Patent ID: 12189115

DESCRIPTION OF EMBODIMENT

A preferred embodiment of the present invention will be described below. It is needless to say that the present invention is not limited to the embodiment below. The present invention can be carried out in various modes within the scope of technical concepts disclosed in this specification. Note that elements common to respective examples are given the same reference numerals.

The preferred embodiment of the present invention will be described in detail below. In the following description, a common reference numeral is used for substantially identical or equivalent elements and parts. Further, as for elements in a pair or a group having the same structure, reference numerals having the same number but different only in suffix alphabet letter are used. Further, when the elements that constitute the pair or the group are collectively called, such a reference numeral that omits each suffix letter is used.

(Optical Deflector (Whole))

FIG.1is a front view of an optical deflector10. The optical deflector10is manufactured from a SOI (Silicon on Insulator) as MEMS (Micro Electro Mechanical Systems). The optical deflector10includes, as main elements, a mirror part11, torsion bars12a,12b, inside piezoelectric actuators13a,13b, a movable frame part14, outside piezoelectric actuators15a,15b, and a fixed frame part16.

A three axis coordinate system consisting of X axis, Y axis, and Z axis is defined for convenience of describing the structure of the optical deflector10. The X axis and the Y axis are defined as directions respectively parallel to the extension directions of the long side and the short side of the fixed frame part16that is a rectangle in front view. The Z axis is defined as a direction parallel to a thickness direction of the optical deflector10.

O is the center of the mirror part11. The center O is also the center of the optical deflector10. Laser light or the like from an unillustrated light source is incident on the center O of the mirror part11.

A rotation axis Ax and a rotation axis Ay as two rotation axes of the mirror part11are orthogonal to each other at the center O of the mirror part11. When the mirror part11faces straight ahead, the rotation axis Ax and the rotation axis Ay are parallel to the X axis and the Y axis, respectively.

The mirror part11is located in the center of the optical deflector10. The torsion bars12a,12bare extending from both sides of the mirror part11along the rotation axis Ay in the Y direction.

The inside piezoelectric actuators13a,13bare arranged on both sides of the mirror part11in the X axis direction, and each end thereof in the Y direction is joined to an intermediate portion of each of the torsion bars12a,12b, respectively. The inside piezoelectric actuators13a,13bare joined to each other at both ends in the Y axis direction to form an elliptical ring long in the Y axis direction as the entire pair so as to surround the mirror part11.

The movable frame part14has an elliptical ring shape vertically long in the Y axis direction to surround the elliptical ring of the inside piezoelectric actuators13a,13b. Each torsion bar12is joined to the inner circumference of the movable frame part14at an end opposite to the side of the mirror part11. Each torsion bar12is joined to the inner circumference of the movable frame part14on the outer circumference of the intermediate portion thereof in the Y axis direction.

The outside piezoelectric actuators15a,15bare arranged on both sides of the mirror part11in the X axis direction. Each outside piezoelectric actuator15is joined to the outer circumference of the movable frame part14and the inner periphery of the fixed frame part16in both end portions, respectively.

When the mirror part11faces straight ahead, a joint portion between the inside piezoelectric actuator13and the movable frame part14, a joint portion between the movable frame part14and the outside piezoelectric actuator15, and a joint portion between the outside piezoelectric actuator15and the fixed frame part16are lined up on a straight line passing through the center O and parallel to the X axis. The inside piezoelectric actuator13has an arc-shaped recess on the inner circumferential side on the straight line to increase the diameter of the round mirror part11.

Each outside piezoelectric actuator15has plural piezoelectric cantilevers21coupled with each other in a meander pattern. The closer each piezoelectric cantilever21to the fixed frame part16, the wider the width thereof becomes. Thus, the closer the piezoelectric cantilever21to the fixed frame part16, the stronger the deformation force thereof, whereas the closer the piezoelectric cantilever21to the movable frame part14, the higher the flexibility thereof becomes.

The inside piezoelectric actuators13and the outside piezoelectric actuators15receive drive voltages from an unillustrated drive device, respectively. The inside piezoelectric actuators13are interposed between the torsion bars12and the movable frame part14to torsionally vibrate the torsion bars12about a rotation axis Ay at a resonance frequency. This causes the mirror part11to turn reciprocally about the rotation axis Ay at a resonance frequency Fy.

It is assumed that the plural piezoelectric cantilevers21equipped in each outside piezoelectric actuator15are numbered, such as No. 1, No. 2, . . . , in order from the side of the fixed frame part16to the side of the movable frame part14. In this case, odd-numbered piezoelectric cantilevers21and even-numbered piezoelectric cantilevers21receive, from the unillustrated drive device, drive voltages with same peak-to-peak but opposite in phase. As a result, the odd-numbered piezoelectric cantilevers21and the even-numbered piezoelectric cantilevers21are curved in phases opposite to each other.

Thus, the curve of each piezoelectric cantilever21is added to the outside piezoelectric actuator15to increase the amount or rotation of a leading end portion of the outside piezoelectric actuator15on the side of the movable frame part14relative to the base end portion thereof on the side of the fixed frame part16. As a result, the outside piezoelectric actuator15causes the movable frame part14to turn reciprocally about a rotation axis parallel to the X axis by the increased amount of rotation on the leading end side at a non-resonant frequency Fx. This causes the mirror part11to turn reciprocally about the rotation axis Ax at the non-resonance frequency Fx. Note that non-resonant frequency Fx<resonance frequency Fy.

(Substrate Layer)

FIG.2Aillustrates a relationship among SOI film thickness, resonance frequency in horizontal axial direction, and swing angle in vertical axial direction. InFIG.2AtoFIG.2C, the SOI film thickness means the thickness of an active layer34(FIG.6and the like) as a substrate layer of the optical deflector10. The resonance in the horizontal axial direction (X axis direction) means reciprocal turning of the mirror part11about the rotation axis Ay. The swing angle in the vertical direction (Y axis direction) means a swing angle of the mirror part11about the rotation axis Ax.

It can be understood fromFIG.2Athat the resonance frequency in the horizontal axial direction and the swing angle in the vertical axial direction have a trade-off relation upon setting the SOI film thickness.

FIG.2Bis an experimental graph of examining a relationship between SOI film thickness and resonance frequency in horizontal axial direction. It can be understood fromFIG.2Bthat the resonance frequency in the horizontal axial direction increases as the SOI film thickness increases.

FIG.2Cis an experimental graph of examining the drive voltage of the outside piezoelectric actuators15and mechanical half angle by setting the SOI film thickness as a parameter. The mechanical half angle is a swing angle to each side with respect to the front of the mirror part11without any damage such as to cut off the torsion bars12when each drive voltage is supplied to the mirror part11to cause reciprocal turning about the rotation axis Ax.

It can be understood fromFIG.2Cthat the mechanical half angle increases as the drive voltage increases or the SOI film thickness decreases.

(Abnormal Vibration)

FIGS.3A to3Dcontains stress distribution maps of examining various abnormal vibrations of the mirror part11by simulation. InFIGS.3A to3D, the whither the area, the higher the stress.

InFIG.3A, an abnormality occurs in a mode in which the inside piezoelectric actuators13torsionally vibrate the torsion bars12.FIG.3Billustrates an enlarged area near the mirror part11at the time of the abnormality in the torsional vibration mode.

At the time of the abnormality in the torsional vibration mode, the influence on the movable frame part14received from the resonance vibration increases as the resonance frequency Fy increases. As a result, the movable frame part14cannot stand it and hence causes abnormal vibration.

As measures against the abnormal vibration as illustrated inFIG.3AandFIG.3B, increasing the rigidity of the movable frame part14or increasing a moment ratio to be described with reference toFIG.4can be considered.

FIG.3CandFIG.3Dillustrate abnormal vibrations of the outside piezoelectric actuators15. InFIG.3C, the outside piezoelectric actuators15a,15bare translationally moved to the +side of the Y axis direction and are colliding with the fixed frame part16. In (d), the piezoelectric cantilevers21of each outside piezoelectric actuator15are inclined in front view.

One of the causes of the abnormal vibrations as inFIG.3CandFIG.3Dis considered that, when the piezoelectric cantilevers21are made thinner to increase the swing angle about the rotation axis Ax, the rigidity of the piezoelectric cantilevers21is reduced, and hence it becomes difficult to transmit, to the movable frame part14, the reciprocal turning force about the rotation axis parallel to the rotation axis Ax by the outside piezoelectric actuators15.

As measures against the abnormal vibrations inFIG.3CandFIG.3D, reducing the weight of the movable elements on the inner circumferential side of the movable frame part14(that is, the mirror part11, the torsion bars12, and the inside piezoelectric actuators13) can be considered.

(Inertia Moment Ratio)

FIG.4is a graph of examining a relationship between the ratio of an inertia moment Iiof the movable frame part14to an inertia moment IMof the mirror part11and the twist angle of the movable frame part14about the rotation axis Ay. The inertia moments IMand Iiof the mirror part11and the movable frame part14are expressed respectively in Equations (1) and (2) below.
IM=mMr2M(1):
Ii=mir2i(2):

In the above equations, subscripts M and i denote the mirror part11(Mirror) and the movable frame part14(inner frame), respectively. m demotes mass. r is distance from the rotation axis Ay.

The inertia moment ratio on the horizontal axis ofFIG.4indicates Ii/IM. On the other hand, since a reaction caused by the reciprocal turning of the mirror part11about the rotation axis Ay occurs in the movable frame part14, the movable frame part14reciprocally turns about the rotation axis Ay in a direction opposite to the mirror part11. The twist angle on the vertical axis indicates the difference ratio of the turning angle of the movable frame part14to the turning angle of the mirror part11at this time. Twist angle=0 means that the same twist angle as the time when there is no reaction is maintained.

It is found fromFIG.4that the twist angle of the movable frame part14approaches 0 as the inertia moment ratio Ii/IMincreases. The abnormal vibration in the torsional vibration mode inFIG.3AandFIG.3Bincreases as the twist angle becomes larger.

(Manufacturing Method)

FIG.5is a diagram illustrating a manufacturing method for the optical deflector10. The manufacturing method will be described in the order of processes.

In STEP1, an SOI wafer30is prepared. The SOI wafer30has a SiO2(silicon dioxide) layer31, a support layer32, a SiO2layer33, the active layer34, and a SiO2layer35in order from bottom (back side) to top (surface) in a lamination direction (=thickness direction). The support layer32and the active layer34are Si layers made of Si (silicon).

Further, in STEP, non-etching regions40and etching regions41are set on the surface of the SOI wafer30. The non-etching regions40include at least forming regions of the movable frame part14. The etching regions41include at least forming regions of the outside piezoelectric actuators15.

In STEP2, etching is carried out on the etching regions41from the surface side of the SOI wafer30. Thus, multiple cavities43are formed on the surface of the SOI wafer30. Each of the cavities43has a bottom formed into a flat plane with an equal depth among the cavities43, and opens to the side of the SiO2layer35.

In STEP3, the SOI wafer30is exposed to a high-temperature atmosphere for a predetermined time to form a SiO2layer45on the exposed surface of the cavities43. As a result, the surface side of the SOI wafer30is covered with the SiO2layer35in the non-etching regions40, and covered with the SiO2layer45on the bottom and sides of each of the cavities43. Thus, consecutive SiO2layers on the surface of the SOI wafer30are covered with the SiO2layer35and the SiO2layer45.

In STEP4, the SOI wafer30is turned upside down and an SOI wafer50is prepared. As a result of turning the SOI wafer30upside down, the SiO2layer31becomes the surface and the SiO2layer35becomes the back side. The SOI wafer50has a SiO2layer51and a support layer52. Further, the SiO2layer35of the SOI wafer30and the support layer52of the SOI wafer50are aligned.

The SOI wafer50may also be manufactured by removing a Box layer and an active layer from an SOI wafer including a support layer and the active layer as Si layers on both sides of a SiO2layer as the BOX layer. In this case, the remaining support layer and a SiO2layer below the support layer become the support layer52and the SiO2layer51, respectively.

In STEP5, the SOI wafer30and the SOI wafer50are joined under a high-temperature atmosphere in the state where the SiO2layer35of the SOI wafer30and the support layer52of the SOI wafer50are aligned in STEP4. Thus, an SOI wafer56is manufactured.

Gas used for the high-temperature atmosphere in STEP5is, for example, (a) hydrogen, (b) Ar (argon), or (c) a mixed gas of hydrogen and Ar. Further, a high-temperature atmosphere holding temperature is not lower than 900° C. and not higher than 1200° C. After the SOI wafer30and the SOI wafer50are joined, the SiO2layer31and the support layer32on the surface side of the joined body are removed to complete the SOI wafer56.

The cavities43are enclosed inside the SOI wafer56. In the SOI wafer56, the surface is the SiO2layer33and the back side is the SiO2layer51. In a laminated structure of the SOI wafer56, the SiO2layer51, the support layer52, the SiO2layer35, the active layer34, and the SiO2layer33are lined up in order from the back side to the surface side in the thickness direction.

In STEP6, upper structural layers58of piezoelectric actuators are formed on the surface of the SiO2layer33as the surface of the SOI wafer56in STEP5. The upper structural layers58include at least a lower electrode layer58a, a PZT(Pb(Zr·Ti)O3layer58b, and an upper electrode layer58cin order upward from the surface of the SiO2layer33.

The bottom of each of the cavities43is formed by etching, whereas the SiO2layer33is included in the SOI wafer30from the beginning. Therefore, unprocessed upper structural layers58are formed on the surface of the SiO2layer33which is flat with almost no unevenness, that is, which can ignore the surface roughness. As a result, each layer included in the upper structural layers58has a uniform film thickness over the entire surface of the SiO2layer33.

In STEP7, the SiO2layer35is used as a stop layer for the SOI wafer56to carry out etching from the side of the upper structural layers58at a depth that reaches the surface of the SiO2layer35. Thus, movable elements of the optical deflector10(where elements on the inner circumferential side of the fixed frame part16are all movable elements) are manufactured. However, since the SiO2layer35is still remaining, the movable elements are combined by the SiO2layer35and the displacement thereof is constrained.

In a process (not illustrated) on the way from STEP7to STEP8, electrode pads69(FIG.6), and an interlayer film80, a wiring layer81, and an insulating layer82(FIG.7) are completed. Thus, the upper structure of the optical deflector10is all completed before next STEP8.

In STEP8, a wax layer61is formed on the surface of the SOI wafer56to make the movable elements buried. Further, a plate62is fixed to the surface of the wax layer61. The wax layer61and the plate62fix the movable elements when etching the back side of the SOI wafer56in order to protect from damage.

In STEP8, after the wax layer61and the plate62are added, a recess63is further formed by etching the SOI wafer56from the back side thereof. The recess63is formed on the inner circumferential side of the fixed frame part16as displacement-allowing space when the movable elements are displaced to the back side upon operating the optical deflector10after completion of the optical deflector10. Note that the recess63penetrates through the support layer52.

Since the depth of the recess63is deep, anisotropic etching is more preferable than isotropic etching as the etching for forming the recess63. Further, it may be either one of dry and wet etchings. This is because that the movable elements are completely buried inside the wax layer61and hence the movable elements never get wet.

(Trace)

FIG.6is a sectional view of the optical deflector10completed through the processes inFIG.5. Between STEP8ofFIG.5andFIG.6, an etching process of the SiO2layer35and the SiO2layer45exposed to the recess63and a subsequent process of removing the wax layer61are included.

In the etching process of the SiO2layer35and the SiO2layer45, the SiO2layer35and the SiO2layer45exposed to the recess63are etched off by an amount corresponding to the thickness of the SiO2layer45on the bottom of the cavities43in the thickness direction of the active layer34uniformly regardless of the location. Thus, all the surfaces of the SiO2layers with the layer surfaces parallel to the back side of the SOI wafer56among surfaces exposed to the recess63(all the exposed surfaces of the SiO2layer35and the SiO2layer45covering the bottom of the cavities43among the surfaces of the SiO2layer45) are removed.

On the other hand, the SiO2layer45on the sides of the cavities43as the SiO2layer with the layer surface parallel to the depth direction of the SOI wafer56among the surfaces exposed to the recess63has a sufficient length in the depth direction. As a result, only part of the back side of the SOI wafer56is etched off without removing all the parts, and remaining parts of the SiO2layer45(SiO2layer45at positions P1) on the sides of the cavities43remain as traces72after the etching process.

After the etching process, the movable elements mutually combined by the SiO2layer45are released. Further, the traces72becomes proof that the optical deflector10was manufactured on the SOI wafer56with the cavities43. This is because, if the optical deflector10was manufactured on an SOI wafer without cavities43, rather than on the SOI wafer56with the cavities43, the recess formed on the back side of the active layer34would not be covered with SiO2to make the active layer34as the substrate layer in the regions of the outside piezoelectric actuators15thinner.

(Examples/Common Points)

FIG.7toFIG.11are sectional views of optical deflectors10ato10eas various examples of the optical deflector10. Note that since the main part of the optical deflector10is left-right symmetric in front view, only the left-half section of a center line Ce of the optical deflector10is illustrated in these sectional views. Further, since SiO2layers such as the SiO2layers33,35and SiO2layers45,51are thin, illustration thereof is omitted. However, the SiO2layer35is illustrated without being omitted only inFIG.11. The reason will be described later.

InFIG.7toFIG.11,80denotes the interlayer film,81denotes the wiring layer,82denotes the insulating layer, and84denotes a reflective layer made of, for example, a metal film that forms a reflective surface of the mirror part11. These are all elements known in the piezoelectric optical deflector10. As for the cavities43, a cavity43positioned more inward than the movable frame part14(on the side of the center O) is indicated as a cavity43i, and a cavity43positioned more outward than the movable frame part14(on the other side of the center O) is indicated as a cavity43o. As a matter of course, the cavity43iand the cavity43obelong to the etching regions41(FIG.5/STEP1).

Note that illustration of the torsion bars12is omitted inFIG.7toFIG.11. However, the thickness of the torsion bars12in the active layer34is set equal to the thickness of the mirror part11in all examples.

The optical deflectors10ato10ehave common features below.

(a) The movable frame part14belongs to the non-etching regions40. Thus, Ii/IMas the inertia moment ratio described above increases to suppress abnormal vibrations described with reference toFIG.3AtoFIG.3D. (b) The outside piezoelectric actuators15belong to the etching regions41and have the cavities43on the back side of the active layer34. Thus, the flexibility of the piezoelectric cantilevers21increase to increase the swing angle of the mirror part11about the rotation axis Ax.

Example 1

In the optical deflector10aofFIG.7, only the forming regions of the movable frame part14among the movable elements are set in the non-etching regions40. As a result, in the active layer34, the cavity43iis formed on the back side of the mirror part11and the torsion bars12, and the cavity43ois formed on the back side of the outside piezoelectric actuators15.

In the optical deflector10a, since the mirror part11becomes lightweight, Ii/IMas the inertia moment ratio further increases to increase the effect of suppressing abnormal vibration.

Example 2

In the optical deflector10bofFIG.8, the forming regions of the inside piezoelectric actuators13and the movable frame part14among the movable elements are set in the non-etching regions40. As a result, in the active layer34, the cavity43iis formed on the back side of the mirror part11and the cavity43ois formed on the back side of the outside piezoelectric actuator15.

Thus, in the optical deflector10b, the swing angle of the optical deflector10about the rotation axis Ax increases. Further, since the mirror part11becomes lightweight, Ii/IMfurther increases to increase the effect of suppressing abnormal vibration.

Example 3

In the optical deflector10cofFIG.9, the forming regions of the mirror part11and the movable frame part14among the movable elements are set in the non-etching regions40. As a result, in the active layer34, the cavity43iis formed on the back side of the inside piezoelectric actuators13and the cavity43ois formed on the back side of the outside piezoelectric actuators15.

In the optical deflector10c, since four parts of the inside piezoelectric actuators13become lightweight, the rigidity of the movable frame part14relatively increases to increase the effect of suppressing abnormal vibrations inFIG.3CandFIG.3D.

Example 4

In the optical deflector10dofFIG.10, the forming regions of the mirror part11, the torsion bars12, and the movable frame part14among the movable elements are set in the non-etching regions40. As a result, the resonance frequency Fy about the rotation axis Ay further increases.

Example 5

In the optical deflector10eofFIG.11, ribs91and92are added to the optical deflector10cofFIG.9on the back sides of the mirror part11and the movable frame part14, respectively. The ribs91and92are joined to the back sides of the mirror part11and the movable frame part14, respectively, through the SiO2layer35. In other words, the ribs91and92are formed by being left as non-etched portions of the support layer52upon forming the recess63.

The rib91increases the rigidity of the mirror part11. As a result, the mirror surface of the mirror part11is prevented from being distorted during reciprocal turning. The rib92further increases the rigidity of the movable frame part14. Thus, the effect of preventing the abnormal vibration of the mirror part11as described with reference toFIG.3AandFIG.3Bincreases.

(Modifications and Supplements)

The SOI wafer is also a Si wafer. Therefore, a first Si wafer of the present invention includes an SOI wafer. In the embodiment, the SOI wafer56is manufactured from two SOI wafers (the SOI wafers30,50). Both or either one of the first Si wafer and a second Si wafer used to manufacture a third Si wafer of the present invention may also be of bare silicon.

A first SiO2layer, a second Si layer, and a third SiO2layer of the present invention correspond to the SiO2layer33, the active layer34, and the SiO2layer35of the embodiment. The third SiO2layer of the present invention corresponds to the SiO2layer45. A fourth SiO2layer and the second Si layer of the present invention correspond to the SiO2layer51and the support layer52, respectively.

In the embodiment, the upper structural layers58include the PZT layer58b. A piezoelectric film layer included in a laminated structure layer of piezoelectric actuators of the present invention can also be made of a material other than PZT.

A movable support part and a fixed support part of the present invention correspond to the movable frame part14and the fixed frame part16of the embodiment, respectively. The movable support part and the fixed support part of the present invention do not have to be frame shapes.

In the embodiment, the rotation axis Ay as a first rotation axis and the rotation axis Ax as a second rotation axis are orthogonal to each other. However, the first rotation axis and the second rotation axis of the present invention do not have to be orthogonal to each other as long as they intersect each other.

The inside piezoelectric actuator13and the outside piezoelectric actuators15of the embodiment correspond to first piezoelectric actuators and second piezoelectric actuators of the present invention, respectively. The first piezoelectric actuators of the present invention do not have to have a semi-elliptical ring shape, and may be straight line-shaped cantilevers. The second piezoelectric actuators of the present invention are not limited to the structure in which the plural piezoelectric cantilevers21are coupled with each other in a meander pattern.

DESCRIPTION OF REFERENCE NUMERALS

10. . . optical deflector,11. . . mirror part,12. . . torsion bar,13. . . inside piezoelectric actuator (first piezoelectric actuator),14. . . movable frame part (movable support part),15. . . outside piezoelectric actuator,16. . . fixed frame part (fixed support part),30. . . SOI wafer (first Si wafer),33. . . SiO2layer (first SiO2layer),34. . . active layer (first Si layer),35. . . SiO2layer (second SiO2layer),40. . . non-etching regions,41. . . etching regions,43. . . cavities,45. . . SiO2layer (third SiO2layer),50. . . SOI wafer (second Si wafer),51. . . SiO2layer (fourth SiO2layer),52. . . support layer (second Si layer),56. . . SOI wafer (third Si wafer),58a. . . lower electrode layer,58b. . . PZT layer,58c. . . upper electrode layer,63. . . recess,72. . . trace,91,92. . . rib.