Actuator and optical scanning device

An actuator includes a driving beam that includes a beam extending in a direction orthogonal to a predetermined axis and supports an object to be driven, a driving source that is formed on a first surface of the beam and causes the object to rotate around the predetermined axis, and a rib formed on a second surface of the beam. A notch is formed in a portion of the driving source corresponding to an end of the rib.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims priority to Japanese Patent Application No. 2018-037056, filed on Mar. 2, 2018, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An aspect of this disclosure relates to an actuator and an optical scanning device.

2. Description of the Related Art

There is a known optical scanning device where a mirror is rotated around a rotational axis to scan light reflected by the mirror by using an actuator including, as a driving source, a piezoelectric element that includes an upper electrode formed on the upper surface of a piezoelectric thin film and a lower electrode formed on the lower surface of the piezoelectric thin film. The actuator also includes an upper wire connected to the upper electrode and a lower wire connected to the lower electrode that are used to apply a voltage to the piezoelectric thin film (see, for example, Japanese Laid-Open Patent Publication No. 2016-001325 and Japanese Patent No. 5876329).

The actuator includes a Micro Electro Mechanical Systems (MEMS) structure for rotating the mirror around the rotational axis, and the MEMS structure greatly deforms in the thickness direction. The MEMS structure may be implemented by a bellows structure to reduce the rigidity in the thickness direction while maintaining the rigidity in the in-plane direction.

FIG. 4 of Japanese Laid-Open Patent Publication No. 2012-123364 discloses a MEMS structure having a bellows structure where a piezoelectric element provided on each cantilever of a bellows is deformed to rotate (or oscillate) a mirror. In the MEMS structure, the cantilever of the bellows warps as a result of expansion and contraction of the piezoelectric element. Because the piezoelectric element expands and contracts not only in the longitudinal direction of the cantilever but also in the lateral direction of the cantilever, the mirror is not only rotated but also translated or displaced in the thickness direction.

When the MEMS structure is used for a projection apparatus that displays an image by scanning a laser beam, the displacement of the mirror in the thickness direction is sufficiently small compared with the scanning range of the image and therefore may not cause a big problem. On the other hand, in an apparatus such as an optical coherence tomography (OCT) apparatus or a Fourier transform infrared spectrometer (FTIR) that uses interference of light, the displacement of the mirror in the thickness direction changes the phase of light reflected by the mirror. Thus, the displacement of the mirror in the thickness direction greatly affects the output of an apparatus such as an OCT apparatus or an FTIR.

SUMMARY OF THE INVENTION

In an aspect of this disclosure, there is provided an actuator including a driving beam that includes a beam extending in a direction orthogonal to a predetermined axis and supports an object to be driven, a driving source that is formed on a first surface of the beam and causes the object to rotate around the predetermined axis, and a rib formed on a second surface of the beam. A notch is formed in a portion of the driving source corresponding to an end of the rib.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below with reference to the accompanying drawings. Throughout the accompanying drawings, the same reference numbers are used for the same components, and repeated descriptions of those components may be omitted.

Embodiment

FIG. 1Ais a top perspective view of an optical scanner100of an optical scanning device according to an embodiment.FIG. 1Bis a bottom perspective view of the optical scanner100ofFIG. 1A.FIG. 2is a top plan view of the optical scanner100ofFIG. 1A. The optical scanner100may be housed in a ceramic package including a package body and a package cover.

The optical scanner100scans an incoming laser beam emitted by a light source by causing a mirror110to rotate (or oscillate). The optical scanner100is, for example, a MEMS mirror where the mirror110is driven by driving sources such as piezoelectric elements. The mirror110of the optical scanner100reflects an incoming laser beam and scans the reflected laser beam two-dimensionally.

As illustrated inFIGS. 1A, 1B, and 2, the optical scanner100includes the mirror110, a mirror support120, coupling beams121A and121B, horizontal driving beams130A and130B, a movable frame160, vertical driving beams170A and170B, and a fixed frame180. The mirror110is supported on an upper surface of the mirror support120.

The horizontal driving beams130A and130B for supporting the mirror110and the mirror support120are connected to the mirror support120and disposed on the corresponding sides of the mirror support120for supporting the mirror110. The horizontal driving beams130A and130B are connected to the mirror support120via the coupling beams121A and121B. The horizontal driving beams130A and130B, the coupling beams121A and121B, the mirror support120, and the mirror110are supported by the movable frame160surrounding these components. The horizontal driving beam130A includes multiple rectangular horizontal beams that extend in the direction of a vertical-rotation axis AXV that is orthogonal to a horizontal-rotation axis AXH, and ends of adjacent horizontal beams are connected to each other via turnaround parts131X2,131X3, and131X4such that the horizontal driving beam130A forms a zig-zag bellows structure as a whole. One end of the horizontal driving beam130A is connected to the inner side of the movable frame160, and another end of the horizontal driving beam130A is connected via a turnaround part131X1and the coupling beam121A to the mirror support120. The horizontal driving beam130B includes multiple rectangular horizontal beams that extend in the direction of the vertical-rotation axis AXV that is orthogonal to the horizontal-rotation axis AXH, and ends of adjacent horizontal beams are connected to each other via turnaround parts131Y2,131Y3, and131Y4such that the horizontal driving beam130B forms a zig-zag bellows structure as a whole. One end of the horizontal driving beam130B is connected to the inner side of the movable frame160, and another end of the horizontal driving beam130B is connected via a turnaround part131Y1and the coupling beam121B to the mirror support120.

The vertical driving beams170A and170B connected to the movable frame160are disposed on the corresponding sides of the movable frame160. The vertical driving beam170A includes multiple rectangular vertical beams173X1and173X2that extend in the direction of the horizontal-rotation axis AXH, and ends of adjacent vertical beams are connected to each other via a turnaround part171X such that the vertical driving beam170A forms a zig-zag bellows structure as a whole. One end of the vertical driving beam170A is connected to the inner side of the fixed frame180, and another end of the vertical driving beam170A is connected to the outer side of the movable frame160. The vertical driving beam170B includes multiple rectangular vertical beams173Y1and173Y2that extend in the direction of the horizontal-rotation axis AXH, and ends of adjacent vertical beams are connected to each other via a turnaround part171Y such that the vertical driving beam170B forms a zig-zag bellows structure as a whole. One end of the vertical driving beam170B is connected to the inner side of the fixed frame180, and another end of the vertical driving beam170B is connected to the outer side of the movable frame160.

The horizontal driving beams130A and130B include, respectively, horizontal driving sources131A and131B that are piezoelectric elements. Also, the vertical driving beams170A and170B include, respectively, vertical driving sources171A and171B that are piezoelectric elements. The horizontal driving beams130A and130B and the vertical driving beams170A and170B function as an actuator that causes the mirror110to rotate (or oscillate) vertically and horizontally to scan a laser beam.

On the upper surfaces of the horizontal driving beams130A and130B, the horizontal driving sources131A and131B are formed for respective horizontal beams that are rectangular units including no curved section. Each of the horizontal driving sources131A is a piezoelectric element formed on the upper surface of the horizontal driving beam130A and includes a piezoelectric thin film, an upper electrode formed on the upper surface of the piezoelectric thin film, and a lower electrode formed on the lower surface of the piezoelectric thin film. Each of the horizontal driving sources131B is a piezoelectric element formed on the upper surface of the horizontal driving beam130B and includes a piezoelectric thin film, an upper electrode formed on the upper surface of the piezoelectric thin film, and a lower electrode formed on the lower surface of the piezoelectric thin film.

In each of the horizontal driving beams130A and130B, a driving voltage with a first waveform and a driving voltage with a second waveform obtained by vertically inverting the first waveform with reference to the median value of the first waveform are applied to horizontal driving sources131A/131B on adjacent horizontal beams to cause the adjacent horizontal beams to warp in opposite vertical directions, and the accumulation of the vertical movement of the horizontal beams is transmitted to the mirror support120. With the movement of the horizontal driving beams130A and130B, the mirror110and the mirror support120rotate (or oscillate) around the horizontal-rotation axis AXH. The direction of this rotation (or oscillation) is referred to as a horizontal direction, and the axis of this rotation (or oscillation) that passes through the center of the light reflection surface of the mirror110is referred to as the horizontal-rotation axis AXH. For example, nonresonant oscillation may be used for the horizontal driving by the horizontal driving beams130A and130B.

For example, the horizontal driving source131A includes horizontal driving sources131A1,131A2,131A3, and131A4that are formed on the first through fourth horizontal beams constituting the horizontal driving beam130A. The horizontal driving source131B includes horizontal driving sources131B1,131B2,131B3, and131B4that are formed on the first through fourth horizontal beams constituting the horizontal driving beam130B. In this case, the mirror110and the mirror support120can be rotated in the horizontal direction by driving the horizontal driving sources131A1,131B1,131A3, and131B3using the first waveform and driving the horizontal driving sources131A2,131B2,131A4, and131B4using the second waveform that is obtained by vertically inverting the first waveform with reference to the median value of the first waveform.

On the upper surfaces of the vertical driving beams170A and170B, the vertical driving sources171A and171B are formed for respective vertical beams173X1,173X2,173Y1, and173Y2that are rectangular units including no curved section. Each vertical driving source171A is a piezoelectric element formed on the upper surface of the vertical driving beam170A and includes a piezoelectric thin film, an upper electrode formed on the upper surface of the piezoelectric thin film, and a lower electrode formed on the lower surface of the piezoelectric thin film. Each vertical driving source171B is a piezoelectric element formed on the upper surface of the vertical driving beam170B and includes a piezoelectric thin film, an upper electrode formed on the upper surface of the piezoelectric thin film, and a lower electrode formed on the lower surface of the piezoelectric thin film.

In each of the vertical driving beams170A and170B, a driving voltage with a first waveform and a driving voltage with a second waveform obtained by vertically inverting the first waveform with reference to the median value of the first waveform are applied to vertical driving sources171A/171B on adjacent vertical beams173X1-173X2or173Y1-173Y2to cause the adjacent vertical beams to warp in opposite vertical directions, and the accumulation of the vertical movement of the vertical beams is transmitted to the movable frame160. With the movement of the vertical driving beams170A and170B, the mirror110connected to the movable frame160rotates (or oscillates) around a rotation axis that is orthogonal to the horizontal-rotation axis AXH. The direction of this rotation (or oscillation) is referred to as a vertical direction, and this rotation axis that passes through the center of the light reflection surface of the mirror110is referred to as a vertical-rotation axis AXV. For example, nonresonant oscillation may be used for the vertical driving by the vertical driving beams170A and170B.

For example, the vertical driving source171A includes vertical driving sources171A1and171A2that are formed on the first and second vertical beams173X1and173X2constituting the vertical driving beam170A. The vertical driving source171B includes vertical driving sources171B1and171B2that are formed on the first and second vertical beams173Y1and173Y2constituting the vertical driving beam170B. In this case, the movable frame160connected to the mirror110can be rotated in the vertical direction by driving the vertical driving sources171A1and171B1using the first waveform and driving the vertical driving sources171A2and171B2using the second waveform that is obtained by vertically inverting the first waveform with reference to the median value of the first waveform.

In the optical scanning device of the present embodiment, the MEMS structure implementing the actuator is formed of a silicon-on-insulator (SOI) substrate including a support layer, a buried oxide (BOX) layer, and an active layer. Each of the fixed frame180and the movable frame160is comprised of the support layer, the BOX layer, and the active layer. On the other hand, each of components such as the horizontal driving beams130A and130B and the vertical driving beams170A and170B other than the fixed frame180and the movable frame160may be comprised of the active layer alone (one layer) or comprised of the BOX layer and the active layer (two layers).

In the optical scanning device of the present embodiment, the horizontal driving sources131A and131B are formed on first surfaces (upper surfaces) of the horizontal beams constituting the horizontal driving beams130A and130B, and ribs132are formed on second surfaces (lower surfaces) of the horizontal beams. The ribs132are positioned in the middle of the horizontal beams, i.e., along the horizontal-rotation axis AXH. Each rib132has a width in the longitudinal direction of the horizontal beam and a length in the lateral direction of the horizontal beam, and the width of the rib132is shorter than the length of the rib132. For example, when a wafer including multiple MEMS structures is diced to manufacture separate MEMS structures, the ribs132formed on the second surfaces (lower surfaces) of the horizontal beams constituting the horizontal driving beams130A and130B can prevent the bellows structures from being excessively vibrated and damaged by a water flow and vibration generated during the dicing.

Also in the optical scanning device of the present embodiment, the vertical driving sources171A and171B are formed on first surfaces (upper surfaces) of the vertical beams173X1,173X2,173Y1, and173Y2constituting the vertical driving beams170A and170B, and ribs172are formed on second surfaces (lower surfaces) of the vertical beams173X1,173X2,173Y1, and173Y2. Each of the ribs172is positioned such that a distance from the joint between the corresponding one of the vertical beams173X1,173X2,173Y1, and173Y2and the corresponding one of the turnaround parts171X and171Y becomes 10 to 20% of the length of the vertical beams173X1,173X2,173Y1, and173Y2. Each of the ribs172has a width in the longitudinal direction of the vertical beams173X1,173X2,173Y1, and173Y2and a length in the lateral direction of the vertical beams173X1,173X2,173Y1, and173Y2, and the width of the ribs172is shorter than the length of the ribs172. The ribs172formed on the second surfaces (lower surfaces) of the vertical beams constituting the vertical driving beams170A and170B can prevent the vertical driving beams170A and170B from unnecessarily warping in a direction (the width or lateral direction of the vertical beams173X1,173X2,173Y1, and173Y2) that is orthogonal to the direction of vertical warping of the vertical driving beams170A and170B and thereby reduce the displacement of the mirror support120in the thickness direction.

In the optical scanning device of the present embodiment, a rib is also formed on the lower surface of the mirror support120that is opposite a surface of the mirror support120on which the mirror110is formed. The rib formed on the lower surface of the mirror support120can prevent the mirror support120from unnecessarily warping.

The ribs172formed on the second surfaces (lower surfaces) of the vertical beams173X1,173X2,173Y1, and173Y2constituting the vertical driving beams170A and170B have a height that is the same as the height (thickness) of the fixed frame180and the movable frame160. When the MEMS structure that functions as an actuator of the optical scanning device is formed of an SOI substrate, the ribs172are formed of the BOX layer and the support layer on the lower surfaces of the vertical beams173X1,173X2,173Y1, and173Y2formed of the active layer. The horizontal beams constituting the horizontal driving beams130A and130B are formed of the active layer, and the ribs132formed on the second surfaces (lower surfaces) of the horizontal beams are formed of the BOX layer and the support layer. The mirror support120is formed of the active layer, and the rib on the lower surface of the mirror support120(the surface that is opposite a surface of the mirror support120on which the mirror110is formed) is formed of the BOX layer and the support layer. Instead of using the support layer of an SOI substrate, the ribs may be formed as steps by etching bulk silicon.

FIG. 3is an enlarged view of a portion of the optical scanner100. The vertical driving sources171B (171B1and171B2), which are piezoelectric elements, are formed on first surfaces (upper surfaces) of the vertical beams173Y1and173Y2constituting the vertical driving beam170B, and the ribs172are formed on second surfaces (lower surfaces) of the vertical beams173Y1and173Y2at positions indicated by dotted lines. Also, in the optical scanner100of the optical scanning device of the present embodiment, notches Z are formed in the vertical driving sources171B1and171B2at positions corresponding to the ends of the ribs172. The notches Z have a semicircular shape in plan view of the vertical beams173Y1and173Y2and are formed by removing portions of the vertical driving sources171B1and171B2on the first surfaces of the vertical beams173Y1and173Y2.

Although the notches Z are described using the vertical driving beam170B inFIG. 3, the vertical driving beam170A has substantially the same configuration.

In the vertical driving beams170A and170B, when voltages are applied to the vertical driving sources171A and171B, the vertical beams173X1,173X2,173Y1, and173Y2warp in the longitudinal direction and also warp slightly in the lateral direction. The warp in the longitudinal is necessary to rotate the mirror support120. On the other hand, the warp in the lateral direction causes the mirror support120to be displaced in the thickness direction in synchronization with the rotating movement.

In the optical scanning device of the present embodiment, the ribs172are formed on the second surfaces (lower surfaces) of the vertical beams173X1,173X2,173Y1, and173Y2constituting the vertical driving beams170A and170B, and the ribs172have a shape that is short in the longitudinal direction of the vertical beams173X1,173X2,173Y1, and173Y2and is long in the lateral direction of the vertical beams173X1,173X2,173Y1, and173Y2. The ribs172can prevent the vertical beams173X1,173X2,173Y1, and173Y2from warping in the lateral direction and reduce the displacement of the mirror support120in the thickness direction.

The optical scanner100of the present embodiment where the warp of the vertical beams173X1,173X2,173Y1, and173Y2in the lateral direction is prevented and the displacement of the mirror support120in the thickness direction is reduced can be used for an apparatus such as an OCT apparatus or an FTIR that uses interference of light for measurement.

For example, in the vertical beam173Y2located first from the outer end, a width WB2of the vertical beam173Y2is 1.83 mm, a length LRof the rib172is 1.79 mm, and the width of the rib172is 0.1 mm. In the vertical beam173Y1located second from the outer end, a width WB1of the vertical beam173Y1is 1.80 mm, a length LRof the rib172is 1.76 mm, and the width of the rib172is 0.1 mm.

The length of each of the ribs172in the lateral direction (or the width direction) of the vertical beams173X1,173X2,173Y1, and173Y2is preferably greater than or equal to 70% of the width of the corresponding one of the vertical beams173X1,173X2,173Y1, and173Y2. With this configuration, the ribs172can sufficiently reduce the displacement of the mirror support120in the thickness direction.

The distance of each of the ribs172from the joint between the corresponding one of the vertical beams173X1,173X2,173Y1, and173Y2and the corresponding one of the turnaround parts171X and171Y is preferably greater than or equal to 10% and less than or equal to 20% of the length of the corresponding one of the vertical beams173X1,173X2,173Y1, and173Y2in the longitudinal direction. This configuration makes it possible to reduce the displacement of the mirror support120in the thickness direction and prevent a decrease in the tilt angle sensitivity of the mirror110(the tilt angle of the mirror110per one voltage applied to the piezoelectric element).

When the ribs172are provided to reduce the displacement of the mirror support120in the thickness direction, the ribs172suppress the warp of the vertical beams173X1,173X2,173Y1, and173Y2in the lateral direction and as a result, a corresponding stress is applied to a joint at an end of each rib172(i.e., an end portion of the rib172that is joined to the vertical beam). When the stress exceeds a critical stress, a crack develops quickly and the rib172may be broken. In the present embodiment, the notches Z are formed in the vertical driving sources171A and171B at positions corresponding to the ends of the ribs172. With this configuration, the width of portions of the vertical driving sources171A and171B directly above the ribs172is reduced and the warp of the vertical beams173X1,173X2,173Y1, and173Y2in the lateral direction is decreased. As a result, the stress caused by the warp is reduced. Also, a stress transmitted from other portions is also dispersed by a silicon active layer between the other portions and the ribs172. Thus, the notches Z can reduce the stress applied to the joints at the ends of the ribs172.

For example, a width Wcoof the notches Z is 0.4 mm, and a depth Dcoof the notches Z is 0.16 mm.

As described above, in the optical scanner100of the optical scanning device of the present embodiment, the ribs172are formed on the second surfaces (lower surfaces) of the vertical beams constituting the vertical driving beams170A and170B. The ribs172can prevent the vertical driving beams170A and170B from unnecessarily warping in a direction (the width or lateral direction of the vertical beams173X1,173X2,173Y1, and173Y2) that is orthogonal to the direction of vertical warping of the vertical driving beams170A and170B and thereby reduce the displacement of the mirror support120in the thickness direction. Also, the notches Z are formed in the vertical driving sources171A and171B at positions corresponding to the ends of the ribs172to reduce the stress applied to the joints at the ends of the ribs172. Also, the ribs172are disposed such that the distance of each of the ribs172from the joint between the corresponding one of the vertical beams173X1,173X2,173Y1, and173Y2and the corresponding one of the turnaround parts171X and171Y becomes greater than or equal to 10% and less than or equal to 20% of the length of the corresponding one of the vertical beams173X1,173X2,173Y1, and173Y2in the longitudinal direction. This configuration makes it possible to reduce the displacement of the mirror support120in the thickness direction, and prevent a decrease in the tilt angle sensitivity of the mirror110(the tilt angle of the mirror110per one voltage applied to the piezoelectric element).

FIG. 4is a graph indicating a relationship between the displacement of the mirror support120in the thickness direction and a rib length.FIG. 4indicates results of a simulation where values of the displacement of the mirror support120in the thickness direction when the mirror support120is tilted ±1 degree are calculated by changing the percentage of the length of the ribs172relative to the width of the vertical beams173X1,173X2,173Y1, and173Y2(rib length/beam width). InFIG. 4, the vertical axis indicates “displacement in thickness direction”, and the horizontal axis indicates “rib length/beam width”. Here, when “rib length/beam width” is zero, it indicates that the ribs172are not provided. When the ribs172are not provided, the displacement in the thickness direction is ±2.6 μm. When the length of the ribs172is substantially the same as the beam width, the displacement in the thickness direction is ±0.03 μm. Thus, the simulation results indicate that the ribs172can reduce the displacement of the mirror support120in the thickness direction. The effect of reducing the displacement in the thickness direction increases as the rib length increases. To obtain a sufficient effect, the length of the ribs172is preferably greater than or equal to 70% of the beam width.

FIG. 5is a graph indicating a relationship between the displacement of the mirror support120in the thickness direction and a rib position.FIG. 5indicates results of a simulation where values of the displacement of the mirror support120in the thickness direction when the mirror support120is tilted ±1 degree are calculated by changing the position of the ribs172. InFIG. 5, the vertical axis indicates “displacement in thickness direction”, and the horizontal axis indicates “distance from turnaround part/beam length”. The “distance from turnaround part” indicates the distance between the ribs172and the joints between the vertical beams173X1,173X2,173Y1, and173Y2and the turnaround parts171X and171Y The effect of reducing the displacement in the thickness direction increases as the position of the ribs172becomes closer to the turnaround parts171X and171Y. However, if the “distance from turnaround part/beam length” becomes less than 10%, the tilt angle sensitivity of the mirror110(the tilt angle of the mirror110per one voltage applied to the piezoelectric element) decreases. Accordingly, the “distance from turnaround part/beam length” is preferably greater than or equal to 10% and less than or equal to 20%.

FIG. 6Ais a drawing illustrating the posture of the vertical driving beam170B when the optical scanner100is driven.FIG. 6Bis a drawing illustrating a stress generated when the optical scanner100is driven as inFIG. 6A. As illustrated inFIG. 6A, the rib172can reduce the warp of a vertical beam of the vertical driving beam170B when the optical scanner100is driven vertically. In this state, a stress in the separation direction is applied to an end of the rib172. Particularly, when the optical scanner100is formed of an SOI substrate, a crack may be formed in the BOX layer whose strength is lower than the silicon active layer, and the vertical driving beam170B or the rib172may be damaged.

FIG. 6Bis an enlarged view of an area X inFIG. 6A. InFIG. 6B, a shade with higher density indicates a higher stress. When the notches Z are not formed in the vertical driving sources171A and171B, a high stress is applied to the joint at the end of the rib172. In this case, the maximum stress applied to the rib172in the separation direction when the mirror110is tilted one degree is 7.8 MPa. When the notches Z are formed in the vertical driving sources171A and171B as in the present embodiment, the width of the portions of the vertical driving sources171A and171B directly above the ribs172decreases. As a result, the warp in the lateral direction decreases and the stress is reduced. Also, the stress caused by the warp in the lateral direction in other portions of the vertical driving sources171A and171B is dispersed by the silicon active layer between the ribs172and the vertical driving sources171A and171B. Thus, the notches Z can reduce the stress applied to the joint at the end of the rib172. In this case, the maximum stress applied to the rib172in the separation direction when the mirror110is tilted one degree is 5.8 MPa. Thus, the stress is reduced by about 30%.

FIG. 7is a graph indicating a relationship between a maximum rib stress and a notch depth.FIG. 7indicates results of a simulation where values of the maximum stress (maximum rib stress) applied to the joint at the end of the rib172when the mirror support120is tilted ±1 degree are calculated by changing the percentage of a depth Dcoof the notch Z relative to the length of the rib172(notch depth/rib length). InFIG. 7, the vertical axis indicates “maximum rib stress”, and the horizontal axis indicates “notch depth/rib length”. As illustrated inFIG. 3, the depth Dcoof the notch Z (notch depth) is represented by a distance between an end of the rib172and a point on the contour of the notch Z that is farthest from the end of the rib172in the longitudinal direction of the rib172. Thus, the notch depth corresponds to the size of the notch Z in the longitudinal direction of the rib172. The stress applied to the rib172can be reduced by making the notch depth greater than or equal to 14% of the rib length. The effect of reducing the stress does not greatly increase even when the “notch depth/rib length” is made greater than 14%. On the other hand, as the notch depth increases, the area of the vertical driving sources171A and171B decreases, and the tilt angle sensitivity of the mirror110decreases. Accordingly, the notch depth is preferably less than or equal to 14% of the rib length and is more preferably about 14% of the rib length.

FIG. 8is a graph indicating a relationship between a maximum rib stress and a notch width.FIG. 8indicates results of a simulation where values of the maximum stress (maximum rib stress) applied to the joint at the end of the rib172when the mirror support120is tilted ±1 degree are calculated by changing the difference between a width Wcoof the notch Z and the width of the rib172(notch width−rib width). InFIG. 8, the vertical axis indicates “maximum rib stress”, and the horizontal axis indicates “notch width−rib width”. The maximum rib stress decreases as the width of the notch Z increases while the difference between the width of the notch Z and the width of the rib172is less than 0.2 mm. However, when the difference becomes greater than or equal to 0.2 mm, the maximum rib stress does not substantially change even if the width of the notch Z is increased further. On the other hand, as the notch width increases, the area of the vertical driving sources171A and171B decreases, and the tilt angle sensitivity of the mirror110decreases. Accordingly, the “notch width−rib width” is preferably less than or equal to 0.2 mm and is more preferably about 0.2 mm.

An actuator and an optical scanning device according to embodiments of the present invention are described above. However, the present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. For example, although an actuator is used for an optical scanning device including a mirror in the above embodiments, the actuator may also be used to drive an object other than a mirror, and the present invention may also be applied to a device that does not include a mirror. An optical scanning device according to an embodiment of the present invention is preferably used for an optical coherence tomography device of a funduscope. In an optical coherence tomography device of a funduscope, resonant driving is not necessary because one of the axes operates at high speed as in a projector, and it is desired that tilt angles can be freely set and adjusted to perform optical scanning. Accordingly, an optical scanner where nonresonant driving is used for both of two axes as in the above embodiments is preferably used for an optical coherence tomography device of a funduscope. An optical scanning device according to an embodiment of the present invention may also be used for a projection device. In the above embodiment, ribs are formed on the lower surfaces of vertical beams constituting a vertical driving beam, and notches are formed in vertical driving sources at positions corresponding to the ends of the ribs. However, the present invention is not limited to this embodiment. Even when notches are not formed in the vertical driving sources, the ribs are preferably positioned such that the “distance from turnaround part/beam length” becomes greater than or equal to 10% and less than or equal to 20%. This configuration makes it possible to reduce the displacement of a mirror support in the thickness direction and prevent a decrease in the tilt angle sensitivity of a mirror (the tilt angle of the mirror per one voltage applied to the piezoelectric element).

An aspect of this disclosure provides an optical scanning device including an actuator whose displacement in the thickness direction is prevented so that the optical scanning device can be used even for a measurement apparatus that uses interference of light.