Patent Publication Number: US-2021165209-A1

Title: Mirror device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present application relates to the field of a mirror device. 
     Description of the Related Art 
     As a ranging system which measures the distance to a target utilizing laser light, light detection and ranging (LiDAR) is known. The distance measurement of the LiDAR is such that the distance to a tar get is measured, using a time-of-flight (TOF) method, by measuring the period of time from when light is emitted from a light emitting device until when a light receiving device receives the light reflected from the target. 
     In the LiDAR, a mirror device fabricated using a micromachining technology is used as a device which controls light. The mirror device is configured of a mirror chip, formed of a reflector, a drive coil, and torsion bars, and a magnet. The mirror chip is formed by thinning one portion of a silicon substrate. The basic operating principle of controlling the angle of a mirror is based on Fleming&#39;s left-hand rule. The drive coil is disposed in a direction perpendicular to a magnetic field, and when current is caused to flow through the drive coil, a force is applied to the drive coil. The force is called a Lorentz force, and the magnitude thereof is proportional to the intensity of current and magnetic field. 
     The mirror is supported by torsion bars. The torsion bars, as well as being the rotation shaft of the mirror, act as torsion springs which suppress the rotation of the mirror. When current flows through the drive coil on the periphery of the mirror, the elastic force of torsion springs generated by the torsion bars, together with a torque which causes the mirror to rotate, works in a direction opposite to the rotation, and the rotation of the mirror stops when the two forces balance with each other. The intensity of the current flowing through the drive coil is changed, and thereby it is possible to control the torque and thus to change the angle of the mirror. 
     A mirror device described in PTL 1 is such that a support substrate is of a frame form, and that a magnetic field generating portion which corresponds to the drive coil is provided outside the support substrate. A movable portion supported by torsion bars is provided inside the frame form of the support substrate, and a reflective layer acting as the mirror is provided in the center of the movable portion, wherein the drive coil is provided in an outer peripheral portion of the movable portion so as to surround the reflective layer. The drive coil is connected by electrical wirings, by way of the torsion bars, to terminals provided on the support substrate. 
     [PTL 1] Japanese Patent No. 4,471 271 
     In order to widen the angle and increase the distance of the detection range of the LiDAR, it is effective to drive through a wide angle the movable portion of the mirror device which scans the light from the light emitting device and also to expand the area of the reflective layer which receives the light reflected from the target. In the mirror device of PTL 1, however, when the area of the reflective layer is increased, the area of the movable portion including the drive coil which compasses the periphery of the reflective layer increases, so that there is a problem in that the weight of the movable portion increases, reducing the deflection angle of the movable portion. 
     In order to increase a deflection angle θ, it is required to lower the moment of inertia. That is, the measure of the deflection angle is inversely proportional to the weight of the movable portion, and the structure is not suitable to establish compatibility between an increase in distance and an increase in angle, so that there is the problem of having to adopt a structure focusing on either one. 
     SUMMARY OF THE INVENTION 
     The present application has been made to solve the above problem, and an object of the present application is to provide a mirror device which, by realizing a large-area reflective layer without increasing the weight of a movable portion, enables the compatibility with a deflection angle. 
     A mirror device disclosed in the present application includes a support substrate of a frame form; a movable portion which is provided inside the frame form of the support substrate; first torsion bars which, being provided between the support substrate and the movable portion, connect the movable portion to the support substrate; a first drive coil which is wound a plurality of turns on the peripheral edge of the principal surface of the movable portion; a smoothing layer which, being provided on the front surface of the movable portion, fills the space between adjacent turns of the drive coil, flattening the front surface of the movable portion; and a reflective layer which, being provided on the front surface of the smoothing layer, reflects light. 
     In the mirror device disclosed in the present application, the front surface of the movable portion including the first drive coil is flattened by the smoothing layer, and the reflective layer is provided on the front surface of the smoothing layer, thus enabling the reflective layer to be provided on the whole region of the movable portion, so that it is possible to realize a large-area reflective layer without increasing the weight of the movable portion. 
     The foregoing and other object, features, aspects, and advantages of the present application will become more apparent from the following detailed description of the present application when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially exploded perspective view showing the configuration of a mirror device according to a first embodiment of the present application. 
         FIG. 2  is a sectional view taken along the line A 1 -A 2  of  FIG. 1 . 
         FIG. 3A  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 3B  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 3C  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 3D  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 4A  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 4B  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 4C  is a process diagram for manufacturing the mirror device according to the first embodiment of the present application. 
         FIG. 5  is a plan view of a mirror device according to a second embodiment of the present application. 
         FIG. 6  is a plan view of the mirror device according to the second embodiment of the present application. 
         FIG. 7  is a plan view of a mirror device according to a third embodiment of the present application. 
         FIG. 8  is a plan view of the mirror device according to the third embodiment of the present application. 
         FIG. 9  is a plan view of a mirror device according to a fourth embodiment of the present application. 
         FIG. 10  is a plan view of the mirror device according to the fourth embodiment of the present application. 
         FIG. 11  is a plan view of the mirror device according to the fourth embodiment of the present application. 
         FIG. 12  is a plan view of the mirror device according to the fourth embodiment of the present application. 
         FIG. 13  is a plan view of a mirror device according to a fifth embodiment of the present application. 
         FIG. 14  is a perspective view of the rear surface a mirror device according to a sixth embodiment of the present application. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will be given, in accordance with the drawings, of a mirror device disclosed in the present application. 
     The drawings are schematic and give a conceptual description of functions structures. Also, in the drawings, portions of the same sign are identical or equivalent to each other. 
     First Embodiment 
       FIG. 1  is a partially exploded perspective view of a mirror device according to a first embodiment, and  FIG. 2  shows an A 1 -A 2  sectional view of a movable portion. As shown in  FIGS. 1 and 2 , the mirror device has a movable portion  1 , torsion bars  2 , and a support substrate  3  configured as one unity. The support substrate  3  is of a frame form. The movable portion  1  is provided inside the frame-formed support substrate  3 . All the principal surface of the movable portion  1  is covered with a reflective layer  4 . A drive coil  5  provided underlying the reflective layer  4  is electrically connected to electrical wirings  6 . Magnetic field generating portions  7  are provided on the perimeter of the support substrate  3 , and the movable portion  1  supported by the torsion bars  2  is oscillated by a Lorentz force produced by causing current to flow through the drive coil  5 . 
     The drawing shows the state in which the magnetic field generating portions  7  are provided outside the support substrate  3 . In the event that a magnetic field is applied to the drive coil  5 , however, it does not matter wherever the magnetic field generating portions  7  are positioned; for example, they may be installed on the rear side of the mirror device. When the magnetic field generating portions  7  are installed on the outer perimeter of the support substrate  3 , the magnetic force decreases further away from the magnetic field generating portions  7 , so that in order to increase the Lorentz force, the magnetic field generating portions  7  are preferably brought as close to the drive coil  5  as possible. 
     The movable portion  1  is of a structure, as shown in  FIG. 2 , in which the drive coil  5  is formed on the principal surface of a parent material  8 , surface irregularities caused by the drive coil  5  are flattened using a smoothing layer  9 , and the reflective layer  4  is formed on the smoothing layer  9 . That is, the drive coil  5  is provided, looped in coils at predetermined intervals, on a front surface of the principal surface of the parent material  8 , thereby forming the space between the wound coils. The space is filled with the smoothing layer  9 , thereby flattening the principal surface. 
     When comparing a heretofore known mirror device and the mirror device according to the first embodiment, in the heretofore known mirror device, the irregularities are caused by the drive coil  5 , so that the reflective layer  4  is provided only inside the region in which the drive coil  5  is wound, while in the mirror device according to the first embodiment, a configuration is adopted such that the surface irregularities caused by the drive coil  5  are flattened using the smoothing layer  9 , and the reflective layer  4  is formed on the front surface of the smoothing layer  9 , thus enabling an increase in the area of the reflective layer  4 . By adopting this kind of configuration, the weight increases by an amount equivalent to that of the smoothing layer  9 . However, there is little affect because the parent material  8  is dominant in weight. The area of the reflective layer increases by an amount equivalent to the region of the front surface portion in which the drive coil  5  is provided, as compared with in the heretofore known mirror device. 
     For example, the axial direction of the torsion bars  2  is set to be longitudinal, and when it is taken that the dimensions of the parent material  8  are 6 mm in length a and 4 mm in width b, a number of turns n of the drive coil  5  is five, a width L of the drive coil  5  is 30 μm, and a space  10  between adjacent turns of the drive coil  5  is 5 μm, an area S in which the drive coil  5  is disposed is 2(L+S)n×{a+b-2(L+S)n}, so that when the numerical values are assigned, the solution about 3.4 mm 2 . Consequently, an area of 20.6 mm 2  obtained by subtracting 3.4 mm 2  from an area a×b of 24 mm 2  of the parent material  8  is used in the heretofore known mirror device, while the area 24 mm 2  of the parent material  8  can be used as the area of the reflective layer  4 , thus enabling the area of the reflective layer  4  to increase by about 16%. 
     Also, the drive coil  5  cannot be formed in the central portion of the parent material  8  in the heretofore known mirror device, but the smoothing enables the drive coil  5  to be provided all the way to the center of the parent material  8 , and the number of turns of the drive coil  5  can be increased, as a result of which drive force increases, enabling an increase in the deflection angle of the mirror device. For example, when the number of turns n of the drive coil  5  is changed from five to ten, the Lorentz force is doubled, so that the deflection angle is doubled. Strictly speaking, however, the drive coil  5  is wound toward the inside of the parent material  8 , so that the length of each turn of coil on and after the second lap shortens by 2(L+S) per lap. Consequently, even though the number of turns is doubled, the Lorentz force is not doubled, and the deflection angle is not doubled either. Also, when the number of turns of the drive coil  5  is increased, coil resistance increases, raising a concern over a deterioration in drive characteristics due to heat generation. However, an optimal number of turns can be set while taking into consideration these facts. 
     Next, a description will be given, referring to  FIGS. 3A to 3D  and  FIGS. 4A to 4C , of a method of manufacturing the mirror device according to the present embodiment. 
     First, as shown in  FIG. 3A , a silicon-on-insulator (SOI) substrate is prepared. The SOI substrate has a structure in which a buried oxide (BOX) layer  12  made of silicon oxide and an active layer  13  made of silicon are sequentially bonded to the front surface a holding substrate  11  made of silicon. The BOX layer  12  serves as a hard mask when processing the rear surface of the SOI substrate as shown in  FIG. 4C . The film thickness is set to, for example, 500 μm for the holding substrate  11 , 1 μm for the BOX layer  12 , and 60 μm for the active layer  13 .  FIGS. 3A to 3D  and  FIGS. 4A to 4C  schematically represent the configuration without regard to the dimensions. 
     In the first embodiment, the thickness of the active layer  13  is set to be the same as that of the parent material of the movable portion  1 . However, the thickness of the active layer  13  can be reduced, for example, by being processed by grinding before the process moves to the step shown in  FIG. 3B . Alternatively, the thickness of the active layer  13  can be conformed to that of the parent material  8  by being etched using an etching unit or an etchant, so that in this step, there is no problem even though the parent material  8  of the movable portion  1  and the active layer  13  differ in thickness from one another. 
     Next, as shown in  FIG. 3B , first oxide films  14  are formed one on each surface of the SOI substrate by thermal oxidation. The first oxide films  14  are used as a mask when implanting ion. The first oxide films  14  formed are selectively removed using photolithography. The thickness of the first oxide films  14  to be formed is set to, for example, 0.5 μm. 
     Subsequently, a process, such as ion implantation, is executed to form piezoresistance. After the piezoresistance is formed, a second oxide film  15  is formed as a field oxide film, as shown in  FIG. 30 . This provides insulation between elements. The film thickness of the second oxide film  15  is set to, for example, 1 μm. 
     Following that, the drive coil  5  and the electrical wirings  6  are formed as in  FIG. 3D . The drive coil  5  and the electrical wirings  6  are formed of, for example, aluminum or copper, wherein the thickness is set to be on the order of 1.8 μm, and the turn intervals of the drive coil  5  are set so that the width L of the drive coil  5  is 30 μm, the space  10  between adjacent turns of the drive coil  5  is 10 μm, and the number of turns n of the drive coil  5  is  15 . 
     When the space  10  between adjacent turns of the drive coil  5  is large, while the thickness of the smoothing layer  9  to be formed in the next step is small, tens to hundreds of nanometer-sized depressed portions occur on the smoothing layer  9  which fills between adjacent turns of the drive coil  5 . For this reason, the space  10  between adjacent turns of the drive coil  5  is preferably small. For example, when the drive coil  5  and the smoothing layer  9  are formed to a thickness of 3 μm and 4 μm, respectively, the space  10  between adjacent turns of the drive coil  5  is changed from 5 μm to 4 μm, thereby shallowing the depressed portions by on the order of 0.03 μm. 
     Next, as shown in  FIG. 4A , the smoothing layer  9  is formed to reduce the irregularities of the front surface of the parent material  8  which are caused by forming the drive coil  5 . The formation of the smoothing layer  9  is such that unevenness caused by the drive coil  5  is filled in with a material applied by, for example, spin coating or spray coating. When large unevenness occurs by forming the drive coil  5 , a material low in viscosity and heavy in weight is used in advance, thereby preventing air bubbles from remaining in uneven portions resulting from the irregularities caused by the drive coil  5 , thus facilitating the smoothing. The thickness of the smoothing layer  9  is desirably such, for example, that the space  10  between adjacent turns of the drive coil  5  is smoothed out, and that the smoothing layer  9  is formed to a certain value of film thickness on the drive coil  5 . 
     A certain value of film thickness, which is referred to here, means that it serves as the insulation between the drive coil  5  and the reflective layer  4 . Furthermore, the smoothing layer  9  also serves as a protective layer with which to protect the drive coil  5 . 
     A material for the smoothing layer  9  includes, for example, epoxy resin or acrylic resin. As a step in which to implement a smoothing process, for example, the step of applying a resist after applying a smoothing material and of selectively etching the smoothing layer  9  formed on the drive coil  5  may be implemented a plurality of times. Also, the method may be used polishing a convex portion of the smoothing material which is raised by the drive coil  5  is polished and flattened. Alternatively, the method may also be used of forming trenches in advance and of embedding metal therein and carrying out etching back by grinding. 
     Next, the reflective layer  4  is formed, as shown in  FIG. 4B . A metallic thin film with a high reflectance in the wavelength of light to be used is desirable for the reflective layer  4 . As the material of the metallic thin film, for example, titanium (Ti), aluminum (Al), copper (Cu), silver (Ag), or gold (Au) is used. 
     Next, as shown in  FIG. 4C , the active layer  13 , the BOX layer  12 , and the holding substrate  11  are etched, fabricating a region a for the movable portion  1 , a region b for the torsion bars  2 , and a region c for the support substrate  3 , thus obtaining the finished mirror device. When etching the holding substrate  11 , the etching starts with the rear surface thereof. 
     When the axial direction of the torsion bars  2  are set to longitudinal, the external dimensions of the movable portion  1  are set to, for example, 6 mm in length, 4 mm in width, and 60 μm in thickness. Also, the external dimensions of the torsion bars  2  are set to, for example, 1 mm in length and 200 μm in width. Here, the movable portion  1  is rectangular in shape, but may be, for example, polygonal, circular, or elliptical. 
     Although the individual configurations according to the present embodiment are not limited to the above described configurations, it goes without saving that the same advantageous effect can be obtained for any external dimensions as long as the conditions are met that the irregularities caused by forming the drive coil  5  are smoothing processed and the reflective layer  4  is provided on the upper layer thereof. Manufacturing method steps in the second and the subsequent embodiments are the same as in the first embodiment and so will be omitted from being described, and a description will be given of differing portions. 
     Second Embodiment 
     The stress produced by torsional deformation when driving the mirror device shows a high value in the vicinity of the centers of the torsion bars  2  and in the vicinity of the corners of the connections between the movable portion  1  and the torsion bars  2 . The high value seen in the vicinity of the centers of the torsion bars  2  results mainly from shearing stress, and the high value seen in the vicinity of the corners of the connections between the movable portion and the torsion bars  2  results mainly from tensile stress. The intensity of the stress changes to some extent depending on the deflection angle of the mirror device or on a material and a film thickness which are used for the smoothing layer  9 . In general, this kind of mirror device carries out repeated movements for a long time, so that the stress acts repeatedly on these portions. As a result, when the smoothing layer  9  is formed in the portions on which the stress concentrates, there is fear that a crack occurs in the smoothing layer  9  and, furthermore, that the drive of the mirror device is adversely affected. In particular, as shown in the first embodiment, when the drive coil  5  is provided all the way to the center of the movable portion  1  and the front surface of the movable portion  1  is flattened by filling the irregularities between adjacent turns of the drive coil  5  with the smoothing layer  9 , the weight increases, albeit only slightly, so that it is necessary to take into account the stress produced by torsional deformation. In a second embodiment, the portions in the vicinity of the corners of the connections between the movable portion  1  and the torsion bars  2 , in the first embodiment, are defined as stress concentration portions, thereby responding to the problems created by the stress concentration. 
       FIGS. 5 and 6  each show a plan view of the mirror device according to the second embodiment. The magnetic field generating portions  7  of the mirror device are omitted from the following plan views. 
       FIG. 5  shows that regions in which neither the reflective layer  4  nor the smoothing layer  9  is provided are set in the movable Portion  1  as the regions correspond to stress concentration portions  16  in the vicinity of the connections between the movable portion  1  and the torsion bars  2 . Also,  FIG. 6  shows that a plurality of slits  17  are formed in each of the regions of the smoothing layer  9  which correspond to the stress concentration portions  16  in the vicinity of the corners of the connections between the movable portion  1  and the torsion bars  2 . The slits  17  shown in  FIG. 6  are shaped to be smaller in width than a laser wavelength used in LiDAR. By so doing, diffraction occurring when light is transmitted and received can be reduced depending on the diffraction limit of the light. As shown in  FIG. 5 , a configuration is such as not to form the smoothing layer  9  in the stress concentration portions  16 , thereby relaxing strain caused by stress concentration. That is, by providing a region where the smoothing layer  9  and the reflecting layer  4  are not provided, two effects can be obtained: an effect of preventing film peeling due to stress and an effect of improving optical characteristics by reducing warpage of the mirror. 
     Also, as shown in  FIG. 6 , the plurality of slits  17  are provided in the regions of the smoothing layer  9  which correspond to the stress concentration portions  16 , thereby enabling a relaxation of stress concentration. 
     The stress concentration portions  16  in which neither the reflective layer  4  nor the smoothing layer  9  is provided are formed by, after forming the reflective layer  4  shown in, for example,  FIG. 4B , selectively removing it using photolithography. Also, after forming the smoothing layer  9  shown in  FIG. 4A , the slits  17  may be formed in portions under which the drive coil  5  does not exist. By so doing, the reflective layer  4  which is to be formed on the drive coil  5  in the next step can be prevented from being electrically connected to the drive coil  5 . 
     The slits  17  do not have to be formed parallel to the torsion bars  2 . For example, the slits  17  may be formed arcuate and be discretely distributed. 
     Third Embodiment 
       FIG. 7  shows one example of a third embodiment. The example in  FIG. 7  is of a structure including a covering layer  18  on the principal surfaces of both the torsion bars  2  and the support substrate  3 . In the third embodiment, as compared with in the first embodiment, the electrical wirings (shown in  FIGS. 5 and 6 ) formed on both the torsion bars  2  and the support substrate  3  are covered with the covering layer  18 , thereby improving the optical property and environment resistance of the mirror device. 
     The covering layer  18  is fabricated after forming the smoothing layer  9  or the reflective layer  4  as shown in, for example,  FIGS. 4B and 4C . 
     The covering layer  18  is such that a film with a low reflectance compared with that of the reflective layer  4  is formed on the front surface of both the support substrate  3  and the electrical wirings  6  formed on the support substrate  3 , both of which are relatively large in surface area among the components around the reflective layer  4 , thereby reducing unnecessary light reflection, enabling an improvement in the optical property of the mirror device. That is, as shown in  FIG. 4C , in the state in which the region a of the movable portion  1 , the region b of the torsion bar  2 , and the region c of the support, substrate  3  are fabricated, the electrical wirings connected to the drive coil  5  are exposed. The electrical wirings are as high in reflectance as the reflective layer  4 , possibly causing false detection of a reflection from the electrical wirings. For this reason, a configuration is adopted such as to cover a high reflectance component with the covering layer  18 . As the covering layer  18 , for example, low reflectance carbon black is formed in portions other than the reflective layer  4 , and thereby the distinguishing of the reflective layer  4  can be carried out with precision, enabling an improvement in the optical property of the mirror device. When providing the covering layer  18  only on the front surface of a limited region, a region to be covered can be set using a mask. 
     The plurality of slits  17  are also provided in stress concentration portions in the vicinity of the centers of the torsion bars  2  and in the vicinity of the corners of the connections between the support substrate  3  and the torsion bars  2 . That is, as shown in  FIG. 8 , the slits  17  are formed in portions shown in the second embodiment, in which there is concern that a crack occurs due to stress, for example, both in the stress concentration portions of the connections between the movable portion  1  and the torsion bars  2  and in the stress concentration portions of the connections between the torsion bars  2  and the support substrate  3 , and thereby it is possible to solve the problems created by implementing the first embodiment. 
     Fourth Embodiment 
       FIG. 9  shows one example of a fourth embodiment.  FIG. 9  is a plan view showing the principal surface of the mirror device. The mirror device according to the fourth embodiment is such that the support substrate  3  is configured of an intermediate frame  22  and an outside frame  30 , wherein the movable portion  1  is connected to the intermediate frame  22  by inside torsion bars  20  acting as first torsion bars, and the intermediate frame  22  is connected to the outside frame  30  by outside torsion bars  23  acting as second torsion bars. That is, a structure is such that the movable portion wherein an inside drive coil  19 , the smoothing layer  9 , and the reflective layer  4  are formed in sequence on the parent material  8  is supported from the intermediate frame  22  by the inside torsion bars  20 , the respective other ends of the inside torsion bars  20  are connected to the inner circumference of the intermediate frame  22  on which are provided an outside drive coil  21  acting as a second drive coil, the outer circumference of the intermediate frame  22  is supported by the outside torsion bars  23  oriented in a direction perpendicular to the inside torsion bars  20 , and the respective other ends of the outside torsion bars  23  are connected to the outside frame  30  of the support substrate  3 . 
     The inside drive coil  19 , being connected to first electrical wirings  24 , is connected tc the outside frame  30  by way of the inside torsion bars  20 , the intermediate frame  22 , and the outside torsion bar  23 , while the outside drive coil  21 , being connected to second electrical wirings  25 , is connected to the outside frame  30  by way of the outside torsion bar  23 . The magnetic field generating portions  7  are provided outside the outside frame  30 , wherein by causing current to flow through the inside and outside drive coils  19  and  21 , the inside and outside torsion bars  20  and  23  are twisted, oscillating the movable portion  1 . In the fourth embodiment, the number of torsion bars  2  is increased as compared with in the first embodiment, thereby enabling biaxial drive, enabling an operation such as Lissajous scanning or raster scanning. 
     The movable portion  1  moves in conjunction with the movement of the inside and outside torsion bars  20  and  23 , and the intermediate frame  22  moves in conjunction with the movement of the outside torsion bars  23 . Consequently, in some cases, the movable portion  1  and the intermediate frame  22  face in different directions from one another. When the outside drive coil  21  is exposed, reception light reflects from the outside drive coil  21 , meaning that a piece of light which does not head in its original direction is sent to a light detector (not shown), and there is a possibility of false detection. For this reason, the outside drive coil  21  is covered with the smoothing layer  9  or the covering layer  18 , thereby reducing the amount of reflection, enabling a reduction in the false detection by the light detector. 
     The mirror device of the structure shown in the fourth embodiment can be fabricated by the manufacturing method shown in  FIGS. 3A to 3D  and  FIGS. 4A to 4C . 
     The response to the stress concentration, shown in the second embodiment, can be combined with the structure of the mirror device according to the fourth embodiment. For example, the configuration of  FIG. 5  in the second embodiment and the configuration of  FIG. 9  in the fourth embodiment are combined together, adopting a configuration such that no reflective layer is provided in the stress concentration portions  16  in the vicinity of the connections between the movable portion  1  and the inside torsion bars  20 , as shown in  FIG. 10 . Alternatively, the configuration of  FIG. 6  in the second embodiment and the configuration of  FIG. 9  in the fourth embodiment are combined together, and thus it is possible, as shown in  FIG. 11 , to adopt a configuration such that slits are provided in the stress concentration portions  16  in the vicinity of the connections between the movable portion  1  and the inside torsion bars  20 . 
     Furthermore, the structure of the covering layer  18  shown in the third embodiment is combined with the structure of the mirror device according to the fourth embodiment, and thus a region in which the covering layer  18  is not formed can be provided in a portion on which stress concentrates. Also, as shown in  FIG. 12 , a configuration is adopted such that the slits  17  are provided in the stress concentration portions of the movable portion  1 , the inside torsion bars  20 , the intermediate frame  22 , the outside torsion bars  23 , and the outside frame  30 , and that portions other than the reflective layer  4  are covered with the covering layer  18 , and thereby it is possible to achieve the advantageous effects acquired by the third and fourth embodiments. 
     Fifth Embodiment 
       FIG. 13  shows one example of a fifth embodiment.  FIG. 13  is a sectional view of the movable portion  1 . The fifth embodiment is of a structure which includes a dummy portion  26 , which is not connected to the drive coil  5  or the inside drive coil  19 , in the central portion of the movable portion  1  in which the drive coil  5  shown in the first to third embodiments or the inside drive coil  19  shown in the fourth embodiment is not formed. As described in the manufacturing method in the first embodiment, when the space  10  between adjacent turns of the drive coil  5  is large, the depressed portions occurring on the smoothing layer  9  are large. 
     In order not to cause the depressed portions to occur, there is a method of forming the smoothing layer  9  to a large thickness, but when the thickness of the smoothing layer  9  is too large, there is fear that the movable portion  1  increases in weight, reducing the deflection angle. For this reason, the dummy portion  26  is provided as an embedded member. In particular, when the drive coil  5  is not provided all the way to the center of the movable portion  1 , the peripheral edge portion of the movable portion  1  is higher than the central portion thereof due to the existence of the drive coil  5 . Because of this, the central portion of the movable portion  1  is depressed. The dummy portion  26  is provided in order to eliminate this depression. By providing the dummy portion  26 , for example, a 1.8-μm depressed portion which should have occurred when a 2.5-mm long space exists can be reduced to 0.04 μm. The depressed portion can be set depending on the thickness of the dummy portion  26  and the viscosity of a material to be used for the smoothing layer  9  or the film thickness of the material to which it is to be formed. The dummy portion  26  may he of any material as long as it is a material with which a desired pattern can be formed. The provision of the dummy portion  26  as the embedded member can also be implemented in the same way, and has the same advantageous effect, as in the first embodiment. 
     Sixth Embodiment 
       FIG. 14  shows one example of a sixth embodiment. FIG.  14  shows the shape of the rear surface of the mirror device, that is, the shape of a surface opposite to the surface on which the reflective layer  4  is formed. In  FIG. 14 , a plurality of lightening portions  27  are formed in the rear surface of the movable portion  1 . In the sixth embodiment, as compared with in the first embodiment, this enables a reduction in the weight of the mirror device, and also, the mirror device can have a structure which is not likely to undergo warpage by creating a portion provided with no lightening portion  27 . The lightening portions  27  which are formed in the rear surface of the movable portion  1  can be formed at the same time, for example, in the step shown in  FIG. 40 , that is, in a step of processing the shape of the rear surface. Also, in the mirror device shown in the fourth embodiment, the lightening portions  27  may be formed in the intermediate frame  22 . By so doing, the intermediate frame  22  can be reduced in weight, and the deflection angle on the second axial side of the mirror device can be increased. The lightening portions  27  to be formed in the rear surface of the intermediate frame  22  can also be formed by carrying out the formation of the lightening portions  27  at the same time as, for example, in the step shown in  FIG. 4C , that is, in the step of processing the shape of the rear surface. The lightening portions  27  to be formed can be fabricated in various patterns, such as a zonal pattern, a matrix pattern, and a honeycomb pattern. 
     Although the present application is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the embodiments. 
     It is therefore understood that numerous modifications which have not been exemplified can be devised without depart in from the scope of the present application. For example, at least one of the constituent components may be modified, added, or eliminated. At least one of the constituent components mentioned in at least one of the preferred embodiments may be selected and combined with the constituent components mentioned in another preferred embodiment.