Patent Document

TECHNICAL FIELD 
     The present invention relates to an actuator and a movable mirror. 
     BACKGROUND ART 
     A movable mirror of such a type that the movable mirror is displaced according to an electrostatic attractive force is expected to be applied in various fields that use light. For example, the movable mirror can be used as a wavefront correction device for adaptive optics which is incorporated in a fundus examination apparatus, an astronomical telescope, and the like. A typical example of the method which employs such a movable mirror that is displaced according to an electrostatic attractive force includes a method of displacing the movable mirror using two parallel flat electrodes. However, a small displacement amount is one of the drawbacks of the parallel flat electrodes. 
     In contrast, a movable mirror which provides a larger displacement amount using comb electrodes has been proposed recently. An example thereof is disclosed in PTL 1. As shown in  FIG. 14 , in this movable mirror, a supporting portion  530  that supports an comb electrode  520  on the moving side and a supporting portion  570  that supports an comb electrode  510  on the stationary side are respectively located above and below in the vertical direction on the drawing sheet. The movable comb electrode and the stationary comb electrode face each other and are disposed alternately. Due to this, since an overlapped area is larger than that of the conventional example that uses parallel flat electrodes, the electrostatic attractive force increases, and the displacement amount can be increased. 
     CITATION LIST 
     Patent Literature 
     PTL 1: US Patent Application Publication No. 2002/0109894 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the structure disclosed in PTL 1, since the comb electrodes and the supporting portions are disposed in the moving direction of the movable comb electrode, the electrostatic attractive force becomes extremely larger than a restoring force of a spring, and a phenomenon called pull-in (retraction) in which comb teeth on the moving side collide with the supporting portion on the stationary side may occur. Thus, this structure has a problem in that it is difficult to obtain a larger displacement amount. 
     With the foregoing in view, an object of the present invention is to provide a technique of suppressing the occurrence of a pull-in phenomenon in a movable mirror that uses an actuator that includes an comb electrode. 
     Solution to Problem 
     The present invention provides an actuator comprising:
         a movable portion that is connected to a reflecting member having a reflective surface;   a movable comb electrode that is disposed at a distance from the reflecting member, is supported by the movable portion, and extends in a direction parallel to the reflective surface;   a supporting portion;   a stationary comb electrode that is supported by the supporting portion, extends in the direction parallel to the reflective surface, and is disposed alternately with the movable comb electrode; and   a voltage controller that applies a voltage to the movable comb electrode and the stationary comb electrode so as to displace the movable comb electrode and the movable portion in a direction normal to the reflective surface,   wherein a portion of the movable portion that supports the movable comb electrode and a portion of the supporting portion that supports the stationary comb electrode are disposed such that the movable comb electrode and the stationary comb electrode pass each other when the movable comb electrode is displaced in the direction normal to the reflective surface.       

     The present invention also provides an actuator comprising:
         a movable portion that is connected to a reflecting member having a reflective surface;   a movable comb electrode that is disposed at a distance from the reflecting member, is supported by the movable portion, and extends in a direction parallel to the reflective surface;   a supporting portion;   a stationary comb electrode that is supported by the supporting portion, extends in the direction parallel to the reflective surface, and is disposed alternately with the movable comb electrode; and   a voltage controller that applies a voltage to the movable comb electrode and the stationary comb electrode so as to displace the movable comb electrode and the movable portion in a direction normal to the reflective surface,   wherein the movable comb electrode and the stationary comb electrode are disposed so as to pass each other when the movable comb electrode is displaced in the direction normal to the reflective surface.       

     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a technique of suppressing the occurrence of a pull-in phenomenon in a movable mirror that uses an actuator that includes an comb electrode. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view schematically showing a structure of the present invention. 
         FIG. 2A  is a top view schematically showing the structure of the present invention. 
         FIG. 2B  is a cross-sectional view schematically showing the structure of the present invention. 
         FIGS. 3A and 3B  are cross-sectional views showing a structure of a reflecting portion according to the present invention. 
         FIGS. 4A to 4D  are schematic views for explaining the way that comb teeth according to the present invention are moved. 
         FIG. 5  is a perspective view schematically showing a structure of a first example. 
         FIG. 6A  is a top view schematically showing the structure of the first example. 
         FIG. 6B  is a cross-sectional view schematically showing the structure of the first example. 
         FIGS. 7A and 7B  are cross-sectional views showing a structure of a reflecting portion according to the first example. 
         FIGS. 8A to 8D  are schematic views for explaining the way that comb teeth according to the first example are moved. 
         FIG. 9  is a perspective view schematically showing a structure of a second example. 
         FIG. 10A  is a top view schematically showing the structure of the second example. 
         FIG. 10B  is a cross-sectional view schematically showing the structure of the second example. 
         FIGS. 11A and 11B  are cross-sectional views showing a structure of a reflecting portion according to the second example. 
         FIGS. 12A to 12D  are schematic views for explaining the way that comb teeth according to the second example are moved. 
         FIGS. 13A to 13D  are another schematic views for explaining the way that comb teeth according to the second example are moved. 
         FIG. 14  is a diagram for explaining a conventional technique. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an electrostatic comb movable mirror according to the present invention will be described with reference to  FIG. 1 . Here,  FIG. 1  is a perspective view of an electrostatic comb movable mirror which is an embodiment of the present invention. A movable mirror  101  shown in  FIG. 1  includes an actuator portion  102  that has a driving function and a reflecting portion  103  that has a reflecting function. 
       FIG. 2A  shows a top view of the actuator portion  102 . In  FIG. 2A , a lateral direction of the drawing sheet will be referred to as an x-direction, a longitudinal direction of the drawing sheet will be referred to as a y-direction, and a vertical direction of the drawing sheet will be referred to as a z-direction. An xy-plane shown in the figure is a plane parallel to a substrate. The actuator portion  102  includes a movable comb electrode  201 , a stationary comb electrode  202 , a movable portion  203 , a spring  204 , and a supporting portion  205  ( 205   a ,  205   b ). 
     The movable portion  203  is coupled with the spring  204  and is connected to the movable comb electrode  201  and the reflecting portion  103 . One end of the spring  204  is fixed to the supporting portion  205   a . The movable comb electrode  201  and the spring  204  are connected to a side wall of the movable portion  203 , and the reflecting portion  103  is connected to an upper surface of the movable portion  203 . 
     The movable comb electrode  201  extends in the y-direction from a side wall of the movable portion  203  parallel to the xz-plane, and the stationary comb electrode  202  extends in the y-direction from a side wall of the supporting portion  205   b  parallel to the xz-plane. Since the side walls of the movable portion and the supporting portion face each other, the movable comb electrode  201  and the stationary comb electrode  202  are disposed to face each other, and the respective comb teeth are arranged alternately. 
       FIG. 2B  shows a cross-sectional view of the movable mirror  101 . Since the heights in the z-direction of a side surface of the movable comb electrode  201  and a side surface of the stationary comb electrode  202  are different, a portion where the comb electrodes do not overlap each other needs to be present. That is, the comb electrodes have a portion where the comb electrodes do not overlap each other in a direction vertical to the reflective surface of the reflecting portion  103 . Here, “different heights” does not mean that the sizes in the z-direction of both comb electrodes are different but means that both comb electrodes are shifted from each other in the z-direction in an initial state where no voltage is applied. This is because the present invention employs a scheme (movable overlap type) that uses a phenomenon in which when comb electrodes are attracted by an electrostatic attractive force, a force acts in a direction where the comb electrodes overlap each other, and the comb electrodes are moved. In this phenomenon, when the comb electrodes overlap each other completely, the comb electrodes are not moved further, it is necessary to decrease an overlapped portion at the initial position and to increase the overlapped portion when a voltage is applied. As shown in the figure, the movable comb electrode  201  and the reflecting portion  103  are located at a predetermined distance in the z-direction, and the stationary comb electrode  202  is not in contact with other members in the z-direction. Thus, even when an electrostatic attractive force occurs and attracts the comb electrodes, any of the comb electrodes does not collide with a member connected to the other comb electrode. As can be understood from  FIGS. 2A and 2B , the movable comb electrode  201  is supported by a predetermined portion of the movable portion  203  in a cantilevered manner to extend in a direction parallel to the reflective surface. Further, the stationary comb electrode  202  is supported by a predetermined portion of the supporting portion  205   b  in a cantilevered manner to extend in a direction parallel to the reflective surface. When the reflecting portion continuously covers a plurality of actuators as will be described later, since the reflecting portion is deformed, there is a possibility that the angle of the reflective surface may not remain constant. In that case, the respective comb electrodes extend in a direction parallel to at least a portion of the reflective surface connected to the movable portion. 
     Although  FIG. 2B  shows the movable comb electrode  201  that is disposed above in the z-direction in relation to the stationary comb electrode  202 , a positional relationship of both comb electrodes may be reversed. 
     The spring  204  extends in the x-direction from a side wall of the movable portion  203  parallel to the yz-plane and is fixed to a side wall of the supporting portion  205   a  parallel to the yz-plane. When the movable portion  203  is displaced in a direction other than the z-direction, the movable comb electrode  201  and the stationary comb electrode  202  may interfere. Thus, it is necessary to suppress displacement in a direction other than the z-direction (in other words, a direction other than the direction normal to the reflective surface of the reflecting portion  103 ). 
     The stationary comb electrode  202  and the spring  204  are fixed by the supporting portions  205   b  and  205   a , respectively. Independent electric potentials are applied to the stationary comb electrode  202  and the movable comb electrode  201 . Thus, an isolation groove  206  is provided so as to electrically isolate the supporting portion  205   b  that belongs to the stationary comb electrode  202  from the supporting portion  205   a  that belongs to the movable comb electrode  202 . Wires are disposed in the isolated supporting portions and are connected to a voltage control circuit  207 . 
     The reflecting portion  103  has an optically reflecting function of reflecting light to be corrected. The reflecting portion  103  has the reflective surface in order to reflect light. The reflecting portion  103  is disposed so as to cover the actuator portion  102  and is coupled with the movable portion  203 . The reflecting portion corresponds to a reflecting member of the present invention. 
       FIG. 3  is a view showing a case where a plurality of actuator portions  102  is disposed. In contrast, the reflecting portion may be a continuous surface  301  that covers the plurality of actuator portions  102  as a whole as shown in  FIG. 3A  and may be independent surfaces  302  that individually cover the respective actuator portions as shown in  FIG. 3B . By individually moving the movable portions  203  of the respective actuators, it is possible to obtain a desired shape. By doing so, since an optical path length of light reflected can be changed by the respective actuator portions, the actuator portions can be used as a wavefront correction device. 
     Next, a method of moving the movable portion  203  will be described with reference to  FIG. 4 .  FIG. 4  is a cross-sectional view of a portion in which the movable comb electrode  201  and the stationary comb electrode  202  are arranged alternately. By applying charges having the opposite signs to the movable comb electrode  201  and the stationary comb electrode  202 , it is possible to move the movable comb electrode  201  in the z-direction (a direction normal to the reflective surface of the reflecting portion  103 ). A working electrostatic attractive force Fz in the z-direction when a potential difference is applied between the movable comb electrode and the stationary comb electrode is expressed as Expression (1) below. 
     
       
         
           
             
               
                 
                   
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     Here, ∈ 0  is a dielectric constant of vacuum, N is the number of gaps between comb electrodes, h is an overlap distance between the movable comb electrode and the stationary comb electrode, V m  is an electric potential of the movable comb electrode, V f  is an electric potential of the stationary comb electrode, and g is a gap width between comb electrodes. 
     For example, when the movable comb electrode  201  and the stationary comb electrode  202  are disposed as shown in  FIG. 4 , the following method may be used to move the movable comb electrode  201  downward in the z-direction. First, as in the state immediately after application of a voltage shown in  FIG. 4A , when charges having the opposite signs are applied to the movable comb electrode  201  and the stationary comb electrode  202 , an electrostatic attractive force (E) is generated, and the electrodes are attracted. As a result, although the movable comb electrode  201  tries to approach the stationary comb electrode  202 , since the electrostatic attractive force is evenly distributed to the left and right side in relation to the x-direction, the movable comb electrode  201  is displaced downward in the z-direction. 
     Subsequently, a balanced state as shown in  FIG. 4B  is created. That is, the movable comb electrode  201  stops at such a position that a restoring force (R) of the spring  204  is balanced with the electrostatic attractive force that moves the movable portion  203 . 
     Subsequently, when a potential difference between the movable comb electrode  201  and the stationary comb electrode  202  is set to 0, a state where no charge is applied is created as shown in  FIG. 4C . After the voltage is removed, the movable comb electrode  201  returns to its initial position according to the restoring force of the spring  204 . A state after this displacement is shown in  FIG. 4D . 
     Although this embodiment describes displacement according to an electrostatic attractive force, displacement may be realized according to an electrostatic repulsive force. 
     In the structure disclosed in PTL 1, when the movable comb electrode  201  is moved, the comb electrode and the supporting portion are disposed in the z-direction which is a moving direction of the movable comb electrode. Thus, an electrostatic attractive force is generated between a distal end surface of the comb electrode and the surface of the supporting portion. When the electrostatic attractive force is extremely larger than the restoring force of the spring, a pull-in phenomenon occurs, and the comb electrode collides with the supporting portion. However, according to the structure of this embodiment, since the supporting portion is not disposed in the z-direction which is the moving direction of the movable comb electrode, a pull-in phenomenon does not occur. That is, in the structure of the present invention, even when an electrostatic attractive force acts, both comb electrodes can pass each other without making collision. Thus, a pull-in phenomenon does not occur, and a short of electrodes does not occur. 
     Since the displacement amount can be predicted by measuring an electrostatic capacitance value, feedback control can be performed. 
     Further, the structure of the present invention can be used in vacuum and may be used in the air. If the structure as disclosed in PTL 1 is used in the air, when comb teeth are moved, the air between the supporting portions causes damping, and a response speed may decrease. However, according to the structure of the present invention, it is possible to suppress the influence of damping. 
     Various modifications and changes can be made to this embodiment within a range without departing from the spirit of the present invention. 
     For example, in this embodiment, although the reflecting portion  103  and the movable portion  203  are connected, a post may be provided between the reflecting portion  103  and the movable portion  203 . In this case, it is necessary to provide the post in such a range that the reflecting portion  103  does not interfere with the comb electrode. 
     Moreover, in this embodiment, the movable portion  203 , the spring  204 , and the supporting portion  205  are formed of silicon that is doped with conductive impurities in order to apply electric potentials to the movable comb electrode  201  and the stationary comb electrode  202 . However, rather than forming these member using conductive materials, a method of feeding current by forming wires or feeding current through bonding wires may be used. 
     First Example 
     Hereinafter, an electrostatic comb movable mirror according to a first example of the present invention will be described with reference to  FIG. 5 . Here,  FIG. 5  is a perspective view of an electrostatic comb movable mirror  501  according to the first example of the present invention. A movable mirror  501  shown in  FIG. 5  includes an actuator portion  502  that has a driving function and a reflecting portion  503  that has a reflecting function. 
       FIG. 6A  shows a top view of the actuator portion  502 . In  FIG. 6A , a lateral direction of the drawing sheet will be referred to as an x-direction, a longitudinal direction of the drawing sheet will be referred to as a y-direction, and a vertical direction of the drawing sheet will be referred to as a z-direction. An xy-plane shown in the figure is a plane parallel to a substrate. The actuator portion  502  includes a movable comb electrode  601 , a stationary comb electrode  602 , a movable portion  603 , a spring  604 , a supporting portion  605  ( 605   a ,  605   b ), and a post  610 . 
     The movable portion  603  is coupled with the spring  604  and is connected to the movable comb electrode  601  and the post  610 . One end of the spring  604  is fixed to the supporting portion  605   a . In this example, the movable portion  603  has a quadrangular prism shape, the movable comb electrode  601  is disposed on two surfaces of the four side walls that are parallel to the xz-plane, and the spring  604  of which one end is fixed to the supporting portion  605   b  is coupled with the two surfaces that are parallel to the yz-plane. Further, the post  610  for transferring displacement of the movable portion  603  to the reflecting portion  503  is provided on the upper surface. 
     The movable comb electrode  601  extends in the y-direction from a side wall of the movable portion  603  parallel to the xz-plane, and the stationary comb electrode  602  extends in the y-direction from a side wall of the supporting portion  605   b  parallel to the xz-plane. Since the side walls of the movable portion and the supporting portion face each other, the movable comb electrode  601  and the stationary comb electrode  602  are disposed to face each other, and the respective comb teeth are arranged alternately. In this example, the movable comb electrode  601  and the stationary comb electrode  602  have a thickness of 200 μm and a length of 200 μm. The number of comb electrodes for one actuator is 40 for the movable comb electrode and 42 for the movable comb electrode, and the number of gaps between the comb electrodes is 80. The thickness of the comb electrode means the size in the z-direction, the length means the size in the y-direction, and the width means the size in the x-direction. 
     Since the heights in the z-direction of a side surface of the movable comb electrode  601  and a side surface of the stationary comb electrode  602  are different, a portion where the comb electrodes do not overlap each other needs to be present. This is because the present invention employs a scheme (movable overlap type) that uses a phenomenon in which when comb electrodes are attracted by an electrostatic attractive force, a force acts in a direction where the comb electrodes overlap each other, and the comb electrodes are moved. In this phenomenon, when the comb electrodes overlap each other completely, the comb electrodes are not moved further, it is necessary to decrease an overlapped portion at the initial position and to increase the overlapped portion when a voltage is applied. 
       FIG. 6B  shows a cross-sectional view of the movable mirror  501  and shows a positional relationship between the movable comb electrode  601  and the stationary comb electrode  602  according to this example. The positional relationship is set such that the movable comb electrode  601  is disposed above the stationary comb electrode  602 . In this case, a shift amount is 10 μm. Further, the movable comb electrode  601  and the stationary comb electrode  602  have a width of 10 μm, and the gap between the two electrodes is 5 μm. As shown in the figure, the movable comb electrode  601  and the reflecting portion  503  are located at a predetermined distance in the z-direction, and the stationary comb electrode  602  is not in contact with other members in the z-direction. Thus, even when an electrostatic attractive force occurs and attracts the comb electrodes, any of the comb electrodes does not collide with a member connected to the other comb electrode. 
     The spring  604  extends in the x-direction from a side wall of the movable portion  603  parallel to the yz-plane and is fixed to a side wall of the supporting portion  605   a  parallel to the yz-plane. When the movable portion  603  is displaced in a direction other than the z-direction, the movable comb electrode  601  and the stationary comb electrode  602  may interfere. Thus, it is necessary to suppress displacement in a direction other than the z-direction. In this example, the spring has such a shape that the spring expands in the xy-direction, whereby spring constants in the x-direction, the y-direction, and the directions of rotation within the xy, yz, and zx-planes are increased to suppress displacement in these directions. In particular, translation in the y-direction and rotation within the xy and yz-planes can be suppressed by increasing the width of the spring in the y-direction. Specifically, the width of the spring in the y-direction is preferably 1/10 or more of the length in the x-direction and 20 times or more than the thickness of the spring. In this example, the spring  604  has a thickness of 5 μm, a length of 500 μm in the x-direction, and a width of 300 μm in the y-direction. 
     The post  610  needs to have sufficient rigidity to accurately transfer displacement of the movable portion  603  to the reflecting portion  503 . Moreover, the post  610  needs to have such a height that the stationary comb electrode  602  and the reflecting portion  503  do not interfere when the movable portion  603  is moved. In this example, the post  610  has a height of 20 μm. 
     The stationary comb electrode  602  and the spring  604  are fixed by the supporting portions  605   b  and  605   a , respectively. Different voltages are applied to the movable comb electrode  601  and the stationary comb electrode  602 . Thus, an isolation groove  606  is provided so as to electrically isolate the supporting portion  605   b  that belongs to the stationary comb electrode  602  from the supporting portion  605   a  that belongs to the movable comb electrode  601 . Wires are disposed in the isolated supporting portions  605  and are connected to a voltage control circuit  607 . 
     The reflecting portion  503  has an optically reflecting function of reflecting light to be corrected. The reflecting portion  503  has the reflective surface in order to reflect light. The reflecting portion  503  is disposed so as to cover the actuator portion  502  and is coupled with the actuator portion  502  via the post  610 . The reflecting portion  503  has a thickness of 5 μm. The reflecting portion corresponds to a reflecting member of the present invention. 
       FIG. 7  is a view showing a case where a plurality of actuator portions  502  is disposed. In contrast, the reflecting portion may be a continuous surface  701  that covers the plurality of actuator portions as a whole as shown in  FIG. 7A  and may be independent surfaces  702  that individually cover the respective actuator portions as shown in  FIG. 7B . In this example, the reflecting portion is disposed so as to cover the plurality of actuator portions as a whole. By individually moving the respective actuator portions  501 , it is possible to obtain a desired shape. By doing so, since an optical path length of light reflected can be changed by the respective actuator portions  501 , the actuator portions can be used as a wavefront correction device. 
     Next, a method of moving the movable portion  603  will be described with reference to  FIG. 8 .  FIG. 8  is a cross-sectional view of a portion in which the movable comb electrode  601  and the stationary comb electrode  602  are arranged alternately. By applying charges having the opposite signs to the movable comb electrode  601  and the stationary comb electrode  602 , it is possible to move the movable comb electrode  601  in the z-direction. 
     For example, when the movable comb electrode  601  and the stationary comb electrode  602  are disposed as shown in  FIG. 8 , the following method may be used to move the movable comb electrode  601  downward in the z-direction. First, as in the state immediately after application of a voltage shown in  FIG. 8A , when charges having the opposite signs are applied to the movable comb electrode  601  and the stationary comb electrode  602 , an electrostatic attractive force (E) is generated, and the electrodes are attracted. As a result, although the movable comb electrode  601  tries to approach the stationary comb electrode  602 , since the electrostatic attractive force is evenly distributed to the left and right side in relation to the horizontal direction (the x-direction), the movable comb electrode  601  is displaced downward in the z-direction. In this example, negative charges are applied to the movable comb electrode  601 , and positive charges are applied to the stationary comb electrode  602 . 
     Subsequently, a balanced state as shown in  FIG. 8B  is created. That is, the movable comb electrode  601  stops at such a position that a restoring force (R) of the spring  604  is balanced with the electrostatic attractive force that moves the movable portion  603 . 
     Subsequently, when a potential difference between the movable comb electrode  601  and the stationary comb electrode  602  is set to 0, a state where no charge is applied is created as shown in  FIG. 8C . After the voltage is removed, the movable comb electrode  601  returns to its initial position according to the restoring force of the spring  604 . A state after this displacement is shown in  FIG. 8D . 
     Although this example describes displacement according to an electrostatic attractive force, displacement may be realized according to an electrostatic repulsive force. 
     Since the displacement amount can be predicted by measuring an electrostatic capacitance value, feedback control can be performed. In this example, closed-loop control (displacement amount feedback) is performed based on the electrostatic capacitance value of the comb electrode. Further, by controlling the displacement amount of the movable comb electrode  601  that extends in the vertical direction according to feedback control, since it is possible to move both comb teeth equally, it is possible to suppress displacement in the direction of rotation within the yz-plane. 
     It is necessary to apply individual voltages to the movable comb electrode  601  and the stationary comb electrode  602 . In this example, the movable portion  603 , the spring  604 , and the supporting portion  605  are formed of conductive silicon that is doped with impurities in order to apply voltages to the electrodes. Further, although wires are formed in order to connect these members to the voltage control circuit  607 , these wires need to be formed of conductive materials, and in this example, copper is used. 
     The reflecting portion  503  has an optically reflecting function and needs to have rigidity appropriate for obtaining a desired shape when the reflecting portion  503  is deformed according to movement of the movable portion  603 . In this example, the reflecting portion  503  is made up of two layers in which the lower layer is a silicon film that determines the shape of the reflecting portion  503  and the upper layer is a gold thin film that determines a reflecting performance. In this case, the gold thin film becomes the reflective surface. 
     Various modifications and changes can be made to this embodiment within a range without departing from the spirit of the present invention. 
     For example, in this example, the movable portion  603 , the spring  604 , and the supporting portion  605  are formed of silicon that is doped with conductive impurities in order to apply electric potentials to the movable comb electrode  601  and the stationary comb electrode  602 . However, rather than forming these member using conductive materials, a method of feeding current by forming wires or feeding current through bonding wires may be used. 
     Further, the dimensions described above are design matters and thus can be set freely. 
     Second Example 
     Hereinafter, an electrostatic comb movable mirror according to a second example of the present invention will be described with reference to  FIG. 9 . Here,  FIG. 9  is a perspective view of an electrostatic comb movable mirror  901  according to the second example of the present invention. A movable mirror shown in  FIG. 9  includes an actuator portion  902  that has a driving function and a reflecting portion  903  that has a reflecting function. 
       FIG. 10A  shows a top view of the actuator portion  902 . In  FIG. 10A , a lateral direction of the drawing sheet will be referred to as an x-direction, a longitudinal direction of the drawing sheet will be referred to as a y-direction, and a vertical direction of the drawing sheet will be referred to as a z-direction. An xy-plane shown in the figure is a plane parallel to a substrate. The actuator portion  902  includes a movable comb electrode  1001 , a stationary comb electrode  1002 , a movable portion  1003 , a spring  1004 , a supporting portion  1005  ( 1005   a ,  1005   b ), and a post  1010 . 
     The movable portion  1003  is coupled with the spring  1004  and is connected to the movable comb electrode  1001  and the post  1010 . One end of the spring  1004  is fixed to the supporting portion  1005   a . In this example, the movable portion  1003  has a quadrangular prism shape, the movable comb electrode  1001  is disposed on two surfaces of the four side walls that are parallel to the xz-plane, and the spring  1004  of which one end is fixed to the supporting portion  1005   a  is coupled with the two surfaces that are parallel to the yz-plane. Further, the post  1010  for transferring displacement of the movable portion  1003  to the reflecting portion  903  is provided on the upper surface. 
     The movable comb electrode  1001  and the stationary comb electrode  1002  have electrodes that are divided in the z-direction. In this example, the comb electrodes are formed from a silicon-on-insulator (SOI) wafer that includes a Si layer, a SiO 2 -buried insulation layer, and a Si layer. The wafer has a size of 4 inches, and the respective layers have a thickness of 100 μm for Si layer, 1 μm for SiO 2 -buried layer, and 100 μm for Si layer. 
     The movable comb electrode  1001  extends in the y-direction from a side wall of the movable portion  1003  parallel to the xz-plane, and the stationary comb electrode  1002  extends in the y-direction from a side wall of the supporting portion  1005   b  parallel to the xz-plane. Since the side walls of the movable portion and the supporting portion face each other, the movable comb electrode  1001  and the stationary comb electrode  1002  are disposed to face each other, and the respective comb teeth are arranged alternately. In this example, the movable comb electrode  1001  and the stationary comb electrode  1002  have a thickness of 200 μm and a length of 200 μm. The number of comb electrodes for one actuator is 40 for the movable comb electrode and 42 for the movable comb electrode, and the number of gaps between the comb electrodes is 80. 
     Since the positional relationship in the z-direction between a side surface of the movable comb electrode  1001  and a side surface of the stationary comb electrode  1002  is different from that of the first example of the present invention, a portion where the comb electrodes do not overlap each other does not necessarily need to be present. This is because in this example, the movable comb electrode  1001  and the stationary comb electrode  1002  are electrically isolated in the z-direction, and a portion where the comb electrodes having the same height do not overlap each other can be created depending on a method of applying a voltage. 
       FIG. 10B  shows a cross-sectional view of the movable mirror  901  and shows a positional relationship between the movable comb electrode  1001  and the stationary comb electrode  1002  according to this example. The positional relationship is set such that the movable comb electrode  1001  and the stationary comb electrode  1002  are disposed at the same height. That is, the comb electrodes overlap each other in a direction vertical to the reflective surface of the reflecting portion  903 . In this example, the movable comb electrode  1001  and the stationary comb electrode  1002  have a width of 10 μm, and the gap between the two electrodes is 5 μm. As shown in the figure, the movable comb electrode  1001  and the reflecting portion  903  are located at a predetermined distance in the z-direction, and the stationary comb electrode  1002  is not in contact with other members in the z-direction. Thus, even when an electrostatic attractive force occurs and attracts the comb electrodes, any of the comb electrodes does not collide with a member connected to the other comb electrode. 
     The spring  1004  extends in the x-direction from a side wall of the movable portion  1003  parallel to the yz-plane and is fixed to a side wall of the supporting portion  1005   a  parallel to the yz-plane. When the actuator portion  902  is displaced in a direction other than the z-direction, the movable comb electrode  1001  and the stationary comb electrode  1002  may interfere. Thus, displacement in a direction other than the z-direction needs to be suppressed by the spring  1004 . In this example, the spring has such a shape that the spring expands in the xy-direction, whereby spring constants in the x-direction, the y-direction, and the directions of rotation within the xy and yz-planes are increased to suppress displacement in these directions. In this example, the spring  1004  has a thickness of 5 μm, a length of 500 μm in the x-direction, and a width of 300 μm in the y-direction. 
     The post  1010  needs to have sufficient rigidity to accurately transfer displacement of the movable portion  1003  to the reflecting portion  903 . Moreover, the post  1010  needs to have such a height that the stationary comb electrode  1002  and the reflecting portion  903  do not interfere when the movable portion  1003  is moved. In this example, the post  1010  has a height of 20 μm. 
     The stationary comb electrode  1002  and the spring  1004  are fixed by the supporting portions  1005   b  and  1005   a , respectively. In order to apply different voltages to the stationary comb electrode  1002  and the movable comb electrode  1001 , an isolation portion  1006  is provided so as to electrically isolate the supporting portion that belongs to the stationary comb electrode from the supporting portion that belongs to the movable comb electrode. Further, since each of the stationary comb electrode and the movable comb electrode has two electrodes above and below in the z-direction with an insulating layer  1011  interposed, four electrodes are disposed in the actuator portion  902  in total. Wires are disposed in these four divided supporting portions  1003  and are connected to a voltage control circuit  1007 . 
     The reflecting portion  903  has an optically reflecting function of reflecting light to be corrected. The reflecting portion  903  has the reflective surface in order to reflect light. The reflecting portion  903  is disposed so as to cover the actuator portion  902  and is coupled with the actuator portion  902  via the post  1010 . The reflecting portion  903  has a thickness of 5 μm. The reflecting portion corresponds to a reflecting member of the present invention. 
       FIG. 11  is a view showing a case where a plurality of actuator portions  902  is disposed. In contrast, the reflecting portion may be a continuous surface  1101  that covers the plurality of actuator portions as a whole as shown in  FIG. 11A  and may be independent surfaces  1102  that individually cover the respective actuator portions as shown in  FIG. 11B . In this example, the reflecting portion  903  is disposed so as to cover the plurality of actuator portions  902  as a whole. By individually moving the respective actuator portions  902 , it is possible to obtain a desired shape. By doing so, since an optical path length of light reflected can be changed by the respective actuator portions, the actuator portions can be used as a wavefront correction device. 
     Next, a method of moving the movable portion  1003  will be described with reference to  FIG. 12 .  FIG. 12  is a cross-sectional view of a portion in which the movable comb electrode  1001  and the stationary comb electrode  1002  are arranged alternately. By applying charges having the opposite signs to the movable comb electrode  1001 , the stationary comb electrode  1002 , and the electrodes divided in the z-direction, it is possible to move the movable comb electrode  1001  vertically in the z-direction. 
     For example, the following method may be used to move the movable comb electrode  1001  downward in the z-direction. First, as in the state immediately after application of a voltage shown in  FIG. 12A , when charges having the opposite signs are applied to the upper electrode of the movable comb electrode  1001  and the lower electrode of the stationary comb electrode  1002 , an electrostatic attractive force (E) is generated, and portions of the electrodes to which charges are applied are attracted. In this example, as shown in  FIG. 12A , negative charges are applied to the upper electrode of the movable comb electrode  1001 , and positive charges are applied to the lower electrode of the stationary comb electrode  1002 . As a result, although the upper electrode of the movable comb electrode  1001  tries to approach the lower electrode of the stationary comb electrode  1002 , since the electrostatic attractive force is evenly distributed to the left and right side in relation to the horizontal direction, the upper electrode of the movable comb electrode  1001  is displaced downward in the z-direction. 
     Subsequently, a balanced state as shown in  FIG. 12B  is created. That is, the movable comb electrode  1001  stops at such a position that a restoring force (R) of the spring  1004  is balanced with the electrostatic attractive force that moves the movable portion  1003 . 
     Subsequently, when a potential difference between the movable comb electrode  1001  and the stationary comb electrode  1002  is set to 0, a state where no charge is applied is created as shown in  FIG. 12C . After the voltage is removed, the movable comb electrode  1001  returns to its initial position according to the restoring force of the spring  1004 . A state after this displacement is shown in  FIG. 12D . 
     On the other hand, the following method may be used to move the movable comb electrode  1001  upward in the z-direction. First, as in the state immediately after application of a voltage shown in  FIG. 13A , when charges having the opposite signs are applied to the lower electrode of the movable comb electrode  1001  and the upper electrode of the stationary comb electrode  1002 , an electrostatic attractive force (E) is generated, and portions of the electrodes to which charges are applied are attracted. In this example, as shown in  FIG. 13A , negative charges are applied to the lower electrode of the movable comb electrode  1001 , and positive charges are applied to the upper electrode of the stationary comb electrode  1002 . As a result, although the lower electrode of the movable comb electrode  1001  tries to approach the upper electrode of the stationary comb electrode  1002 , since the electrostatic attractive force is evenly distributed to the left and right side in relation to the horizontal direction, the lower electrode of the movable comb electrode  1001  is displaced upward in the z-direction. 
     Subsequently, a balanced state as shown in  FIG. 13B  is created. That is, the movable comb electrode  1001  stops at such a position that a restoring force (R) of the spring  1004  is balanced with the electrostatic attractive force that moves the movable portion  1003 . 
     Subsequently, when a potential difference between the movable comb electrode  1001  and the stationary comb electrode  1002  is set to 0, a state where no charge is applied is created as shown in  FIG. 13C . After the voltage is removed, the movable comb electrode  1001  returns to its initial position according to the restoring force of the spring  1004 . A state after this displacement is shown in  FIG. 13D . 
     Although this example describes displacement according to an electrostatic attractive force, displacement may be realized according to an electrostatic repulsive force. 
     Since the displacement amount can be predicted by measuring an electrostatic capacitance value, feedback control can be performed. In this example, closed-loop control (displacement amount feedback) is performed based on the electrostatic capacitance value of the comb electrode. Further, by controlling the displacement amount of the movable comb electrode  1001  that extends in the vertical direction according to feedback control, since it is possible to move both comb teeth equally, it is possible to suppress displacement in the direction of rotation within the yz-plane. 
     It is necessary to apply individual voltages to the movable comb electrode  1001  and the stationary comb electrode  1002 . In this example, the movable portion  1003 , the spring  1004 , and the supporting portion  1005  are formed of conductive silicon that is doped with impurities in order to apply voltages to the electrodes. Further, although wires are formed in order to connect these members to the voltage control circuit  1007 , these wires need to be formed of conductive materials, and in this example, copper is used. 
     The reflecting portion has an optically reflecting function and needs to have rigidity appropriate for obtaining a desired shape when the reflecting portion is deformed according to movement of the movable portion  1003 . In this example, the reflecting portion  903  is made up of two layers in which the lower layer is a silicon film that determines the shape of the reflecting portion and the upper layer is a gold thin film that determines a reflecting performance. In this case, the gold thin film becomes the reflective surface. 
     Various modifications and changes can be made to this embodiment within a range without departing from the spirit of the present invention. 
     For example, in this example, the movable portion  1003 , the spring  1004 , and the supporting portion  1005  are formed of silicon that is doped with conductive impurities in order to apply electric potentials to the movable comb electrode  1001  and the stationary comb electrode  1002 . However, rather than forming these member using conductive materials, a method of feeding current by forming wires or feeding current through bonding wires may be used. 
     Further, the dimensions described above are design matters and thus can be set freely. 
     The structure of the present invention enables a movable mirror to having a fast response speed, and can be used as a wavefront correction device for adaptive optics which is incorporated in a fundus examination apparatus, an astronomical telescope, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-8935, filed on Jan. 19, 2012, which is hereby incorporated by reference herein in its entirety.

Technology Category: g