, the third linear portions  445 A and  445 B, and the fourth linear portions  446 A and  446 B so as to extend along a direction parallel to the Y-axis. In addition, a third movable unit may be provided on an outer side of the second movable unit  404  to surround the second movable unit  404 , and a second piezoelectric film for causing the second movable unit  404  to swing around the Y-axis may be provided in the third movable unit. For example, the second piezoelectric film extends along a direction parallel to the X-axis. The magnetic field generation portion  409  is omitted. Respective configurations in the first, second, and third embodiments, or the modification example may be applied to respective configurations in another embodiment or another modification example. 
     REFERENCE SIGNS LIST 
       20 : mirror device (optical device),  21 : base,  21   a : first surface (main surface),  21   b : second surface (main surface),  22   a : mirror surface (optical function unit),  22   b : movable unit,  224 : rib portion,  26 : first elastic support portion,  27 : second elastic support portion,  100 : SOI substrate (semiconductor substrate),  101 : support layer (first semiconductor layer),  101   a : surface,  102 : device layer (second semiconductor layer),  102   a : surface,  103 : intermediate layer (insulating layer),  104 : first resist layer,  105 : depression,  105   a : bottom surface,  105   b : side surface,  106 : second resist layer,  281 ,  283 : fixed comb electrode,  281   a ,  283   a : fixed comb finger,  282 ,  284 : movable comb electrode,  282   a ,  284   a : movable comb finger,  301 : optical device,  305 : base,  307 : movable unit,  311 : first torsion bar (elastic support portion),  312 : second torsion bar (elastic support portion),  309 : SOI substrate (semiconductor substrate),  375 : main body rib portion,  376 : frame rib portion,  377 : connection rib portion,  321   c : first rib portion,  322   c : second rib portion,  323   c : third rib portion,  324   c : fourth rib portion,  331 : first movable comb electrode,  331   a : first movable comb finger,  332 : second movable comb electrode,  332   a : second movable comb finger,  333 : third movable comb electrode,  333   a : third movable comb finger,  334 : fourth movable comb electrode,  334   a : fourth movable comb finger,  335 : fifth movable comb electrode,  335   a : fifth movable comb finger,  336 : sixth movable comb electrode,  336   a : sixth movable comb finger,  341 : first fixed comb electrode,  341   a : first fixed comb finger,  342 : second fixed comb electrode,  342   a : second fixed comb finger,  343 : third fixed comb electrode,  343   a : third fixed comb finger,  344 : fourth fixed comb electrode,  344   a : fourth fixed comb finger,  345 : fifth fixed comb electrode,  345   a : fifth fixed comb finger,  346 : sixth fixed comb electrode,  346   a : sixth fixed comb finger,  371 : optical function unit,  391 : handle layer (first semiconductor layer),  392 : device layer (second semiconductor layer),  393 : intermediate layer (insulating layer),  401 : optical device,  402 : support portion (base),  403 : first movable unit,  403   c : rib portion,  404 : second movable unit,  405 ,  406 : first torsion bar (elastic support portion),  407 ,  408 : second torsion bar (elastic support portion),  410 : mirror surface (optical function unit),  414 ,  415 : coil,  447 : rib portion,  460 : SOT substrate (semiconductor substrate),  461 : support layer (first semiconductor layer),  462 : device layer (second semiconductor layer),  463 : intermediate layer (insulating layer).