Optical deflector including mirror with extended reinforcement rib coupled to protruded portions of torsion bar

An optical deflector includes a mirror with a reflective layer on its front-side surface, a first support frame adapted to support the mirror, at least one torsion bar coupled between the first support frame and the mirror; and a reinforcement rib provided on a rear-side surface of the mirror. The torsion bar has a pair of protruded portions arranged symmetrically with respect to the torsion bar in the vicinity of a coupling portion between the torsion bar and the mirror. The reinforcement rib has a central portion and a pair of extension portions extended symmetrically from the central portion and coupled to the protruded portions, respectively, of the torsion bar.

This application claims the priority benefit under 35 U. S. C. §119 to Japanese Patent Application No. JP2015-051862 filed on Mar. 16, 2015, which disclosure is hereby incorporated in its entirety by reference.

BACKGROUND

Field

The presently disclosed subject matter relates to an optical deflector used in an optical scanner for a projector, a laser headlamp, a bar code reader, a laser printer, a laser head amplifier, a head-up display unit and the like.

Description of the Related Art

Recently, optical deflectors used in optical scanners have been micro electro mechanical system (MEMS) devices manufactured by semiconductor manufacturing technology and micro machine technology.

A first prior art optical deflector is constructed by a mirror supported by torsion bars to a support frame. Also, provided between the support frame and the torsion bars are actuators serving as cantilevers. Thus, the mirror can be rocked around an axis by the actuators.

In the above-described first prior art optical deflector, since the thickness of the mirror is the same as that of the torsion bars, the mirror is very thin. Therefore, the moment of inertia of the mirror is so small that the resonant frequency of the mirror is very large. As a result, the mirror can be driven at a higher speed than a required speed.

In the first prior art optical deflector, however, since the mirror is very thin, the rigidity of the mirror is very small. Therefore, when the rocking angle of the mirror is large, a relatively large stress as a repulsive force would be spread isotropically and broadly into the mirror from the torsion bars (see:FIG. 5A). As a result, the entire mirror would be greatly deformed in a bowl shape, so that the dynamic face-deflection peak-to-valley amount would be very large. Thus, the optical scanning characteristics of reflected light of the mirror would not satisfy the required optical scanning characteristics in optical scanners for high definition projectors. At worst, the mirror entirely would break down. Note that the required dynamic face-deflection peak-to-valley amount is defined by one-tenth of a wavelength (λ=450 nm) of a laser beam irradiated onto the mirror.

A second prior art optical deflector is further constructed by a ring-shaped reinforcement rib provided on a rear surface of the mirror of the first prior art optical deflector. In this case, the size of the ring-shaped reinforcement rib is smaller than that of the mirror. Therefore, the substantial thickness of the mirror is larger than that of the torsion bars (see: FIG. 11 of US2014/0071512A1).

In the above-described second prior art optical deflector, due to the presence of the ring-shaped reinforcement rib, the rigidity of the mirror is larger than that of the mirror of the first prior art optical deflector. Therefore, when the rocking angle of the mirror is large, a relatively large stress as a repulsive force spread from the torsion bars into the mirror would be interrupted by the ring-shaped reinforcement rib (see:FIG. 5B). In other words, no substantial stress occurs in a central portion of the mirror within the ring-shaped reinforcement rib. As a result, the dynamic face-deflection peak-to-valley amount of the mirror is smaller than that of the first prior art optical deflector.

In the above-described second prior art optical deflector, however, the above-mentioned relatively large stress would still broadly spread into portions of the mirror between the torsion bars and the ring-shaped reinforcement rib. Therefore, the portions of the mirror between the torsion bars and the ring-shaped reinforcement rib would be distorted, so that the optical scanning characteristics of reflected light from the mirror would still deteriorate. Additionally, the ring-shaped reinforcement rib would be peeled off.

In a third prior art optical deflector, protruded portions are provided at the mirror of the second prior art optical deflector along a rocking direction in the vicinity of a coupling portion between the mirror and the torsion bars, and extension portions of the reinforcement rib are coupled to the protruded portions of the mirror (see: WO2014/122781A1).

In the above-described third prior art optical deflector, due to the presence of the extension portions of the ring-shaped reinforcement rib, the rigidity of the mirror is larger than that of the mirror of the second prior art optical deflector. Therefore, when the rocking angle of the mirror is large, a relatively large stress as a repulsive force spread from the torsion bars into the mirror would also be interrupted by the extension portions of the ring-shaped reinforcement rib (see:FIG. 5C). In other words, no substantial stress occurs in portions of the mirror beyond the extended ring-shaped reinforcement rib in addition to the central portion of the mirror. As a result, the dynamic face-deflection peak-to-valley amount of the mirror is smaller than that of the second prior art optical deflector.

In the above-described third prior art optical deflector, however, the above-mentioned relatively large stress would still spread into portions of the mirror surrounded by the extension portions of the ring-shaped reinforcement rib. Therefore, the portions of the mirror surrounded by the extension portions of the ring-shaped reinforcement rib would be distorted, so that the optical scanning characteristics of reflected light from the mirror would still deteriorate. Additionally, the ring-shaped reinforcement rib would be peeled off.

SUMMARY

The presently disclosed subject matter seeks to solve one or more of the above-described problems.

According to the presently disclosed subject matter, an optical deflector includes a mirror, a mirror with a reflective layer on its front-side surface, a first support frame adapted to support the mirror, at least one torsion bar coupled between the first support frame and the mirror, and a reinforcement rib provided on a rear-side surface of the mirror. The torsion bar has a pair of protruded portions arranged symmetrically with respect to the torsion bar in the vicinity of a coupling portion between the torsion bar and the mirror. The reinforcement rib has a central portion and a pair of extension portions extended symmetrically from the central portion and coupled to the protruded portions, respectively, of the torsion bar.

Thus, according to the presently disclosed subject matter, when the rocking angle of the mirror is large, although a relatively large stress as a repulsive force spread from the torsion bar is concentrated at a portion of the torsion bar in the vicinity of the protruded portions thereof, no substantial stress occurs in a portion of the mirror surrounded by the extension portions of the reinforcement rib. Therefore, the portion of the mirror surrounded by the extension portions reinforcement rib would not be distorted, i.e., the entire mirror would not be distorted, so that the optical scanning characteristics of reflected light from the mirror would improve. Additionally, the ring-shaped reinforcement rib would not be peeled off.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

InFIG. 1, which illustrates an embodiment of the two-dimensional optical deflector according to the presently disclosed subject matter, reference numeral10designates a two-dimensional optical deflector,20designates a laser light source, and30designates a control unit for controlling the optical deflector10and the laser light source20.

The optical deflector10is constructed by a circular mirror1with a reflective layer1aon its front surface for reflecting incident light L from the laser light source20, a pair of torsion bars2aand2bcoupled to the mirror1along an X-axis, an inner frame (movable support frame)3surrounding the mirror1and the torsion bars2aand2bfor supporting the mirror1, a semi-ring shaped inner piezoelectric actuator4acoupled between the torsion bars2aand2band supported by an inner coupling portion3aof the inner frame3, and a semi-ring shaped inner piezoelectric actuator4bcoupled between the torsion bars2aand2band supported by an inner coupling portion3bof the inner frame3. In this case, the inner frame3has a circular inner circumference along the inner piezoelectric actuators4aand4b, and a rectangular outer circumference. The flexing direction of the inner piezoelectric actuator4ais opposite to that of the inner piezoelectric actuator4b, so that the inner piezoelectric actuators4aand4bserve as cantilevers for rocking the mirror1around the X-axis.

Also, the optical deflector10includes an outer frame (fixed support frame)5and a pair of meander-type outer piezoelectric actuators6aand6bcoupled between coupling portions5aand5bof the outer frame5and outer coupling portions3cand3dof the inner frame3and serving as cantilevers for rocking the mirror1around a Y-axis on the plane of the mirror1centered at the center O of the mirror1. The outer piezoelectric actuators6aand6bare arranged opposite to each other with respect to the mirror1.

The mirror1can be square, rectangular, polygonal or elliptical. In this case, the inner-circumference of the inner frame3is adapted to the shape of the mirror1.

Referring toFIGS. 2A and 2B, in addition toFIG. 1, protruded portions2a-1and2a-2are provided symmetrically at both sides of the torsion bar2ain the vicinity of a coupling portion between the mirror1and the torsion bar2a, and protruded portions2b-1and2b-2are provided symmetrically at both sides of the torsion bar2bin the vicinity of a coupling portion between the mirror1and the torsion bar2b. Also, provided on the rear surface of the mirror1is a reinforcement rib7formed by a ring-shaped central portion71, extension portions7a-1and7a-2extended symmetrically from the ring-shaped central portion71on the side of the torsion bar2a, and extension portions7b-1and7b-2extended symmetrically from the ring-shaped central portion71on the side of the torsion bar2b. In this case, the size (i.e., radius) of the ring-shaped central portion71is smaller than that of the mirror1. Also, the ring-shaped central portion71can be circular or elliptical. Further, the extension portions7a-1and7a-2are coupled to the protruded portions2a-1and2a-2, respectively, of the torsion bar2a, and the extension portions7b-1and7b-2of the torsion bar2bare coupled to the protruded portions2b-1and2b-2, respectively, of the torsion bar2b.

InFIGS. 2A and 2B, the mirror1is constructed by the monocrystalline silicon (“Device”) layer303and the aluminum (Al) reflective layer311(see:FIG. 3); the torsion bars2aand2balong with the protruded portions2a-1,2a-2,2b-1and2b-2are constructed by the monocrystalline silicon (“Device”) layer303(see:FIG. 3); and the reinforcement rib7as well as by the ring-shaped central portion71, and the extension portions7a-1,7a-2,7b-1and7b-2are constructed by the monocrystalline silicon (“Handle”) layer301, the intermediate silicon dioxide (“Box”) layer, the intermediate silicon dioxide (“Box”) layer302and the silicon dioxide layer304(see:FIG. 3). However, the torsion bars2aand2band the reinforcement rib7can be made of a single material substrate such as a monocrystalline silicon substrate as illustrated inFIG. 2C.

In more detail, the torsion bars2aand2bhave ends coupled to the outer circumference of the mirror1and other ends coupled to the inner circumference of the inner frame3. Therefore, the torsion bars2aand2bare twisted by the inner piezoelectric actuators4aand4bto rock the mirror1around the X-axis.

The outer frame5is rectangular-framed to surround the inner frame3associated with the meander-type outer piezoelectric actuators6aand6b.

The meander-type outer piezoelectric actuator6ais constructed by piezoelectric cantilevers6a-1,6a-2,6a-3and6a-4which are serially-coupled from the coupling portion5aof the outer frame5to the coupling portion2cof the inner frame3. Also, each of the piezoelectric cantilevers6a-1,6a-2,6a-3and6a-4is in parallel with the X-axis. Therefore, the piezoelectric cantilevers6a-1,6a-2,6a-3and6a-4are folded at every cantilever or meandering from the outer frame5to the inner frame3, so that the amplitudes of the piezoelectric cantilevers6a-,6a-2,6a-3and6a-4can be changed along directions perpendicular to the Y-axis.

Similarly, the meander-type outer piezoelectric actuator6bis constructed by piezoelectric cantilevers6b-1,6b-2,6b-3and6b-4which are serially-coupled from the coupling portion5bof the outer frame5to the outer coupling portion2dof the inner frame3. Also, each of the piezoelectric cantilevers6b-1,6b-2,6b-3and6b-4are in parallel with the X-axis. Therefore, the piezoelectric cantilevers6b-1,6b-2,6b-3and6b-4are folded at every cantilever or meandering from the outer frame5to the inner frame3, so that the piezoelectric cantilevers6b-1,6b-2,6b-3and6b-4can be changed along directions perpendicular to the Y-axis.

Provided on the outer frame5are pads P which are connected to the control unit30. The control unit30applies a drive voltage VX1to the inner piezoelectric actuator4aand applies a drive voltage VX2to the inner piezoelectric actuator4b. The drive voltages VX1and VX2are sinusoidal, and the drive voltage VX1is opposite in phase to the drive voltage VX2. For example, the frequency fXof the drive voltages VX1and VX2is one resonant frequency frsuch as 25 kHz depending upon a resonant structure formed by the mirror1, the torsion bars2aand2band the inner piezoelectric actuators4aand4b. Thus, the mirror1is rocked around the X-axis. On the other hand, the control unit30applies a drive voltage VY1to the odd-numbered piezoelectric cantilevers6a-1,6a-3,6b-1and6b-3, and applies a drive voltage VY2to the even-numbered piezoelectric cantilevers6a-2,6a-4,6b-2and6b-4. The drive voltages VY1and VY2are sinusoidal or saw-tooth-shaped, and the drive voltage VY1is opposite in phase to the drive voltage VY2. For example, the frequency fYof the drive voltages VY1and VY2is 60 Hz, much lower than the resonant frequency fr. Thus, the mirror1is rocked around the Y-axis.

The control unit30includes a microprocessor, a digital signal processor, or a field programmable gate array (FPGA).

A method for manufacturing the optical deflector10ofFIG. 1is explained next with reference toFIG. 3which is a cross-sectional view of the optical deflector10ofFIG. 1.

First, a silicon-on-insulator (SOI) structure constructed by a monocrystalline silicon support (“Handle”) layer301, an intermediate (buried) silicon dioxide (“Box”) layer302, and a monocrystalline silicon active (“Device”) layer303is prepared. Also, by a thermal oxidation process, a silicon dioxide layer304is formed on the support layer301, and a silicon dioxide layer305is formed on the active layer303.

Next, the upper electrode layer308and the PZT layer307are patterned by a photolithography and etching process. Then, the lower electrode layer306and the silicon dioxide layer305are patterned by a photolithography and etching process.

Next, an about 500 nm thick silicon dioxide interlayer309is formed on the entire surface by a plasma chemical vapor deposition (PCVD) process.

Next, contact holes are perforated in the silicon dioxide interlayer309by a photolithography and dry etching process. The contact holes correspond to the piezoelectric actuators4aand4b, the piezoelectric cantilevers6a-1,6a-2,6a-3,6a-4,6b-1,6b-2,6b-3,6b-4, and the pads P.

Next, wiring layers310made of AlCu (1% Cu) are formed by a photolithography process, a sputtering process, and a lift-off process. The wiring layers310are electrically connected between the upper electrode layers308of the piezoelectric actuators4aand4b, and the piezoelectric cantilevers6a-1,6a-2,6a-3,6a-4,6b-1,6b-2,6b-3and6b-4and their corresponding pads P.

Next, the silicon dioxide layer304is etched by a photolithography and dry etching process, so that the silicon dioxide layer304is left in an area corresponding to the inner frame3, the outer frame5and the reinforcement rib7.

Next, the support layer301is etched by a dry etching process using the silicon dioxide layer304as an etching mask. Then, the silicon dioxide layer302is etched by a wet etching process using the support layer301as an etching mask.

Finally, an aluminum (Al) reflective layer311is formed by an evaporation process on the active layer303, and is patterned by a photolithography and etching process, thus completing the optical deflector.

When a simulation using the Oofelie-Multiphysics V4.4 (trademark) simulation software provided by Open Engineering was performed upon the optical deflector10ofFIG. 1, a stress distribution as illustrated inFIG. 4was obtained. InFIG. 4, the stronger stress, whether it is a compressive stress or a tensile stress, is shown darker in the illustration where a very large stress is indicated by X. InFIG. 4, a relatively large stress as indicated by X0is concentrated at a portion of the torsion bar2a(2b) in the vicinity of the protruded portions2a-1and2a-2(2b-1and2b-2), so that the spread of the relatively large stress is terminated approximately at a coupling portion between the torsion bar2a(2b) and the mirror1. Therefore, no substantial stress occurs in the portion of the mirror1surrounded by the extension portions7a-1and7a-2(7b-1and7b-2). Thus, the entire mirror1would not be distorted, so that the optical scanning characteristics of reflected light from the mirror1would improve. Additionally, the ring-shaped reinforcement rib7would not be peeled off.

On the other hand, in the first prior art optical deflector as illustrated inFIG. 5A, since the thickness of a mirror101is the same as that of torsion bars102aand102b, a relatively large stress as indicated by XA is spread isotropically and broadly into the mirror101. As a result, the mirror101would be greatly deformed in a bowl shape. Also, in the second prior art optical deflector as illustrated inFIG. 5B, a ring-shaped reinforcement rib103is added to the rear-side of the mirror101ofFIG. 5A, so that a relatively large stress indicated by XB is interrupted by the ring-shaped reinforcement rib103. Thus, no substantial stress occurs in the mirror101within the ring-shaped reinforcement rib103. Further, in the third prior art optical deflector as illustrated inFIG. 5C, protruded portions101a-1,101a-2,101b-1and101b-2are provided at the mirror101, and extension portions103a-1,103a-2,103b-1and103b-2of the reinforcement rib103are coupled to the protruded portions101a-1,101a-2,101b-1and101b-2, respectively, of the mirror101, so that a relatively large stress indicated by XC is interrupted by the extension portions103a-1,103a-2,103b-1and103b-2of the reinforcement rib103. In any ofFIGS. 5A, 5B and 5C, however, the relatively large stress occurs in a part of the mirror101, so that the part of the mirror101would be distorted. Thus, the optical scanning characteristics of reflected light from the mirror101would deteriorate. At worst, the ring-shaped reinforcement rib103ofFIGS. 5B and 5Cwould be peeled off.

FIG. 6Ais a detailed front-side view of the mirror1and torsion bar2bofFIG. 1, andFIG. 6Bis a detailed rear-side view of the mirror1and torsion bar2bofFIG. 1.

As illustrated inFIG. 6A, the protruded portions such as2b-1and2b-2have root portions2b-1rand2b-2rand end portions2b-1eand2b-2ewider than the root portions2b-1rand2b-2r. As a result, the extension portions7b-1and7b-2are surely coupled to the end portions2b-1eand2b-2e, respectively. However, the width of the protruded portions2b-1and2b-2can be uniformly wide as illustrated inFIG. 4.

As illustrated inFIG. 6B, the extension portion7b-1(7b-2) has a first concave (or inwardly-curved) portion7b-2C toward the torsion bar2bto which the extension portion7b-1(7b-2) is coupled, and a second concave (or inwardly-curved) portion7b-1C′ (7b-2C′) toward the torsion bar2barranged between the protruded portion2b-1(2b-2) of the torsion bar2band the mirror1.

As illustrated inFIG. 7, which is an enlargement of the first inwardly-curved portion7b-1C of the extension portion7b-1ofFIG. 6B, when the torsion bar2bis twisted, a stress as indicated by Y1is spread from the torsion bar2bthrough its protruded portion2b-1to the first inwardly-curved portion7b-1C of the extension portion7b-1. In this case, the stress indicated by Y1would be relaxed by the first inwardly-curved portion7b-1C, so that the extension portion7b-1would not be peeled off from the protruded portion2b-1at the first inwardly-curved portion7b-1C.

As illustrated inFIG. 8A, which is an enlargement of the second inwardly-curved portion7b-1C′ of the extension portion7b-1ofFIG. 6B, when the torsion bar2bis twisted, a stress as indicated by Y2is spread from the torsion bar2bthrough its protruded portion2b-1to the second inwardly-curved portion7b-1C′. Also, a stress as indicated by Y3is spread from the torsion bar2bthrough the mirror1to the second inwardly-curved portion7b-1C′. Note that the stress indicated by Y3is much smaller than the stress indicated by Y2. In this case, the stresses indicated by Y2and Y3are relaxed by the second inwardly-curved portion7b-1C′. As a result, as illustrated inFIG. 8B, which is a partial enlargement ofFIG. 8A, the extension portion7b-1would not be peeled off at an external edge Z2of the protruded portion2b-1coupling with the extension portion7b-1and at an external edge Z3of the mirror1coupling with the extension portion7b-1.

The ring-shaped central portion71ofFIG. 2Acan be modified into central portions71′ and71″ as illustrated inFIGS. 9A and 9B. InFIG. 9A, the central portions71′ is constructed by two parallel linear portions each with the extension portions7a-1and7a-2and the extension portions7b-1and7b-2. InFIG. 9B, the central portions71″ is constructed by a well crib portion whose four ends are connected to the extension portions7a-1,7a-2,7b-1and7b-2.

InFIG. 1, instead of the meander-type piezoelectric actuators6aand6b, a pair of outer torsion bars can be coupled between the outer support frame5and the inner support frame3, and two pairs of piezoelectric actuators can be coupled between the outer support frame5and the outer torsion bars to rock the inner support frame3through the outer torsion bars along the Y-axis.

Also, the two-dimensional optical deflector ofFIG. 1can be applied to a one-dimensional optical deflector where the outer support frame5and the actuators6aand6bare removed.

Further, in the above-described embodiment, only one torsion bar can be provided instead of the pair of torsion bars2aand2bfor rocking the mirror1around the X-axis. Similarly, only one meander-type actuator or only one outer torsion bar can be provided instead of the pair of meander-type actuators6aand6bor the pair of outer torsion bars.

Furthermore, in the above-described embodiment, electrostatic actuators or electromagnetic actuators can be provided instead of the piezoelectric actuators.

It will be apparent to those skilled in the art that various modifications and variations can be made in the presently disclosed subject matter without departing from the spirit or scope of the presently disclosed subject matter. Thus, it is intended that the presently disclosed subject matter covers the modifications and variations of the presently disclosed subject matter provided they come within the scope of the appended claims and their equivalents. All related or prior art references described above and in the Background section of the present specification are hereby incorporated in their entirety by reference.