Abstract:
Disclosed is an optical scanning device, including a mirror, a first drive beam configured to swing the mirror around a first axis, and a second drive beam configured to swing the mirror around a second axis, wherein the second drive beam is provided in such a manner that a plurality of beams extending in a direction intersecting with a direction of the second axis are joined with adjacent beams at edge portions thereof, and thereby has a zigzag shape, and each of the plurality of beams includes a rib extending in a direction of a width of the beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a division and claims benefit under 35 U.S.C. 120 of copending U.S. patent application Ser. No. 13/330,767, filed Dec. 20, 2011, which claims the benefit of priority to Japanese Patent Application No. 2010-286758 filed on Dec. 22, 2010, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to an optical scanning device, and more specifically to an optical scanning device that supports a mirror supporting part from both sides in an axis direction by a pair of torsion beams, and drives the mirror supporting part so as to swing around the axis by twisting the torsion beams. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventionally, an optical scanning device is known that includes a movable plate that reflects an incident light, a torsion beam that supports the movable plate in a rotatable way around an axial direction, and a drive part that gives a drive force in a twisting direction to the torsion beam, wherein a rib is formed at least in the vicinity of a connection between the movable plate and the torsion beam, as disclosed in Japanese Patent Application Laid-Open Publication No. 2010-128116 (which is hereinafter called “Patent Document 1”). 
         [0006]    The optical scanning device disclosed in Patent Document 1 aims at suppressing a dynamic distortion of a reflecting plane without increasing a weight of the movable plate. 
         [0007]    In recent years, optical scanning devices tend to be required to have higher resolution. To implement the higher resolution, a resonant frequency is needed to be raised, for which a rigidity of a torsion beam (which may be called a “torsion bar”) is needed to be increased. 
         [0008]    Here, if a width of the torsion beam is broadened to improve the rigidity, since a deformed state of the torsion beam differs depending on a distance from a center axis of the torsion, a problem of nonlinearity of a displacement is caused. 
         [0009]    In Patent Document 1, since such a problem of the nonlinear oscillation is not considered at all, if the resonant frequency is raised, the problem of the nonlinearity of the displacement is caused. Moreover, in the configuration described in Patent Document 1, an effect of preventing a mirror deformation is obtained, but a stress caused by the oscillation is not blocked, so if the resonant frequency is raised, the mirror deformation is not prevented. 
       SUMMARY OF THE INVENTION 
       [0010]    Embodiments of the present invention provide a novel and useful optical scanning device solving one or more of the problems discussed above. 
         [0011]    More specifically, embodiments of the present invention provide an optical scanning device to be able to reduce an occurrence of a nonlinear oscillation and a generated stress, and to be able to prevent a deformation of a mirror even if driven at a high resonant frequency. 
         [0012]    According to one aspect of the present invention, an optical scanning device is provided, the device including:
       a mirror;   a mirror supporting part to support the mirror on an upper surface; and   a pair of torsion beams to support the mirror supporting part from both sides in an axis direction and to drive the mirror supporting part so as to swing the mirror supporting part around the axis by being twisted themselves,   wherein each of the torsion beams includes a slit approximately parallel to the axis direction.       
 
         [0017]    Additional objects and advantages of the embodiments are set forth in part in the description which follows, and in part will become obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1A  is a top perspective view showing an example of an optical scanning device of a first embodiment of the present invention; 
           [0019]      FIG. 1B  is a bottom perspective view showing an example of the optical scanning device of the first embodiment of the present invention; 
           [0020]      FIG. 2A  is a enlarged view showing a part A of  FIG. 1A ; 
           [0021]      FIG. 2B  is a enlarged view showing a part B of  FIG. 1A ; 
           [0022]      FIG. 3A  is a view showing a configuration inside a movable frame of a comparative example; 
           [0023]      FIG. 3B  is a perspective view showing an enlarged torsion beam of the optical scanning device of the comparative example; 
           [0024]      FIG. 3C  is a cross-sectional view of the torsion beam of the optical scanning device of the comparative example; 
           [0025]      FIG. 3D  is a view showing a beam with a square cross section; 
           [0026]      FIG. 4A  is a view showing an example of a frequency/displacement characteristic of a linear oscillation; 
           [0027]      FIG. 4B  is a view showing an example of a frequency/displacement characteristic of a nonlinear oscillation; 
           [0028]      FIG. 4C  is a view showing an example of a frequency/displacement characteristic when the nonlinear oscillation strongly appears; 
           [0029]      FIG. 5A  is a view showing a configuration inside a movable frame of the optical scanning device of the first embodiment; 
           [0030]      FIG. 5B  is an enlarged view of the torsion beam of the optical scanning device of the first embodiment; 
           [0031]      FIG. 5C  is a view showing a cross-sectional configuration of the torsion beam of the optical scanning device of the first embodiment; 
           [0032]      FIG. 6A  is a view showing performance results of an optical scanning device of a comparative example without a slit; 
           [0033]      FIG. 6B  is a view showing performance results of an optical scanning device of a first example including a slit; 
           [0034]      FIG. 7A  is a view showing a displacement/frequency characteristic of the optical scanning device of the first embodiment; 
           [0035]      FIG. 7B  is a view showing a displacement/frequency characteristic when a squareness ratio of a torsion beam divided by a slit is changed; 
           [0036]      FIG. 7C  is a view showing a displacement/frequency characteristic of an optical scanning device of a comparative example; 
           [0037]      FIG. 8A  is an enlarged view showing the upper side of a torsion beam when a short slit is provided in the torsion beam; 
           [0038]      FIG. 8B  is an enlarged view showing the back side of the torsion beam when the short slit is provided in the torsion beam; 
           [0039]      FIG. 8C  is a stress distribution map showing the back side of the torsion beam when the short slit is provided in the torsion beam; 
           [0040]      FIG. 9A  is a configuration view showing the upper side of a connection between a mirror supporting part and a torsion beam of an optical scanning device of the first embodiment; 
           [0041]      FIG. 9B  is a configuration view showing the back side of the connection between the mirror supporting part and the torsion beam of the optical scanning device of the first embodiment; 
           [0042]      FIG. 10  is a map showing a stress distribution at an edge of the slit of the torsion beam in the optical scanning device of the first embodiment; 
           [0043]      FIG. 11A  is a view showing an example of a deformation distribution of a mirror of an optical scanning device having a configuration without a mirror deformation prevention structure; 
           [0044]      FIG. 11B  is a view showing an example of a stress distribution of the mirror of the optical scanning device having the configuration without the mirror deformation prevention structure; 
           [0045]      FIG. 12  is a view to illustrate a mirror deformation prevention structure of the optical scanning device of the first embodiment; 
           [0046]      FIG. 13A  is a perspective view showing a rib structure of the mirror supporting part of the optical scanning device of the first embodiment; 
           [0047]      FIG. 13B  is a plan view showing the rib structure of the mirror supporting part of the optical scanning device of the first embodiment; 
           [0048]      FIG. 14A  is a view showing a mirror deformation amount of the optical scanning device of the first embodiment; 
           [0049]      FIG. 14B  is a view showing a stress distribution of the optical scanning device of the first embodiment including a projecting part of a connecting rib; 
           [0050]      FIG. 14C  is a view showing a stress distribution in a mirror plane; 
           [0051]      FIG. 15A  is a cross-sectional configuration view of a torsion beam of an optical scanning device of a second embodiment; 
           [0052]      FIG. 15B  is a plane configuration view of the back side of the optical scanning device of the second embodiment; 
           [0053]      FIGS. 15C and 15D  are views showing the relationship among a projecting amount X of a connecting rib, a mirror flatness λ in a maximum inclination and a nonlinear coefficient μ; 
           [0054]      FIG. 16A  is a perspective view showing a configuration on the upper side of an optical scanning device using a movable frame and not including a rib on the back side; 
           [0055]      FIG. 16B  is a view showing a configuration on the back side of the optical scanning device using the movable frame and not including the rib on the back side; 
           [0056]      FIG. 16C  is a view showing a horizontal driving state of the optical scanning device using the movable frame and not including the rib on the back side; 
           [0057]      FIG. 17A  is a perspective view showing a configuration on the upper side of an optical scanning device using a movable frame and including a rib on the back side; 
           [0058]      FIG. 17B  is a view showing a configuration on the back side of the optical scanning device using the movable frame and including the rib on the back side; 
           [0059]      FIG. 17C  is a view showing a horizontal driving state of the optical scanning device using the movable frame and including the rib on the back side; 
           [0060]      FIG. 18A  is a perspective view showing a configuration on the upper side of the optical scanning device of the first embodiment; 
           [0061]      FIG. 18B  is a perspective view showing a configuration on the back side of the optical scanning device of the first embodiment; 
           [0062]      FIG. 18C  is an enlarged view showing a crosstalk prevention structure of the optical scanning device of the first embodiment; 
           [0063]      FIG. 19  is a stress distribution map in horizontal driving of the optical scanning device of the first embodiment; 
           [0064]      FIG. 20A  is a plan configuration view showing an optical scanning device without a frequency change prevention structure; 
           [0065]      FIG. 20B  is a cross-sectional configuration view of a movable frame and a resonant drive beam of the optical scanning device without the frequency change prevention structure shown in  FIG. 20A ; 
           [0066]      FIG. 20C  is a view showing a drive state of the resonant drive beam; 
           [0067]      FIG. 20D  is a view showing the relationship between an integrated drive time of the resonant drive beam and a resonant frequency change rate; 
           [0068]      FIG. 21A  is a stress distribution map on the upper side in a horizontal driving of an optical scanning device without a frequency change prevention structure; 
           [0069]      FIG. 21B  is a stress distribution map on the back side in the horizontal driving of the optical scanning device without the frequency change prevention structure; 
           [0070]      FIG. 21C  is an enlarged view showing a supporting part of a drive beam of a stress distribution on the back side in the horizontal driving of the optical scanning device without the frequency change prevention structure; 
           [0071]      FIG. 22A  is a plane configuration view of an optical scanning device with a first frequency change prevention structure; 
           [0072]      FIG. 22B  is an enlarged view showing a root part shown in  FIG. 22A ; 
           [0073]      FIG. 23A  is a stress distribution map on the upper side of a first drive beam of an optical scanning device of a first embodiment including a first frequency change prevention structure in a horizontal driving; 
           [0074]      FIG. 23B  is a stress distribution map on the back side of the first drive beam of the optical scanning device of the first embodiment including the first frequency change prevention structure in a horizontal driving; 
           [0075]      FIG. 23C  is an enlarged view of a root part of the first drive beam shown in  FIG. 23B ; 
           [0076]      FIG. 24  is a view showing the relationship between an integrated drive time and a resonant frequency change rate caused by a resonant drive of the optical scanning device of the first embodiment; 
           [0077]      FIG. 25A  is a plan view showing the upper side of an optical scanning device of a first embodiment including first and second frequency change prevention structures; 
           [0078]      FIG. 25B  is a plan view showing the back side of the optical scanning device of the first embodiment including the first and second frequency change prevention structures; 
           [0079]      FIG. 25C  is an enlarged plan view showing an inside of a movable frame of the optical scanning device of the first embodiment; 
           [0080]      FIG. 25D  is an enlarged view showing a stress distribution of a lateral edge of the first drive beam of the optical scanning device of the first embodiment; 
           [0081]      FIG. 26A  is a stress distribution map of an optical scanning device without a frequency change prevention structure; 
           [0082]      FIG. 26B  is a stress distribution map of an optical scanning device with only a first frequency change prevention structure; 
           [0083]      FIG. 26C  is a stress distribution map of an optical scanning device of a first embodiment with first and second frequency change prevention structures; 
           [0084]      FIG. 27  is a view showing stress measurement results in a root of the first drive beam with respect to each frequency change prevention structure shown in  FIGS. 26A through 26C ; 
           [0085]      FIG. 28  is a view showing an example of an optical scanning device of a second embodiment; 
           [0086]      FIG. 29  is a view showing an example of an optical scanning device of a third embodiment; 
           [0087]      FIG. 30  is a view showing an example of an optical scanning device of a fourth embodiment; and 
           [0088]      FIG. 31  is a view showing an example of an optical scanning device of a fifth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0089]    A description is given below, with reference to drawings of embodiments of the present invention. 
       First Embodiment  
       [0090]    (Overall Structure) 
         [0091]      FIGS. 1A and 1B  are perspective views showing an example of a configuration of an optical scanning device of a first embodiment of the present invention.  FIG. 1A  is a top perspective view showing an example of the optical scanning device of the first embodiment.  FIG. 1B  is a bottom perspective view showing an example of the optical scanning device of the first embodiment. 
         [0092]    In  FIGS. 1A and 1B , the optical scanning device of the present invention includes a mirror  10 , a mirror supporting part  20 , torsion beams  30 , coupling beams  40 , first drive beams  50 , a movable frame  60 , second drive beams  70 , crosstalk preventing ribs  80 , and a fixed frame  90 . The torsion beams  30  include a slit  31 . Moreover, as shown in  FIG. 1A , the first drive beams  50  include a drive source  51 , and the second drive beams  70  include a drive source  71 . Furthermore, as shown in  FIG. 1B , a rib is provided on the back side of the mirror supporting part  20 , and harmonic superposition preventing ribs  72  are provided on the back side of the second drive beams  70 . 
         [0093]    In  FIGS. 1A and 1B , the mirror  10  is supported on the upper surface of the mirror supporting part  20 , and the mirror supporting part  20  is connected to edges of the torsion beams  30  located on both sides of the mirror supporting part  20 . The torsion beams  30  form a rocking axis, extend in an axial direction, and support the mirror supporting part  20  from both sides in the axial direction. By twisting the torsion beams  30 , the mirror  10  supported by the mirror supporting part  20  swings and performs an operation of deflecting a reflected light of an incident light on the mirror  10 . The torsion beams  30  are coupled and supported by the coupling beams  40 , and connected to the first drive beams  50 . The first drive beams  50 , the coupling beams  40 , the torsion beams  30 , the mirror supporting part  20  and the mirror  10  are surrounded by the movable frame  60 . The first drive beams  50  are supported by the movable frame  60  at one side, and extend inward so as to be connected to the coupling beams  40 . Two first drive beams  50  are provided so as to sandwich the mirror  10  and the mirror supporting part  20  in a direction perpendicular to the torsion beams  30 . A thin film of a piezoelectric device is formed on the upper surface of the first drive beam  50  as the drive source  51 . Since the piezoelectric device expands and contracts depending on the polarity of an applied voltage, by alternately applying different voltages in phase to the left first drive beam  50  and the right first drive beam  50 , the first drive beams  50  on the left and right of the mirror alternately oscillate up and down oppositely, by which the mirror  10  can be swung around the axis, making the torsion beams  30  serve as a rocking axis or a rotation axis. The direction where the mirror  10  swings around the torsion beams  30  is hereinafter called a “horizontal direction”. Generally speaking, the horizontal direction corresponds to a horizontal direction of a screen for projection. In the screen, a lateral direction is generally called the horizontal direction. For example, resonant drive may be used for a horizontal drive by the first drive beams  50 , and the mirror  10  may be driven and swung at high speed. Moreover, one edge of each of the second drive beams  70  is coupled to the outside of the movable frame  60 . By applying different polarities of voltages to the adjacent drive sources  71  per rectangle unit, the adjacent rectangular beams are recurved in vertically opposite directions, and an integration of an up-and-down motion of the rectangular beams can be transmitted to the movable frame  60 . Then the mirror  10  can be swung in a direction perpendicular to the horizontal direction, in a vertical direction. As mentioned above, the vertical direction generally corresponds to a vertical direction of a screen for projection, a longitudinal direction of the screen. For example, the second drive beams  70  may generate the drive force by non-resonant oscillation. 
         [0094]    The optical scanning device of the present embodiment may be implemented by various materials and processing methods as long as the optical scanning device has the above mentioned configuration and the configuration is practicable. For example, the optical scanning device of the present embodiment may be implemented by Micro Electro Machine System (MEMS) technology by using semiconductor fabrication. For example, if a Silicon On Insulator (SOI) substrate is used, by processing the substrate so as to leave only a silicon substrate on the upper side as thin beam parts, and by processing the substrate so as to also leave a silicon substrate on the back side as thick frames and ribs, a structural body of the optical scanning device can be produced readily. 
         [0095]    The optical scanning device of the present embodiment can be configured as a piezoelectric dual-axis drive type actuator mountable on a micro projector, and can be inexpensively produced to have a small size and a high performance. Here, for example, the “small size” means a height not more than 7 mm, and the “high performance” means that a high-speed drawing can be performed on an A3 size screen at a 50 cm distance at an XGA resolution (i.e., eXtended Graphic Array, a resolution of 1024*768 pixels) or 720 p. The optical scanning device of the present embodiment may be, for example, configured as a small-size and high-performance dual axis drive type micro mirror actuator including a non-resonant drive type actuator mechanism that swings in a vertical direction at a mechanical angle ±9 degrees at 60 Hz in a saw-tooth pattern and a resonant drive type actuator mechanism that swings in a horizontal direction at a mechanical angle ±12 degrees at a resonant frequency 25 kHz. 
         [0096]      FIGS. 2A and 2B  are enlarged views showing A part and B part of  FIG. 1A .  FIG. 2A  is an enlarged view showing the A part of  FIG. 1A .  FIG. 2B  is an enlarged view showing the B part of  FIG. 1A . 
         [0097]    In  FIG. 2A , four of mirror horizontal angle sensors  100  are provided on the coupling beams  40 . The mirror horizontal angle sensors  100  are sensors that detect an inclination angle of the mirror  10  in a horizontal direction. Since the coupling beams  40  reflect the inclination angle of the mirror  10  in the horizontal direction, by providing the mirror horizontal angle sensors  100  on the coupling beams  40 , the inclination angle of the mirror  10  of the horizontal direction can be detected. The mirror horizontal angle sensors  100  may be, for example, configured to have a thin film of a piezoelectric device, to detect a voltage excited in the thin film of the piezoelectric device according to the inclination angle, and to detect the inclination angle of the horizontal direction. 
         [0098]    In  FIG. 2B , a mirror vertical angle sensor  101  is provided on the second drive beam  70 . Since a drive in a vertical direction is reflected in a motion of the second drive beams  70 , for example, the mirror vertical angle sensor  101  may be provided on the second beam  70 . For example, the mirror vertical angle sensor  101  may be also configured to use a piezoelectric device as mentioned above. 
         [0099]    Next, with respect to details of the optical scanning device of the present embodiment, descriptions are given for respective component parts sequentially. Here, in the optical scanning device of the first embodiment, if there is a practical example such as measurement results, the example is taken and referred to for each component part. 
         [0100]    (Slit Structure) 
         [0101]      FIGS. 3A through 3D  are views showing a comparative example to illustrate a slit structure of the optical scanning device of the present embodiment. FIGS. 
         [0102]      3 A through  3 D show an optical scanning device of the comparative example including torsion beams  130  in which a slit  31  is not formed, different from the present embodiment. Here, in  FIGS. 3A through 3D , numerals similar to  FIGS. 1 and 2  are put to components similar to ones of the optical scanning device of the present embodiment, and different numerals are put to components different from the optical scanning device of the present embodiment. 
         [0103]      FIG. 3A  is a view showing a configuration within a movable frame  60  of the optical scanning device of the comparative example. As shown in  FIG. 3A , the optical scanning device of the comparative example differs from the optical scanning device of the present embodiment in that the slits  31  are not formed in torsion beams  130 , inside the movable frame  60 . 
         [0104]      FIG. 3B  is an enlarged perspective view showing the torsion beams  130  of the optical scanning device of the comparative example. As shown in  FIG. 3B , the torsion beams  130  have a lamellar shape with a greater width compared to a thickness. Since high resolution is required in recent years, scanning many pixels per unit time is needed, and speeding up the swinging drive for scanning is needed. To implement speeding up, the resonant frequency to drive the mirror  10  has to be raised. To do this, rigidity of the torsion beams  130  have to be increased. This is because if the optical scanning device is configured by using a semiconductor process with MEMS technology, since the thickness of the thin parts is determined by the rigidity related to swinging sensitivity of the second drive beams  70  and a primary resonant frequency fo, and all of the thin parts are configured to be constant, the horizontal width of the torsion beams  130  needs to be increased in order to raise the rigidity. 
         [0105]      FIG. 3C  is a cross-sectional view of the torsion beam  130  of the optical scanning device of the comparative example. As shown in  FIG. 3C , the cross-section of the torsion beam  130  has a rectangle shape with a greater width compared to the thickness. This is because, as mentioned above, the beam width of the torsion beam  130  is broadened and the rigidity is improved. Then, when the torsion beam  130  is twisted, a center part Ct, an edge Eg and a middle part Md deform differently depending on the position. If the mirror angle is largely changed by twisting, the differences of the deformation state among the positions Ct, Md and Eg appear as a nonlinear displacement. 
         [0106]      FIGS. 4A through 4C  are views showing frequency/displacement characteristics of a linear resonant oscillation and a nonlinear resonant oscillation.  FIG. 4A  is a view showing an example of a frequency/displacement characteristic of a linear resonant oscillation. As shown in  FIG. 4A , the linear resonant oscillation performs a symmetric oscillation, making a resonant frequency fa the center. 
         [0107]      FIG. 4B  is a view showing a frequency/displacement characteristic of a nonlinear oscillation. As shown in  FIG. 4B , in the nonlinear resonant oscillation, a balance of the right and left breaks, and a mountain of the resonant frequency leans to the right or left. In  FIG. 4B , the mountain of the resonant frequency leans toward the right. 
         [0108]      FIG. 4C  is a view showing examples of frequency/displacement if the nonlinear resonant oscillation intensely appears. As shown in  FIG. 4C , the biggest problem when the nonlinear resonant oscillation intensely appears is that a displacement at the drive frequency f does not change even if the drive voltage is changed in a range from V1 to V3, making the drive frequency f constant. In other words, because the peak is inclined, the displacement is increased or decreased in the inclined direction even though the drive voltage is increased or decreased, so there is a phenomenon where the displacement is not increased and decreased at all at a point of the frequency f. This prevents a projection size of a laser light from being changed freely by adjusting an applied voltage. 
         [0109]    Next, the description is given, with reference to  FIG. 3D .  FIG. 3D  is a view showing a beam with a square cross section. In  FIG. 3D , the width is shown by W, and the thickness is shown by T. The most efficient measures to prevent the nonlinearity are to change the cross-sectional shape of the beam from the rectangle shown in  FIG. 3C  to the square shown in  FIG. 3D . 
         [0110]    However, in order to make the cross-sectional shape a square while keeping a drive frequency constant, the thickness T needs to be increased from the rectangular shape in  FIG. 3C . However, if the thickness T is simply increased, the thickness of the second drive beams  70  that are the vertical non-resonant drive structure is also increased, and desired vertical drive voltage sensitivity cannot be obtained. 
         [0111]    Therefore, in the optical scanning device of the present embodiment, the slit  31  is provided in the torsion beam  30 ; the cross sections on both sides of the slit  31  are respectively made a square; the width of the torsion beam  30  is broadened as a whole; and the torsion beam  30  is configured to maintain the rigidity. 
         [0112]      FIGS. 5A through 5C  are views to explain the torsion beams  30  of the optical scanning device of the first embodiment.  FIG. 5A  is a view showing an inside configuration of the movable frame  60  of the optical scanning device of the first embodiment. In  FIG. 5A , each of the torsion beams  30  of the present embodiment includes the slit  31 . 
         [0113]      FIG. 5B  is an enlarged view showing the torsion beam  30  of the optical scanning device of the first embodiment. As shown in  FIG. 5B , the torsion beam  30  of the optical scanning device of the present embodiment includes the slit  31  parallel to the axial direction. In  FIG. 5B , since only a single slit  31  is provided in the center of the torsion beam  30 , the slit  31  is provided in a position corresponding to the rocking axis. Moreover, the slit  31  does not reach the inside edge or the outside edge of the torsion beam  30 , and is not configured to divide the torsion beam  30 . 
         [0114]      FIG. 5C  is a view showing an example of a cross-sectional configuration of the torsion beam  30  of the optical scanning device of the first embodiment. As shown in  FIG. 5C , by forming the slit  31  in the center of the torsion beam  30 , the torsion beam  30  is divided into the left torsion beam  30 L and the right torsion beam  30 R in the cross section including the slit  31 . Both the left torsion beam  30 L and the right torsion beam  30 R have a cross section similar to a square. Accordingly, a rotation center  30 LC of the left torsion beam  30 L and a rotation center  3 ORC of the right torsion beam  30 R both become the center of respective torsion beams  30 L,  30 R, and a difference by a displacement does not occur, by which the nonlinear oscillation can be reduced. Furthermore, the torsion beam  30  is assumed to swing around an assumed rotation center  31 C as a whole, making it possible for the mirror  10  to swing in a horizontal direction in a desired way. 
         [0115]    In this manner, according to the optical scanning device of the present embodiment, by providing the slit  31  parallel to the axial direction in the torsion beam  30 , and by making the respective divided torsion beams  30 L,  30 R have a shape similar to a square in a cross section including the slit  31 , generation of nonlinear oscillation is suppressed. 
       First Example 
       [0116]      FIGS. 6A and 6B  are views showing performance results of an optical scanning device of a first example.  FIG. 6A  is a view showing performance results of an optical scanning device of a comparative example without the slit  31 .  FIG. 6B  is a view showing performance results of an optical scanning device of the first example with the slit  31 . 
         [0117]      FIG. 6A  shows moments in a case where the optical scanning device of the comparative example is displaced linearly and nonlinearly.  FIG. 6A  shows if the moments between the linearity and the nonlinearity disagree, the nonlinearity is intense, and if the moments between the linearity and the nonlinearity agree, the nonlinearity does not occur. In  FIG. 6A , the moments of the linearity and the nonlinearity do not overlap, the optical scanning device of the comparative example including the torsion beams  130  without the slits  31  shows a characteristic with intense nonlinearity. 
         [0118]    On the other hand,  FIG. 6B  shows moments in a case where the optical scanning device of the first example having a configuration similar to the first embodiment is displaced linearly and nonlinearly. In  FIG. 6B , the moments of the linearity and the nonlinearity overlap with each other, which shows that nonlinearity does not occur. 
         [0119]      FIGS. 7A through 7C  are views showing a displacement/frequency characteristic of the optical scanning device of the first example and the comparative example.  FIG. 7A  is a view showing the displacement/frequency characteristic of the optical scanning device of the first example.  FIG. 7B  shows a displacement/frequency characteristic when a squareness ratio of the torsion beams  30 L,  30 R divided by the slit  31  changes.  FIG. 7C  is a view showing the displacement/frequency characteristic of the optical scanning device of the comparative example. 
         [0120]    As shown in  FIG. 7A , the optical scanning device of the first example includes only minimal nonlinearity, and has a characteristic that can increase or decrease displacement by frequency f, depending on increase or decrease of a drive voltage. This makes it possible to increase or decrease an irradiation area of light by increasing or decreasing the drive voltage. 
         [0121]    Here, as shown in  FIG. 7B , even if the slit  31  is provided, when the squareness ratio is changed with respect to the right torsion beam  30 R and the left torsion beam  30 L of the slit  31 , and a generated stress is eased up, the nonlinearity occurs. However, the nonlinearity weakens, and increasing or decreasing the displacement according to the drive voltage when frequency f is constant becomes possible. 
         [0122]    Therefore,  FIGS. 7A and 7B  prove that by providing the slit  31  in the torsion beam  30 , if the frequency is constant and the drive voltage is increased or decreased, increasing or decreasing the displacement is possible. 
         [0123]    On the other hand,  FIG. 7C  proves that where the nonlinearity is intense, even if the frequency is made constant and the drive voltage is increased or decreased, the displacement does not change, and the size of the scanning area cannot be changed. 
         [0124]    In this way, according to the optical scanning device of the first example, by providing the slit  31  approximately parallel to the axial direction in the center of the torsion beam  30 , reducing the nonlinearity is possible. In this case, the slit  31  agrees with the rotation axis or the rocking axis. 
         [0125]    Here, it is also possible to provide plural slits  31  in the torsion beam  30  symmetrically about the rotation axis. However, if the number of the slits  31  is increased to two, three and more symmetrically about the rotation axis, the nonlinearity further weakens, but the rigidity as the axis beam also weakens. In order to reinforce the rigidity, if many ribs  21  are provided on the back side of the mirror supporting part  20 , in that case, the gravity center of the mirror moves downward from the rotation axis in a thickness direction, which generates a pendulum motion. Hence, if the number of the slits  31  is increased, the number of the slits  31  needs to be increased considering a balance with the rigidity. Here, even if the slit  31  is only a single slit, the pendulum motion itself occurs. However, because torsional rigidity of the torsion beam  30  is strong enough, even when the mirror  10  swings at a mirror inclination of ±12 degrees mechanical angle, a displacement amount of the pendulum motion is minute, and there is no problem. 
         [0126]    (Displacement Expansion Structure by Stress Dispersion) 
         [0127]      FIGS. 8A through 8C  are views to illustrate points to be considered where the slit  31  is provided in the torsion beam  30  in the optical scanning device of the first embodiment for nonlinearity oscillation measures.  FIG. 8A  is an enlarged view of the upper side of the torsion beam  30  including a short slit  131 .  FIG. 8B  is an enlarged view of the back side of the torsion beam  30  including the short slit  131 .  FIG. 8C  is a view showing a stress distribution on the back side of the torsion beam  30  including the short slit  131 . 
         [0128]      FIGS. 8A and 8B  show a case where an edge of the slit  131  provided in the torsion beam  30  contacts an edge face of a rib  121  provided on the back side of the mirror supporting part  20 . In such a case, as shown in  FIG. 8C , stress concentrates on the edge of the slit  131  and damage easily occurs, which causes a problem of not being able to incline the mirror  10  sufficiently. Such a phenomenon also occurs if the edge of the slit  131  does not reach the edge face of the rib  121 . 
         [0129]      FIGS. 9A and 9B  are views showing an example of a configuration of a connection between the mirror supporting part  20  and the torsion beam  30  of the optical scanning device of the first embodiment. To prevent the generation of the stress concentration on the edge of the slit  31  illustrated in  FIGS. 8A through 8C , the optical scanning device of the first embodiment adopts a configuration shown in  FIGS. 9A and 9B . 
         [0130]      FIG. 9A  is a view showing an example of a configuration on the upper side of the connection between the mirror supporting part  20  and the torsion beam  30  of the optical scanning device of the first embodiment.  FIG. 9B  is a view showing an example of a configuration on the back side of the connection between the mirror supporting part  20  and the torsion beam  30  of the optical scanning device of the first embodiment. 
         [0131]    In  FIG. 9A , an edge  31 E of the slit  31  provided in the torsion beam  30  cuts more inward than an outer edge of the rib  21  on the back side of the mirror supporting part  20 , a part of the rib  21  is configured to be exposed from the slit  31 . Thus, by reaching the edge  31 E of the slit  31  more inward than the outer edge of the rib  21 , the rib  21  reinforces the edge  31 E of the slit  31 , and can absorb and reduce the stress generated at the edge  31 E of the slit  31 . 
         [0132]    In  FIG. 9A , a thin film of a black resist  32  is formed between the mirror  10  and the torsion beam  30 . The black resist  32  is formed to prevent light from being reflected from a space between the mirror  10  and the torsion beam  30  outside the mirror  10 , if the light is irradiated in a range beyond the mirror  10 . For example, the black resist  32  may be formed by application. 
         [0133]    As shown in  FIG. 9B , the slit  31  reaches inside of the rib  21  located in the connection between the mirror supporting part  20  and the torsion beam  30 . 
         [0134]      FIG. 10  is a view showing a stress distribution in the edge  31 E of the slit  31  of the torsion beam  30  in the optical scanning device of the first embodiment.  FIG. 10  shows that the stress generated at the edge  31 E of the slit  31  does not concentrate on the edge  31 E but disperses in the torsion beam  30 . If  FIG. 10  is compared to  FIG. 8C , the difference is made clear. 
         [0135]    In this way, by the edge of the slit  31  on the mirror supporting part  20  provided in the torsion beam  30  reaching more inward than the outside edge of the rib  21 , and by making a configuration where the slit  31  cuts into the mirror  10  side, the stress generated at the slit edge  31 E can be dispersed into an area other than the slit edge  31 E, and inclining the mirror  10  in a large displacement becomes possible. 
         [0136]    (Mirror Deformation Prevention Structure) 
         [0137]      FIGS. 11A and 11B  are views showing an example of mirror deformation and a stress distribution in an optical scanning device of a configuration without a mirror deformation prevention structure.  FIG. 11A  is a view showing an example of a deformation distribution of the mirror  10  of the optical scanning device of the configuration without the mirror deformation prevention structure.  FIG. 11B  is a view showing an example of a stress distribution of the mirror  10  of the optical scanning device of the configuration without the mirror deformation prevention structure. 
         [0138]    In  FIG. 11A , a vertical line passing through the center of the mirror  10  becomes a rocking axis. As shown in  FIG. 11A , deformations in the farthest parts from the center on a diameter perpendicular to the rocking axis, and in parts symmetrical about the rocking axis between the farthest parts are large. 
         [0139]      FIG. 11B  is similar to  FIG. 11A  in that a vertical line passing through the center of the mirror  10  is a rocking axis.  FIG. 11B  shows that parts with high stress generated in the mirror  10  are connections with the torsion beams  30 . 
         [0140]      FIG. 12  illustrates a mirror deformation prevention structure of the optical scanning device of the first embodiment. In  FIG. 12 , parts with a large deformation of the mirror  10  are shown as A-F. In the optical scanning device of the first embodiment, by providing ribs  21  connecting such parts with the large mirror deformation to each other on the back side of the mirror supporting part  20 , and by further providing ribs  21  on the connections of the border between the torsion beam  30  and the mirror supporting part  20 , a maximum mirror deformation prevention effect is obtained with a minimum number of ribs. 
         [0141]      FIGS. 13A and 13B  are views showing a rib structure on the back side of the mirror supporting part  20  of the optical scanning device of the first embodiment.  FIG. 13A  is a perspective view showing a rib structure of the mirror supporting part  20  of the optical scanning device of the first embodiment.  FIG. 13B  is a plan view showing a rib structure of the mirror supporting part  20  of the optical scanning device of the first embodiment. 
         [0142]    In  FIGS. 13A and 13B , the ribs  21  are provided so as to connect the points A-F having high stress. More specifically, the rib structure includes the arc-like ribs  23  that respectively connect A with B, and C with D arcuately, chordal ribs  24  that connect the both edges of the arc-like ribs  23  to each other and reinforce the arc-like ribs  23 , a transverse rib  26  that connects E with F in a direction perpendicular to the rocking axis, and longitudinal ribs  25  that connect A with C, and B with D in a direction parallel to the rocking axis. With such ribs  23 - 26 , the deformation of the mirror  10  can be directly suppressed. 
         [0143]    However, as shown in  FIG. 11B , it is thought that the stress of the torsion beam  30  comes from the connection with the mirror supporting part  20  to the mirror  10 , and acts on the mirror  10  so as to be deformed, so measures for the stress is needed. Therefore, in the optical scanning device of the present embodiment, connecting ribs  22  are also provided on the connection between the torsion beam  30  and the mirror supporting part  20 . Furthermore, as shown in  FIG. 13B , by projecting the connecting rib  22  farther toward the torsion beam  30  than is the edge of the mirror  10 , a stress transmission from the torsion beam  30  is effectively blocked. Because such projecting parts of the connecting ribs  22  are near the rotation axis (or rocking axis), inertia does not increase, which is advantageous for a high speed drive. 
         [0144]      FIGS. 14A through 14C  are views showing an example of a mirror deformation amount and a stress distribution of the optical scanning device of the first embodiment.  FIG. 14A  is a view showing an example of the mirror deformation amount of the optical scanning device of the first embodiment.  FIGS. 14A  proves that if the rib  21  is provided on the back side of the mirror supporting part  20  as the mirror deformation prevention structure, the deformation amount of the mirror  10  is almost zero, and the mirror  10  is sufficiently flat. 
         [0145]      FIG. 14B  is a view showing the stress distribution of the optical scanning device of the first embodiment, including the projecting part of the connecting rib  22 .  FIG. 14B  proves that by projecting the ribs  21  (i.e., connecting ribs  22 ) toward the torsion beam  30  beyond the edge of the mirror  10 , the stress concentrates on the projecting part, which becomes a stress relaxation part of the torsion beam  30 . 
         [0146]      FIG. 14C  is a view showing a stress distribution in a mirror  10  plane. As shown in  FIG. 14C , stress is only minimally generated in the mirror  10  plane. This is because the stress from the torsion beam  30  is absorbed in the projecting part of the connecting ribs  22 , and is not transferred to the mirror  10 . 
       Second Example 
       [0147]      FIGS. 15A through 15D  are views showing a configuration and performance results of an optical scanning device of a second example.  FIG. 15A  is a view showing a cross-sectional configuration of a torsion beam  30  of the optical scanning device of the second example. As shown in  FIG. 15A , a slit  31  is provided in the center of a torsion beam  30 , and torsion beams  30 L,  30 R having a cross-sectional shape similar to a square on both sides of the slit  31 . The left torsion beam  30 L and the right torsion beam  30 R have the same cross-sectional configuration, and the width is expressed as W, and the thickness is expressed as T. 
         [0148]      FIG. 15B  is a view showing a plane configuration on the back side of the optical scanning device of the second example. As shown in  FIG. 15B , the optical scanning device of the second example has a configuration similar to the optical scanning device of the first embodiment described in  FIGS. 13A and 13B . Specifically, the optical scanning device of the second example includes connecting ribs  22  that project toward the torsion beam  30  more than does the circumference of the mirror  10 , and a projecting amount of the connecting rib  22  from the mirror  10  is expressed as X mm. 
         [0149]      FIG. 15C  is a view showing the relationship between a mirror flatness λ in the maximum inclination and a nonlinearity coefficient β. In  FIG. 15C , W means a width of one side of a torsion beam; T means a torsion beam thickness; W/T means a squareness ratio of one side of the torsion beam; X means a rib projecting amount; λ means a mirror flatness in the maximum inclination; and β means a nonlinearity coefficient. Moreover, characteristics shown by solid lines express the mirror flatness λ, and characteristics shown by broken lines express the nonlinearity coefficient β. 
         [0150]    In the characteristics shown by the broken lines in  FIG. 15C , as the squareness W/T of the torsion beam is small and close to one (i.e., close to a square), and as the projecting amount X mm decreases, the nonlinearity coefficient β decreases. However, values of the nonlinearity coefficient β change are relatively small, even though the rib projecting amount X changes. 
         [0151]    On the other hand, in the characteristics shown by the solid lines in  FIG. 15C , the mirror flatness λ takes local minimum values around W/T=1.8, X=0.1 mm. At X=0.1 mm, the nonlinearity coefficient β does not exactly have the optimal values, but as mentioned above, because changes of the nonlinearity coefficient β are not so large and a configuration that projects the ribs  21  aims at the mirror deformation prevention, W/T=1.8, X=0.1 mm are made the optimal values. 
         [0152]      FIG. 15D  is a view showing the relationship among the squareness W/T of the torsion beam  30 , the mirror flatness λ in the maximum inclination, an axis beam maximum stress σ in the maximum inclination, and the nonlinearity coefficient β. In  FIG. 15D , from the results of  FIG. 15C , the projecting amount X is fixed at X =0.1 mm. Then, changes in the mirror flatness λ in the maximum inclination, the axis beam maximum stress σ in the maximum inclination, and the nonlinearity coefficient β are measured, changing the squareness W/T of both sides  30 R,  30 L of the torsion beam  30 . 
         [0153]    As shown in  FIG. 15D , the nonlinearity coefficient β decreases as the squareness W/T decreases and approaches one (i.e., as approaching a square). The results can be said to be natural because bringing both of the sides  30 R,  30 L of the torsion beam  30  close to a square is fundamentally performed for the nonlinearity measures. 
         [0154]    On the other hand, the mirror flatness λ in the maximum inclination takes the minimum value at W/T=1.76. In addition, though the axis beam maximum stress σ in the maximum inclination decreases as W/T increases, there is no problem as long as the maximum stress σ is not more than an allowable stress of the torsion beam  30 . The axis beam maximum stresses σ in the maximum inclination shown in  FIG. 15D  are all values without any problems. 
         [0155]    From  FIG. 15C , an optimal range of the rib projecting amount X is 0.05≦X≦0.15 mm, and X=0.1 mm is the optimal value. 
         [0156]    Also, from the characteristics of the axis beam maximum stress a in the maximum inclination and the nonlinearity coefficient β shown in  FIG. 15D , the optimal range of W/T is 1.7≦W/T≦1.9, and the optimal value is W/T=1.76. 
         [0157]    In this way, by adjusting the rib projecting amount X and W/T of both sides  30 R,  30 L of the torsion beam  30 , the maximum stress σ applied to the torsion beam  30  is made a magnitude without a problem, and the mirror flatness λ and the nonlinearity coefficient β can be reduced. 
         [0158]    (Crosstalk Prevention Structure to Vertical Driving Beam in Horizontal Driving) 
         [0159]      FIG. 16A through 16C  are views to illustrate crosstalk generated if there is a movable frame  60  without a rib on the back side in the optical scanning device of the first embodiment.  FIG. 16A  is a perspective view showing a configuration on the upper side of an optical scanning device using a movable frame  60  without a rib on the back side.  FIG. 16B  is a view showing a configuration on the back side of the optical scanning device using the movable frame  160  without the rib on the back side.  FIG. 16C  is a view showing a horizontal driving state of the optical scanning device using the movable frame  160  without the rib on the back side. 
         [0160]    As shown in  FIGS. 16A and 16B , if the optical scanning device is configured by using the movable frame  160  without the rib, the movable frame  160  is configured as a beam with the same thickness as the other beams. 
         [0161]    As shown in  FIG. 16C , if the optical scanning device is configured by using the movable frame  160  without the rib, the second drive beams  70  that are vertical drive beams largely deform by horizontal driving by the torsion beams  30 . In other words, so-called crosstalk that affects the vertical driving when the horizontal driving occurs. 
         [0162]      FIGS. 17A through 17C  are views to illustrate crosstalk that is generated even if a movable frame  60  including a rib on the back side is used.  FIG. 17A  is a perspective view showing a configuration on the upper side of the optical scanning device using the movable frame  60  with the rib on the back side.  FIG. 17B  is a perspective view showing a configuration on the back side of the optical scanning device using the movable frame  60  with the rib on the back side.  FIG. 17C  is a view showing a horizontal drive state of the optical scanning device using the movable frame  60  with the rib on the back side. 
         [0163]    As shown in  FIGS. 17A and 17B , by using the movable frame  60  including a rib on the back side, the movable frame  60  has a degree of thickness, and is configured as a frame with high rigidity. 
         [0164]    However, as shown in  FIG. 17C , when the optical scanning device is driven horizontally by using the first drive beams  50 , the second drive beams  70  that are a vertical drive beams still deform. 
         [0165]      FIGS. 18A through 18C  are views to illustrate a crosstalk prevention structure to the vertical drive beams during the horizontal drive of the optical scanning device of the first embodiment.  FIG. 18A  is a perspective view showing a configuration on the upper side of the optical scanning device of the first embodiment.  FIG. 18B  is a perspective view showing a configuration on the back side of the optical scanning device of the first embodiment.  FIG. 18C  is an enlarged view showing the crosstalk prevention structure of the optical scanning device of the first embodiment. 
         [0166]    As shown in  FIG. 18A , the crosstalk prevention structure is not provided on the upper side of the optical scanning device. 
         [0167]    On the other hand, as shown in  FIG. 18B , on the back side of the optical scanning device of the first embodiment, a movable frame  60  including a rib is provided, and plural crosstalk preventing ribs  81 - 83  are provided on a connection  80  between the second drive beam  70  and the movable frame  60 . Here, in  FIG. 18B , the second drive beams  70  include ribs  72  in places other than the connection  80  with the movable frame. The ribs  72  are for harmonic superposition prevention when the optical scanning device is driven in a vertical direction, and differ from the ribs for the crosstalk prevention. For example, when the second drive beams  70  are driven at 60 Hz, sometimes the harmonics of multiple numbers of 60 Hz such as 120 Hz, 240 Hz, 360 Hz and the like are superimposed. The ribs  72  for harmonic superposition prevention are provided to prevent such superposition of the harmonics. 
         [0168]    As shown in  FIG. 18C , the connection  80  between the movable frame  60  and the second drive beam  70  includes plural crosstalk preventing ribs  81 - 83  that extend in an axial direction of the horizontal drive and in a direction perpendicular to the axial direction. The crosstalk preventing rib  81  is a rib that extends continuously from the movable frame  60  parallel to the torsion beams  30  and the second drive beams  70 . Moreover, the crosstalk preventing rib  82  is provided extending in a width direction of the second drive beams  70 , symmetrically with the harmonic superposition preventing rib  72 . The crosstalk preventing rib  83  is provided extending continuously from the movable frame  60  parallel to the crosstalk rib  82 . Furthermore, the crosstalk preventing ribs  81 ,  83  are configured to form a triangular hollow  84  outside the movable frame  60 . 
         [0169]    In this manner, by providing the crosstalk preventing ribs  81 - 83  between the edge of the second drive beam  70  that is the vertical drive beam and the movable frame  60 , a transmission of the oscillation in the horizontal drive to the second drive beams  70  can be prevented. In particular, by forming a triangle with the rib  81  and rib  83 , the triangular hollow  84  can absorb stress generated by the horizontal drive, and reducing the influence on the second drive beams  70  of the vertical drive beam is possible. 
         [0170]      FIG. 19  is a view showing a stress distribution during the horizontal driving of the optical scanning device of the first embodiment including the crosstalk preventing ribs  81 - 83 . As shown in  FIG. 19 , stress acts on the mirror  10  driving horizontally, but stress does not occur in the second drive beams  70  of the vertical drive beam. Thus, by providing the crosstalk preventing ribs  81 - 83  between the vertical drive beam edge and the movable frame  60 , transmission of the swinging oscillation in the horizontal resonant drive to the vertical drive beams can be blocked. 
         [0171]    (Frequency Change Prevention Structure) 
         [0172]      FIGS. 20A through 20D  are views to illustrate a frequency change caused by driving an optical scanning device without a frequency change prevention structure, though similar to the optical scanning device of the first embodiment.  FIG. 20A  is a view showing a plane configuration of the optical scanning device without the frequency change prevention structure. In  FIG. 20A , the optical scanning device without the frequency change prevention structure has a shape in which resonant drive beams  150  in a horizontal direction extend from an inside wall of the movable frame  60 . The resonant drive beam  150  extends vertically at a length L from the inside wall. 
         [0173]      FIG. 20B  is a view showing a cross-sectional configuration of the movable frame  60  and the resonant drive beam  150  without the frequency change prevention structure shown in  FIG. 20A . As shown in  FIG. 20B , the movable frame  60  is made up of a whole SOI substrate including a thick silicon substrate, and the resonant drive beam  150  is made up of a thin silicon substrate via a buried oxide film  61 . In addition, the resonant drive beam  150  includes a drive source composed of a thin film of a piezoelectric device on the surface. In this way, a part including a rib such as the movable frame  60  is made up of the whole SOI substrate composed by laminating the thick silicon substrate on the back side, the oxide film, and the thin silicon substrate on the upper side. On the other hand, a part constructing a beam such as the resonant drive beam  150  is made up of only the thin silicon substrate on the upper side. In this respect, the optical scanning device of the first embodiment is similar. 
         [0174]      FIG. 20C  is a view showing a state of driving the resonant drive beam  150 . The drive source  151  repeats expansion and contraction depending on the polarity of the drive voltage, by which the resonant drive beam  150  oscillates up and down. At this time, because the buried oxide film  61  sandwiched by the resonant drive beam  150  and the movable frame  60  becomes a supporting point of the up and down drive, and the buried oxide film  61  is a member like a glass with few elasticity, the buried oxide film  61  has a high brittleness and is easily damaged. Accordingly, sometimes a crack  62  occurs by the up and down driving of the resonant drive beam  150 , and the oxide film  61  is damaged. 
         [0175]      FIG. 20D  is a graph showing an example of the relationship between an integrated drive time of the resonant drive beam  150  and a resonant frequency change rate. As shown in  FIG. 20D , if the resonant drive beam  150  is continuously driven, the crack  62  occurs at the supporting point of the oxide film  61  at a certain time Tc, and an apparent length L of the resonant drive beam  150  increases to (L+α), which causes the frequency to shift lower and to change. 
         [0176]      FIGS. 21A through 21C  are views showing a stress distribution of an optical scanning device without a frequency change prevention structure during the horizontal driving.  FIG. 21A  is a view showing a stress distribution on the upper side of the optical scanning device without the frequency change prevention structure in the horizontal driving.  FIG. 21B  is a view showing a stress distribution on the back side of the optical scanning device without the frequency change prevention structure in the horizontal driving.  FIG. 21C  is an enlarged view showing a stress distribution of a supporting point part of the drive beam on the back side of the optical scanning device without the frequency change prevention structure in the horizontal driving. 
         [0177]    As shown in  FIGS. 21A and 21B , the resonant drive beams  150  are coupled to the movable frame  60  in a state of extending vertically from the movable frame  60 . Also,  FIG. 21C  proves that stress is in a state easily occurring in a root part  63  that becomes a supporting point of the resonant drive beams  150 . 
         [0178]      FIGS. 22A and 22B  are views to illustrate a first frequency change prevention structure of an optical scanning device of the first embodiment.  FIG. 22A  is a view showing a plane configuration of an optical scanning device of the present embodiment including a frequency change prevention structure. In  FIG. 22A , a root part  52  connecting with the movable frame  60  of the first drive beam  50  is not connected to the inside wall of the movable frame  60  vertically; a curved shape part  53  having a rounded structure short of the movable frame  60  is formed; and the root part  52  is coupled to the movable frame  60  via the curved shape part  53 . In other words, a plane configuration of the first drive beam  50  has the curved shape part  53  that is cut inward at a position near the movable frame  60  but not reaching the movable frame  60  in a side connecting the movable frame  60  with the coupling beam  40 . 
         [0179]      FIG. 22B  is an enlarged view showing the root part  52  shown in  FIG. 22A . In  FIG. 22B , the curved shape part  53  that is cut inward is formed at a distance D from the supporting point  64  that is a border between the movable frame  60  and the first drive beam  50 . Because the curved shape part  53  has an effect of dispersing and relaxing stress, by forming the curved shape part  53  more inward than the supporting point  64  of the movable frame  60 , the stress that concentrates on a supporting point  64  (see  FIG. 23C ) can be dispersed to the curved shape part  53 . This makes it possible to protect a part of the oxide film  61  of the movable frame  60 , and to make the part of the oxide film  61  difficult to be damaged even if driven continuously. 
         [0180]      FIGS. 23A through 23C  are views showing a stress distribution in the horizontal drive of the optical scanning device of the first embodiment including the first frequency change prevention structure.  FIG. 23A  is a view showing the stress distribution on the upper side of the first drive beam  50  in the horizontal drive of the optical scanning device of the first embodiment.  FIG. 23B  is a view showing the stress distribution on the back side of the first drive beam  50  in the horizontal drive of the optical scanning device of the first embodiment. In  FIGS. 23A and 23B , the curved shape part  53  is formed in the root part  52  of the first drive beam  50 . 
         [0181]      FIG. 23C  is an enlarged view showing the root part  52  of the first drive beam  50  shown in  FIG. 23B . In  FIG. 23C , the curved shape part  53  is formed at a position more inside than the supporting point  64  and closer to the movable frame  60  than to the coupling beam  40 . The stress distribution occurs in an area more inside than the curved shape part  53 , and does not reach the part of the supporting point, as shown in  FIG. 23C . Here, if a distance D between the supporting point  64  and the closest position to the supporting point  64  of the curved shape part  53  is so short that a generated stress cannot be separated from the supporting point of the up and down driving, but so long as swinging sensitivity decreases, there may be caused a concern of not being capable of meeting a required specification. Therefore, the distance D needs to be set at a proper value, for example, which may be set at 0.1 mm. 
         [0182]      FIG. 24  is a view showing an integrated drive time and a resonant frequency change rate of the optical scanning device of the first embodiment.  FIG. 24 , different from the example of  FIG. 20D , shows that even though the integrated drive time becomes long, the resonant frequency change rate is constant, and the resonant frequency is kept constant. 
         [0183]    Thus, according to the optical scanning device of the first embodiment, with respect to a planar shape of the first drive beam  50  that performs a resonant drive, by forming the curved shape part  53  cutting inward at a position near the movable frame  60  but not reaching the movable frame  60 , it is possible to prevent a stress concentration on the supporting point  64  of a border between the movable frame  60  and the first drive beam  50 , to prevent the oxide film  61  of the movable frame  60  from being damaged, and to keep a drive frequency constant. 
         [0184]      FIGS. 25A through 25D  are views to illustrate an optical scanning device of the first embodiment further including a second frequency change prevention structure adding to the first frequency change prevention structure.  FIG. 25A  is a plane configuration view on the upper side of the optical scanning device of the first embodiment including the first and second frequency change prevention structure.  FIG. 25B  is a plane configuration view on the back side of the optical scanning device of the first embodiment including the first and second frequency change prevention structure. In  FIGS. 25A and 25B , a planar shape of the first drive beams  50  differs from the shape shown in  FIGS. 22 and 23  in that sides of the first drive beam  50  include not only the curved shape part  53  but also a constricted part  54  that cuts toward the mirror  10 . In this way, by providing not only the curved shape part  53  but also the constricted part  54  that cuts inward, the stress of the first drive beam  50  can be further moved inward and dispersed. 
         [0185]      FIG. 25C  is an enlarged plan view showing an inside of the movable frame  60  of the optical scanning device of the first embodiment.  FIG. 25D  is an enlarged view showing a stress distribution of a side part of the first drive beam  50 . As shown in  FIG. 25C , with respect to the first drive beam  50 , the curved shape part  53  and the constricted part  54  continue and form the side part of the first drive beam  50 . Also, as shown in  FIG. 25D , by providing the constricted part  54  on the coupling beam  40  side near the mirror  10 , the stress is shifted toward the constricted part  54 , and the stress is remarkably reduced in the root of the movable frame  60  side. In other words, by forming the constricted part  54  distant from the supporting point  64  of the movable frame  60 , the stress in the supporting point  64  can be widely moved to the constricted part and can be distinctly reduced. 
         [0186]      FIGS. 26A through 26C  are views showing stress distributions of optical scanning devices of respective embodiments in a comparative way.  FIG. 26A  is a view showing a stress distribution of an optical scanning device without a frequency change prevention structure.  FIG. 26B  is a view showing a stress distribution of an optical scanning device with only a first frequency change prevention structure.  FIG. 26C  is a view showing a stress distribution of an optical scanning device with the first and second frequency change prevention structures. 
         [0187]    In  FIG. 26A , a stress is applied to the supporting point  64  that is a root of the resonant drive beam  150 , and a stress distribution that may cause damage is shown. 
         [0188]    On the other hand, in  FIG. 26B , by providing the curved shape part  53  more inward than the supporting point  64 , a stress is generated more inward than in the curved shape part  53 , and the stress reaching the supporting point  64  can be prevented. 
         [0189]    Furthermore, in  FIG. 26C , by providing the constricted part  54  on the mirror  10  side, which is the coupling beam  40  side, stress is moved to the constricted part  54 , and the stress is hardly generated more to the exterior than the curved shape part  53 . 
         [0190]      FIG. 27  is a graph showing stress measurement results in the supporting point of the first drive beam  50  of the respective frequency change prevention structures shown in  FIGS. 26A through 26C . As shown in  FIG. 27 , compared to the beam shape without the frequency change prevention structure of  FIG. 26A , by providing the first frequency change prevention structure, the beam shape of  FIG. 26B  greatly decreases the root stress. In addition, by further providing the second frequency change prevention structure, the beam shape of  FIG. 26C  further decreases the generated stress in the root than does the beam shape of  FIG. 26B . 
         [0191]    In this manner, by providing the frequency change prevention structure of the curved shape part  53  and the constricted part  54  for the first drive beam  50 , destruction of the oxide film  61  of the supporting point  64  of the movable frame  60  can be prevented, and the optical scanning device can be driven by keeping the frequency constant even if driven continuously for a long time. 
       Second Embodiment  
       [0192]      FIG. 28  is a view showing an example of an optical scanning device of a second embodiment. In the optical scanning device of the second embodiment, only a structure of a rib  21 A provided on the back side of the mirror supporting part  20  differs from the optical scanning device of the first embodiment. Hence, with respect to the other components, the same numerals as the description hereinbefore are used and the descriptions are omitted. 
         [0193]    The rib  21 A of the optical scanning device of the second embodiment includes coupling ribs  22 A, arc-like ribs  23 A, chordal ribs  24 A, longitudinal ribs  25 A, and a transverse rib  26 A, which have a similar structure to those in the optical scanning device of the first embodiment. The optical scanning device of the second embodiment differs from that of the first embodiment in that penetration ribs  27 A extended from the longitudinal rib  25 A cross the chordal ribs  24 A, and further reach the inside wall of the arc-like ribs  23 A. 
         [0194]    According to the optical scanning device of the second embodiment, by further providing the penetration ribs  27 A that penetrate the chordal ribs  24 A and reach the arc-like ribs  23 A, deformation of the mirror  10  can be further reduced. 
       Third Embodiment 
       [0195]      FIG. 29  is a view showing an example of an optical scanning device of a third embodiment. The optical scanning device of the third embodiment differs from that of the first embodiment in that a rib  21 B includes longitudinal ribs  25 B and penetration ribs  27 B that connect a point A with a point D, and a point B with a point C respectively and form a shape crossing in an X-like shape. Since the other coupling ribs  22 B, arc-like ribs  23 B, chordal ribs  24 B and transverse ribs  26 B have a configuration similar to corresponding ribs of the optical scanning device of the second embodiment, the descriptions are omitted. 
         [0196]    According to the optical scanning device of the third embodiment, implementing a mirror deformation prevention structure strong against diagonal stress is possible. 
       Fourth Embodiment  
       [0197]      FIG. 30  is a view showing an example of an optical scanning device of a fourth embodiment. The optical device of the fourth embodiment differs from those of the first and second embodiments in that a rib  21 C includes connecting ribs  22 C, arc-like ribs  23 C and chordal ribs  24 C that are formed as a single large mass. In this manner, constructing the connecting ribs  22 C in an integrated manner with the arc-like rib  23 C and the chordal rib  24 C is possible. Because the mirror supporting part  20  is reinforced more solidly, an effect of preventing the mirror deformation can be certainly enhanced. Here, since the configuration of the longitudinal ribs  25 C and the transverse ribs  26 C is similar to those of the first and second embodiments, the descriptions are omitted. 
       Fifth Embodiment  
       [0198]      FIG. 31  is a view showing an example of an optical scanning device of a fifth embodiment. The optical scanning device of the fifth embodiment differs from that of the third embodiment in that a rib  21 D includes connecting ribs  22 D, arc-like ribs  23 D and chordal ribs  24 D that are formed as a single large mass. In this case also, because the mirror supporting part  20  is reinforced more solidly, an effect of preventing the mirror deformation can be surely improved. Here, since the configuration of the longitudinal ribs  25 D and the transverse ribs  26 D are similar to that of the third embodiment, the descriptions are omitted. 
         [0199]    In this way, according to embodiments of the present invention, it is possible to reduce generation of a nonlinear oscillation and to prevent a mirror deformation. 
         [0200]    The embodiments of the present invention can be applied to an image projection apparatus such as a projector that projects an image by deflecting light. 
         [0201]    Embodiment (1) is an optical scanning device including:
       a mirror;   a mirror supporting part to support the mirror on an upper surface; and   a pair of torsion beams to support the mirror supporting part from both sides in an axis direction and to drive the mirror supporting part so as to swing the mirror supporting part around the axis by being twisted themselves,   wherein each of the torsion beams includes a slit approximately parallel to the axis direction.       
 
         [0206]    Embodiment (2) is the optical scanning device as described in Embodiment (1),
       wherein the slit is a single slit and provided in a center of the torsion beam.       
 
         [0208]    Embodiment (3) is the optical scanning device as described in Embodiment (1),
       wherein a rib is provided on a back side of a connection between the mirror supporting part and the torsion beam.       
 
         [0210]    Embodiment (4) is the optical scanning device as described in Embodiment (3),
       wherein the rib projects toward the torsion beam more outward than an edge of the mirror.       
 
         [0212]    Embodiment (5) is the optical scanning device as described in Embodiment (3),
       wherein an inner edge of the slit reaches more inward than an outer edge of the rib, and a part of the rib is exposed from the slit.       
 
         [0214]    Embodiment (6) is the optical scanning device as described in Embodiment (3),
       wherein the rib has arc-like walls along a circumference of the mirror supporting part extending from the connection, and a chordal wall connecting edges of the arc-like walls to each other formed on the back side of the mirror supporting part.       
 
         [0216]    Embodiment (7) is the optical scanning device as described in Embodiment (6),
       wherein the rib is provided at each side of the mirror supporting part, and further includes a longitudinal part connecting the chordal wall of one side to the chordal wall of the other side, extending parallel to the axis direction.       
 
         [0218]    Embodiment (8) is the optical scanning device as described in Embodiment (3),
       wherein the rib further includes a transverse part extending by passing a center of the mirror.       
 
         [0220]    Embodiment (9) is the optical scanning device as described in Embodiment (1), further including:
       a movable frame surrounding the mirror, the mirror supporting part and the pair of torsion beams;   a pair of first drive beams, the first drive beams having first edges connected to and supported by inside walls of the movable frame, being opposite to each other in a direction perpendicular to the axis, and being configured to generate a driving force to swing the mirror supporting part in a first direction by an up and down driving; and   coupling beams to couple second edges of the first drive beams with the torsion beams and to transmit the driving force to the torsion beams,   wherein a planar shape of the first drive beam includes a curved shape part cut inward in a side connecting the movable frame with the coupling beam in a position near the movable frame.       
 
         [0225]    Embodiment (10) is the optical scanning device as described in Embodiment (9),
       wherein the planar shape of the first drive beam includes a constricted shape part cut inward most in a location near the coupling beam in the side.       
 
         [0227]    Embodiment (11) is the optical scanning apparatus as described in Embodiment (9), further including
       second drive beams coupling to the movable frame from outside and configured to swing the mirror supporting part in a second direction through the movable frame; and   a crosstalk preventing rib crossing in the first and the second directions on a back side of a connection between the movable frame and the second drive beams.       
 
         [0230]    Embodiment (12) is the optical scanning device as described in Embodiment (11),
       wherein the movable frame includes an outer wall extending in a direction different from the first and the second directions in the connection, and   wherein the outer wall and the crosstalk preventing rib form a triangular hollow.       
 
         [0233]    Embodiment (13) is the optical scanning device as described in Embodiment (9),
       wherein the first drive beam generates a drive force by resonant oscillation, and the second drive beam generates a drive force by non-resonant oscillation.       
 
         [0235]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.