Patent Publication Number: US-2022236556-A1

Title: Light deflector and manufacturing method

Description:
TECHNICAL FIELD 
     The present invention relates to a light deflector for MEMS and a manufacturing method therefor. 
     BACKGROUND ART 
     There is known a light deflector for piezoelectric MEMS (For example, Patent literature 1). 
     In the light deflector of Patent Literature 1, a joint edge of a minor part and each of torsion bars is formed by radius parts recessed inward to prevent damage to the joint portion of the mirror part and each of the torsion bars. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-Open No. 2017-151476 
     SUMMARY OF INVENTION 
     Technical Problem 
     A light deflector for general piezoelectric MEMS uses an SOI (Silicon on Insulator) wafer as the substrate. The plane index of the principal surface of a general SOI wafer is ( 100 ) or ( 110 ). 
     On the other hand, when the frequency of reciprocal turning of a mirror part about the axis of torsion bars increases, it is preferred to increase the rigidity of the torsion bars in a twisting direction. Therefore, the light deflector is manufactured to adjust the crystal orientation of the torsion bars in the axial direction to &lt; 100 &gt;. 
     However, when the crystal orientation of the torsion bars in the axial direction is set to &lt; 100 &gt;, the normal direction becomes &lt; 110 &gt;as a cleavage direction of silicon single crystal at a point on the curved line of the radius part. Since the cleavage direction is a direction easy to form a crack and cause the crack to grow, it will be easier to form a crack at the point and break the torsion bar. 
     It is an object of the present invention to provide a light deflector and a manufacturing method therefor capable of preventing the cleavage of a radius part provided in a joint edge portion of each of torsion bars. 
     Solution to Problem 
     A light deflector of the present invention includes: 
     a mirror part which can turn reciprocally about a predetermined rotation axis; 
     a pair of torsion bars extending from both sides of the mirror part along the rotation axis of the mirror part; 
     a frame part which surrounds the mirror part and the pair of torsion bars; 
     a plurality of piezoelectric actuators which intervenes between each torsion bar and the frame part to cause torsional vibration of each torsion bar about the rotation axis so as to reciprocally turn the mirror part about the rotation axis; 
     first radius parts each of which is formed by a cylindrical curved surface recessed inward in a joint edge portion between each torsion bar and the mirror part; and 
     second radius parts each of which is formed by a cylindrical curved surface recessed inward in a joint edge portion between each piezoelectric actuator on a side facing the mirror part and each torsion bar, wherein 
     a substrate layer of the mirror part, the torsion bars, and the piezoelectric actuators is a common silicon single crystal layer, 
     a plane index of a principal surface of the silicon single crystal layer is either one of ( 100 ) and ( 110 ), 
     an axial direction of the torsion bars is &lt; 100 &gt;as a crystal orientation of the silicon single crystal layer, and 
     at least a predetermined section of either one of the first radius parts and the second radius parts is so formed that unevenness with respect to the cylindrical curved surface is 600 nm or less. 
     According to the present invention, either one of the predetermined sections of the first radius parts and second radius parts can be so formed that unevenness with respect to the cylindrical curved surface is 600 nm or less. Thereby, at least either one cleavage can be suppressed. 
     Preferably, both ends of the predetermined section in the light deflector of the present invention are set outside of both ends of a center divided section when the cylindrical curved surface is divided into three equal sections. 
     Since the axial direction of the torsion bars is the crystal orientation of &lt; 100 &gt;of the silicon single crystal layer, a cleavage direction in each of the first radius parts and second radius parts exists within the center divided section when the cylindrical curved surface is divided into three equal sections. According to this structure, the suppression of cleavage in a range including a cleavage direction in at least either one of the first radius parts and the second radius parts can be guaranteed. 
     Preferably, in the light deflector of the present invention, 
     the at least ether one is so formed that a contour line when each radius part is cut out on a plane parallel to the principal surface is set as a waviness curve, 
     an average line of waviness curves is set by a method of least squares, 
     an adjacent peak to peak section of the waviness curve is set as a cycle, 
     a distance from the average line at each position of the waviness curve is set as the amount of waviness, and 
     a difference between the maximum amount of waviness and the minimum amount of waviness of the predetermined section in each cycle is 600 nm or less over the whole cycle included in the predetermined section. 
     According to this structure, the amount of waviness of the predetermined section can be suppressed to suppress the cleavage of the first radius parts and the second radius parts. 
     Preferably, the plane in the light deflector of the present invention is at least either one surface. 
     The surface of each of the first radius parts and the second radius parts is located in a shallow position from the surface of the light deflector. According to this structure, it is easy to detect a contour line of each of the first radius parts and the second radius parts. 
     Preferably, the at least either one in the light deflector of the present invention is the second radius parts. 
     The torsion bars receive a torsional force about the rotation axis from each of the piezoelectric actuators and transmit it to the mirror part. Therefore, a stronger torsional force is applied to the second radius parts than to the first radius parts during the operation of the light deflector. 
     According to this structure, the life of the torsion bars can be extended by suppressing unevenness by giving priority to the first radius parts over the second radius parts. 
     A manufacturing method of the present invention includes: 
     a coating process in which the surface of a substrate including an active layer of silicon single crystal with a plane index of a principal surface being either one of ( 100 ) and ( 110 ) is covered with a photoresist film with a film thickness of not thinner than 5 μm and not thicker than 10 μm; 
     an exposure process in which the surface side of the substrate after the coating process is exposed through a photomask including a contour pattern of contours of the mirror part, the torsion bars, the piezoelectric actuators, the first radius parts, and the second radius parts of the light deflector; and 
     a contouring process in which the surface side of the substrate is etched to form, on the active layer, the contours of the mirror part, the torsion bars, the piezoelectric actuators, the first radius parts, and the second radius parts of the light deflector. 
     According to the manufacturing method of the present invention, it can guarantee the amount of waviness of the curved surface of each of the first radius parts and second radius parts within 600 nm to suppress the cleavage of the radius part in the joint portion of the torsion bar. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a front view of a MEMS light deflector. 
         FIG. 2A  is a front view of an SOI wafer used in the manufacture of the light deflector. 
         FIG. 2B  is a front view of another SOI wafer used in the manufacture of the light deflector. 
         FIG. 3  is an enlarged diagram of a range including torsion bars. 
         FIG. 4  is a distribution image of surface stress of a torsion bar by simulation. 
         FIG. 5  is a distribution image of cross-sectional stress of the torsion bar by simulation. 
         FIG. 6  is a microscopic observation image of destruction in such a light deflector that no measures have been taken against the amount of waviness of a radius part. 
         FIG. 7  is a SEM observation image of a fracture surface of  FIG. 6  observed from a predetermined direction. 
         FIG. 8  is an explanatory chart of the amount of waviness when applying the JIS standard for surface roughness to a curved surface of the radius part. 
         FIG. 9  is a graph illustrating a relationship between the amount of waviness of the radius part and limit deflection angle. 
         FIG. 10  is a process diagram of a manufacturing method for the light deflector. 
         FIG. 11  is a cross-sectional view of the light deflector manufactured by the manufacturing method of  FIG. 10 . 
         FIG. 12A  is a SEM observation image in which a section of a resist film layer exceeding 10 μm as a stipulated film thickness and the amount of waviness when the radius part is formed by the resist film layer have been observed. 
         FIG. 12B  is a SEM observation image in which a section of the resist film layer within 10 μm as the stipulated film thickness and the amount of waviness when the radius part is formed by the resist film layer have been observed. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     A preferred embodiment of the present invention will be described in detail below. In the following description, a common reference numeral is used for substantially identical or equivalent elements and parts. Further, as for elements or parts having the same structure but different only in arrangement position, a reference numeral with the same numeral but different only in alphabet letter attached is used. Further, when not distinguishing individually between the elements or parts having the same numeral but different only in alphabet letter attached, only the numeral is collectively attached to the elements or parts by omitting the alphabet letter(s) attached. 
     [MEMS Light Deflector] 
       FIG. 1  is a front view of a MEMS light deflector  10  (as seen from the front side). The light deflector  10  includes, as main elements, a mirror part  11 , torsion bars  12   a,    12   b,  inside piezoelectric actuators  13   a,    13   b,  a movable frame part  14 , outside piezoelectric actuators  15   a,    15   b,  and a fixed frame part  16 . 
     In the following, a three axis coordinate system consisting of X axis, Y axis, and. Z axis is defined for convenience of description. The X axis and the Y axis are set to a horizontal direction and a vertical direction of the light deflector  10  in front view. The Z axis is set to a thickness direction of the light deflector  10 . O is the center of the round. mirror part  11 . 
     The mirror part  11  can turn reciprocally about rotation axes  22   x  and  22   y  orthogonal to each other at the center O. The rotation axes  22   x  and  22   y  are parallel to the X axis and the Y axis, respectively, when the mirror part  11  faces straight ahead. 
     A pair of torsion bars  12   a,    12   b  are extending from both sides of the mirror part  11  along the rotation axis  22   y.  A pair of inside piezoelectric actuators  13   a,    13   b  are joined to each other in the X axis direction to have an elliptical ring shape vertically long as a whole. The elliptical ring is surrounding the mirror part  11  and the torsion bars  12 . 
     The movable frame part  14  has an elliptical ring shape larger than the elliptical ring formed by the pair of inside piezoelectric actuators  13   a,    13   b.  The elliptical ring is surrounding the elliptical ring of the pair of inside piezoelectric actuators  13   a,    13   b.  Each inside piezoelectric actuator  13  is joined to the inner circumference of the movable frame part  14  on the straight line of the X axis direction that passes through the center O. 
     Each of the outside piezoelectric actuators  15  intervenes between the movable frame part  14  and the fixed frame part  16 . Each outside piezoelectric actuator  15  is equipped with plural cantilevers  19  the vertical direction of which is aligned to the Y axis. Cantilevers  19  adjacent to each other in the X axis direction are coupled to each other either on one edge or the other edge in the Y axis direction. The coupling points are alternate from one to the other in the Y axis direction in order of alignment of the cantilevers  19  in the X axis direction. Thus, the plural cantilevers  19  in each outside piezoelectric actuator  15  constitute a coupling body having a meander pattern as a whole. 
     The outside piezoelectric actuators  15  are driven by a drive voltage from an unillustrated drive voltage supply unit to cause the movable frame part  14  to turn reciprocally about the rotation axis (≠ rotation axis  22   x ) in the X axis direction that passes through the center O at a non-resonant frequency. Thus, the mirror part  11  reciprocally turns about the rotation axis  22   x  at the non-resonant frequency. 
     The inside piezoelectric actuators  13  are driven by another drive voltage from the unillustrated drive voltage supply unit to cause the torsion bars  12  to torsionally vibrate about the rotation axis  22   y  at a resonance frequency. Thus, the mirror part  11  reciprocally turns about the rotation axis  22   y  at the resonance frequency. 
     [SOI Wafer] 
       FIG. 2A  and FIG,  2 B are front views of  501  wafers  25   a  and  25   b  used in the manufacture of the light deflector  10 , respectively. The front of each SOI water  25  is a principal surface on the surface side of the SOI wafer  25 . 
     The coordinate axes illustrated in  FIG. 2A  and  FIG. 2B  represent crystal orientations inside an active layer  28   d  ( FIG. 11 ) formed of silicon single crystal(in each SOI wafer  25 . An orientation flat  26  represents the crystal orientation of the active layer  28   d  (silicon single crystal layer) of the SOI wafer  25 . 
     Multiple light deflectors  10  are cut out of each SOI wafer  25 . In  FIG. 2A  and  FIG. 2B , the outline of each light deflector  10  is represented by a rectangular dashed line. The long and short sides of the rectangle correspond to the horizontal side (long side) and the vertical side (short side) of the fixed frame part  16  of the light deflector  10  in the front view of  FIG. 1 , respectively. 
     An orientation flat  26 a is in a direction of &lt; 100 &gt;. Therefore, the plane index of the principal surface of the SOI wafer  25   a  is ( 100 ). On the other hand, an orientation flat  26   b  is in a direction of &lt; 110 &gt;. Therefore, the plane index of the principal surface of a support layer  28 b is ( 110 ). 
     As will be described with reference to  FIG. 11  later, a substrate layer of the light deflector  10  is made up of the active layer  28   d  of the SOI wafer  25 . The dashed rectangle in  FIG. 2  represents the outline of each light deflector  10 . Therefore, the axial direction extension direction) of the torsion bars  12  are &lt; 100 &gt;in both of the SOI wafers  25   a  and  25   b.  This is because adjusting the axial direction of the torsion bars  12  to &lt; 100 &gt;is advantageous to increase the resonance frequency of the mirror part  11  about the rotation axis  22   y.  When the axial direction of the torsion bars  12  is adjusted to &lt; 100 &gt;, the torsional rigidity of the torsion bars  12  increases. 
     Therefore, in the SOI wafer  25   a,  the axial direction of the torsion bars  12  becomes a direction orthogonal to the orientation flat  26   a.  On the other hand, in the SOI wafer  25   b,  the axial direction of the torsion bars  12  becomes a direction at an angle of 45° with respect to the orientation flat  26   b.    
     [Radius Part] 
       FIG. 3  is an enlarged diagram of a range including the torsion bars  12  in  FIG. 1 . In such a central section, the movable frame part  14 , and the mirror part  11 , the torsion bars  12 , and the inside piezoelectric actuators  13  internally surrounded by the movable frame part  14 , are included. In  FIG. 3 , the three axis coordinate system of X axis-Y axis-Z axis, and directions of crystal orientations are illustrated. The X axis and the Y axis match with &lt; 100 &gt;. A direction at an angle of 45° with respect to both the X axis and the Y axis matches with &lt; 110 &gt;. 
     Each torsion bar  12  is joined to a circumferential portion of the mirror part  11  on the center O side, and joined to the movable frame part  1 A on the side opposite to the center O. Each inside piezoelectric actuator  13  is joined to each of the torsion bars  12   a,    12   b  in each of both edge portions having a semi-elliptical ring shape, and joined to the inner circumference of the movable frame part  14  in the central portion of the outer circumference thereof having the shape. 
     Radius parts  31  are formed in joint edge portions, in which each torsion bar  12  is joined to the mirror part  11 , the inside piezoelectric actuator  13 , and the movable frame part  14 , to increase the strength. Each of the radius parts  31  is formed by a curved surface recessed inward. 
     To be more precise, radius parts  31 aa and  3  lba constitute joint edge portions between each torsion bar  12  and the mirror part  11 . Radius parts  31   ab  and  31   bb  constitute joint edge portions between each torsion bar  12  and each inside piezoelectric actuator  13  on the side of the mirror part  11 . Radius parts  31   ac  and  31   bc  constitute joint edge portions between each torsion bar  12  and each inside piezoelectric actuator  13  on the side of the movable frame part  14 . Radius parts  31   ad  and  31   bd  constitute joint edge portions between each torsion bar  12  and the inner circumference side of the movable frame part  14 . 
     Although the radius parts  31  are formed to reinforce the corners, a problem arises in this light deflector  10 . In other words, as described above with reference to  FIG. 2A  and  FIG. 2B , the curved surface of each radius part  31  formed in each joint edge portion of each torsion bar  12  contains a surface element facing the cleavage direction of &lt; 110 &gt;, that is, a surface element the normal line of which becomes the direction of &lt; 110 &gt;to adjust the axial direction of the torsion bar  12  to &lt; 100 &gt;of the active layer  28 d. As a result, it will be easier to make cleavage progress from the portion of the surface element in the curved surface 
       FIG. 4  is a distribution image of surface stress of a torsion bar  12  by simulation.  FIG. 5  is a distribution image of cross-sectional stress of the torsion bar  12  by simulation. It means that the whither the area, the higher the stress. 
     It is found from  FIG. 4  and FIG,  5  that stress in portions corresponding to the radius parts  31   aa,    31   ba,    31   ab,  and  31   bb  ( FIG. 3 ) is remarkably higher than that in the other portions. 
       FIG. 6  is a microscopic observation image of the destruction of one torsion bar  12  in such a light deflector  10  that no measures have been taken against the amount of waviness of radius parts  31  to be described later.  FIG. 7  is a SEM observation image of a fracture surface  37  of  FIG. 6  observed from a predetermined direction. 
     In  FIG. 6 , the torsion bar  12   b  is broken between the mirror part  11  and the inside piezoelectric actuator  13  due to the destruction of the radius parts  31 . In  FIG. 7 , a surface  35  (front side surface) in the neighborhood of the fracture surface  37  and a curved surface  36  are projected together with the fracture surface  37 . 
     [Amount of Waviness] 
       FIG. 8  is an explanatory chart on the amount of waviness when applying the JIS standard for surface roughness to the radius parts  31 . The JIS standard is specifically “JIS B 0601-1994,” but it is assumed that other equivalent standards are also included. 
     In “JIS B 0601-1994,” measured cross-sectional curve, cross-sectional curve, reference length, roughness curve, peak, trough, highest peak, lowest trough, and average line are defined for surface roughness. Each word/phrase in  FIG. 8  conforms to the definitions thereof 
     The measured cross-sectional curve is a contour line when the curved surface of a radius part  31  is cut out in a predetermined cross section parallel to the X-Y plane (for example, the cross section passing through the center of thickness in the Z axis). Here, both ends of the contour line are set to positions in which the contour of the radius part  31  is parallel to &lt; 100 &gt;. The measured cross-sectional curve is divided into three equal sections in a direction of the reference length (the horizontal axis of  FIG. 8 ). The length of each divided section in the direction of the reference length is denoted by L. 
     Here, “waviness curve,” “cycle,” and “amount of waviness” are defined separately from the JIS standard described above. 
     The waviness curve shall be a contour line when the radius part  31  is cut out on a plane parallel to the principal surface of the SOI wafer  25 . In the JIS standard described above, the average line by the method of least squares is set with respect to the measured cross-sectional curve. In the calculation of the next amount of waviness, the average line by the method of least squares is set with respect to the waviness curve. Then, distance of each position on the waviness curve from the average line is defined as the amount of waviness. 
     The cycle shall be a section between the highest peaks when the roughness curve is replaced with the waviness curve in  FIG. 8 . Further, the plane mentioned above is the surface of the radius part  31 . Since the surface of the radius part  31  is located in a shallow position from the surface of the light deflector  10 , it is easy to measure the contour line of the radius part  31 . 
     In  FIG. 8 , Ta and Ba denote the maximum amount of waviness and the minimum amount of waviness in a cycle in which a difference between the maximum amount of waviness and the minimum amount of waviness is maximized in the entire cycle of the radius part  31 . Tb and Bb denote the maximum amount of waviness and the minimum amount of waviness in a cycle in which a difference between the maximum amount of waviness and the minimum amount of waviness is maximized among circles included in the center section of the three equal sections. There is a relation of Tb-Bb difference &lt;Ta-Ba difference. 
       FIG. 9  is a graph illustrating a relationship between the amount of waviness of the radius part  31  and H limit deflection angle. Here, the “H limit deflection angle” is the maximum deflection angle of the mirror part  11  about the rotation axis  22   y  when the torsion bar  12  is broken. In order to prevent the breakage of the torsion bar  12 , the light deflector  10  must be used by setting the deflection angle of the mirror part  11  about the rotation axis  22   y  to an angle less than the H limit deflection angle. 
     Note that there are two kinds of deflection angles of the mirror part  11 , that is, a deflection angle about the rotation axis  22   y  and a deflection angle about the rotation axis  22   x.  When distinguishing between both deflection angles, the deflection angle about the rotation axis  22   y  is called the “H deflection angle,” and the deflection angle about the rotation axis  22   x  is called the “V deflection angle.” Then, when the mirror part  11  faces straight ahead, it is defined that the H. deflection angle and the V deflection angle are both 0°. Note further that each of the deflection angles is a mechanical deflection angle. 
     In  FIG. 9 , each numerical value on the vertical axis means a relative value of the H limit deflection angle. H limit deflection angle=1.0 means the maximum allowed value of the H limit deflection angle when the light deflector  10  is used as an in-vehicle product. When the light deflector  10  is actually sold as a product, there is a need to guarantee H limit deflection angle=0.8 or more, that is, 20% less than the maximum allowed value. To do that, the amount of waviness of the curved surface of the radius part  31  has only to be set within 600 nm as can be found from  FIG. 9 . 
     [Suppression of Cleavage] 
     A structure to suppress the cleavage of the radius parts  31  in the light deflector  10  will be described. For all cycles included in predetermined radius parts  31  (particularly, at least either the radius parts  31   aa,    31   ba  or the radius parts  31   ab,    31   bb ), a difference between the maximum amount of waviness and the minimum amount of waviness in each cycle is set equal to or less than 600 nm. Thus, even when each radius part  31  has a portion to turn the normal line in the direction of the crystal orientation of &lt; 110 &gt;, cleavage from the portion can be effectively prevented. Note that at least either one of them is typically the radius parts  31   ab,    31   bb.    
     In other words, the above measures are so taken that each radius part  31  is formed by a cylindrically curved surface recessed inward as the joint edge portion between respective elements. The predetermined section of the radius part  31  is so formed that the unevenness against the cylindrically curved surface is 600 nm or less. 
     Both ends of the predetermined section (for example, which correspond to both ends of an extraction length in  FIG. 8 ) are set outside of both ends of a center divided section when the cylindrical curved surface is divided into the three equal sections. The center divided section corresponds to the center curved section among the three curved sections in the reference length direction (vertical axis direction) in the roughness curve of  FIG. 8 . 
     The cylindrical curved surface includes an ideal side of a cylinder (a side with an unevenness of 0). Further, the cylindrical curved surface shall include a side whose unevenness falls within a predetermined first threshold value for the ideal side of the cylinder, and a curved surface whose unevenness falls within the predetermined first threshold value and whose amount of upheaval or amount of depression per unit length in any direction of the side falls within a second threshold value. 
     The reason for setting both ends of a predetermined section outside of both ends of the center divided section is as follows. Namely, this is because the axial direction of the torsion bar  12  is the crystal orientation of &lt; 100 &gt; of the silicon single crystal layer, and hence the cleavage direction in the radius part  31  exists within the center divided section when the cylindrical curved surface is divided into three equal sections. 
     [Manufacturing Method] 
       FIG. 10  is a process diagram of a manufacturing method for the light deflector  10  to make the amount of waviness of the radius parts  31  within 600 nm, which is particularly an example of an SOI processing process. 
     In STEP 1 , an SOI wafer  25  (the SOI wafer  25   a  in  FIG. 2A  or the SOI wafer  25   b  in  FIG. 2B ) is prepared. 
     Next, in STEP 2 , an element forming layer  42  is formed on a surface  40  of the SOI wafer  25 . Specifically, the element forming layer  42  is an electrode layer  42   a,  a piezoelectric layer  42   b,  and an electrode layer  42   c  in  FIG. 11 . 
     STEP 3  corresponds to a coating process. In STEP 3 , the operation of spin rotation  45  is performed while dripping a photosensitizer  44  on the SOT water  25  with the surface  40  thereon from a nozzle  43 . Thus, the photosensitizer  44  spreads evenly on the surface of the element forming layer  42 . 
     In a conventional photosensitizer coating process, AZ4620 (viscosity: 400 cSt) is used as the photosensitizer. Further, the spin rotation  45  is 1000 rpm to 2000 rpm. By doing this, the film thickness of a resist film layer formed on the surface of the element forming layer  42  gets thicker than 10 μm. When the rotational speed of the spin rotation  45  increases to make the film thickness of the resist film layer  48  thinner, the film thickness unevenness increases in turn. 
     On the contrary, in STEP 3 , AZ6130 (viscosity: 70 cSt) is used as the photosensitizer  44 . Further, the spin rotation  45  is 500 rpm to 1000 rpm. By doing this, the resist film layer  48  the film thickness of which is made uniform is formed on the surface side of the element forming layer  42  in STEP 4 . Thus, the film thickness of the resist film layer  48  becomes not thinner than 5 μm and not thicker than 10 μm. 
     Note that if the film thickness is thinner than 5 μm, a MEMS uneven step structure cannot be covered. On the other hand, if the film thickness exceeds 10 μm, the photosensitizer  44  cannot be coated evenly on the surface of the element forming layer  42 , and hence the condition that the amount of waviness of the curved surface of the radius part  31  is within 600 nm cannot be satisfied. 
     In STEP 5 , a photomask  50  covers the SOI wafer  25  with the resist film layer  48  thereon. The photomask  50  has patterns  52  according to the number and arrangement of light deflectors  10  to be manufactured from one SOI wafer  25   
     In STEP 5 , ultraviolet light  51  is further irradiated from the surface side of the photomask  50  as an exposure process of the present invention. Areas of the resist film layer  48  corresponding to the patterns  52  of the photomask  50  are exposed by the ultraviolet light  51 . 
     FIG,  11  is a cross-sectional view of the light deflector  10  manufactured further through an etching process after STEPS, The SOI wafer  25  has a structure in which an SiO2 layer  28   a,  the support layer  28   b,  an SiO2 layer  28   c,  the active layer  28   d,  and an SiO2 layer  28   e  are laminated from the back side to the surface side. The element forming layer  42  has a structure in which the electrode layer  42   a,  the piezoelectric layer  42   b,  and the electrode layer  42   c  are laminated in this order from the side of the SOI wafer  25 . 
     The mirror part  11  has a metal layer  54  that covers the surface of the SiO2 layer  28   e,  The surface of the metal layer  54  is a reflective surface of light incident on the mirror part  11  from an unillustrated light source. The active layer  28   d  forms a common substrate layer of the mirror part  11 , the inside piezoelectric actuators  13 , and the outside piezoelectric actuators  15 . 
       FIG. 12A  is a SEM observation image in which a section of the resist film layer  48  exceeding  10  p.m as the stipulated film thickness of the resist film layer  48  and the amount of waviness of the curved surface of the radius part  31  manufactured with the film thickness have been observed.  FIG. 12B  is a SEM observation image in which a section of the resist film layer  48  within 10 μan as the stipulated film thickness of the resist film layer  48  and the amount of waviness of the curved surface of the radius part  31  manufactured with the film thickness have been observed. 
     From the comparison between the SEM observation images of  FIG. 12A  and  FIG. 12B , it can be understood that, when the film thickness of the resist film layer  48  is set less than stipulated 10 μm, the curved surface of the radius part  31  manufactured with the film thickness can be kept within desired 600 nm. 
     Modifications and Supplements 
     The light deflector  10  of the embodiments of the two-axis type, that is, a light deflector which two-dimensionally scans scanning light from the mirror part. However, the light deflector of the present invention may also be of the one-axis type, drat is, a light deflector rhich one-dimensionally scans scanning light from the mirror part. 
     The light deflector  10  of the embodiment is such that each torsion bar reaches the inner circumference of the movable frame part  14  (movable frame) beyond the joint portion with each inside piezoelectric actuator  13  as the piezoelectric actuator in the Y axis direction and is joined to the inner circumference thereof. However, in the light deflector of the present invention, the torsion bar does not have to be joined to the movable frame part  14  as a support part. 
     In the embodiment, the photosensitizer  44  using AZ6130 (viscosity: 70 cSt) as a photoresist film is adopted as the resist film layer  48 . However, the photosensitizer to form the photoresist film of the present invention is of any type as long as the surface side of the substrate before the exposure process can be covered with a uniform photoresist film of not thinner than 5 μM and not thicker than 10 μm. 
     The radius parts  31   aa  and  31   ba  of the embodiment correspond to first radius parts of the present invention. The radius parts  31   ab  and  31   bb  correspond to second radius parts of the present invention. In the embodiment, the entire surface  40  of the SOI wafer  25  is covered with the resist film layer  48  the film thickness of which is not thinner than 5 μm and not thicker than 10 μm as described in STEP 4  of  FIG. 10 . Therefore, in addition to the radius parts  31   aa,    31   ba,    31   ab,    31   bb,  the amount of waviness of the radius parts  31   ac,    31   bc,    31   ad,    31   bd  on the other side, and further radius parts of coupling portions of the cantilevers  19  coupled with each other as a meander pattern are kept within 600 nm. In the present invention, the radius parts whose amount of waviness is within 600 nm are only radius parts  31   ab  and  31   bb  is allowed. 
     In the embodiment, the entire surface  40  of the SOI wafer  25  is covered with the resist film layer  48  the entire film surface thickness of which is thinner than 5 μm and not thicker than 10 μm as described in STEP 4  of  FIG. 10 . Therefore, a difference between the maximum amount of waviness and the minimum amount of waviness in each cycle included in the entire length of the roughness curve of each radius part can be set to 600 nm or less without limiting each cycle included in a range of the extraction length of the roughness curve of the radius part. 
     DESCRIPTION OF REFERENCE NUMERALS 
       10  . . . light deflector,  11  . . . mirror part,  12  . . . torsion bar,  13  . . . inside piezoelectric actuator,  14  . . . movable frame part,  22 x,  22   y  . . . rotation axis,  25  . . . S 01  wafer,  28   d  . . . active layer,  31  . . . radius part,  36  . . . curved surface,  40  . . . surface,  42  . . . element forming layer,  44  . . . photosensitizer,  45  . . . spin rotation,  48  . . . resist film layer,  52  . . . pattern.