Electromagnetically Driven Two-Axis Optical Beam Steering MEMS Mirror andIts Dependence of Actuation on Magnetic Field

YOSHIYUKI WATANABE, YUTAKA ABE, SHINNOSUKE IWAMATSU, SEIYA KOBAYASHI,YOSHIYUKI TAKAHASHI, and TOSHIYUKI SATO

Yamagata Research Institute of Technology, Japan

SUMMARY

This paper describes the development of an electro-magnetically driven two-axis MEMS mirror which steersan optical beam, and the dependence of the tilting angleson the magnet shape and size and the initial gap betweenplanar coils and the magnet surface. A reflective Au mirror(1.8 × 1.8 mm2) can be tilted bidirectionally with an elec-tromagnetic force induced by the current of the planar coilsand the magnetic field of a permanent magnet. A newlydeveloped MEMS mirror device (10 × 10 × 0.2 mm3) wasset on a printed circuit board (15 × 15 × 1.0 mm3), and theboard was fixed on a holder in which a magnet was inset.The utilized magnets were cubic (6, 8, and 15 mm squareand 5 mm thick), cylindrical (6 and 8 mm in diameter and5 mm thick), and spherical (8 mm in diameter) to investigateefficient actuation. The initial gaps of the planar coils andmagnet surface were 0, 500, 1000, and 2000 µm. Themagnetic flux density and its gradient decreased with dis-tance from the magnet surface. The tilting angles of theMEMS mirror increased with decreasing size of the squaremagnet and shorter distance, and were largest when usinga magnet 6 mm square and a 500-µm gap, in which condi-tion the maximum tilting angles in the X and Y directionswere 2.95 and 3.68 deg/mA, respectively. In addition, weobtained 3D-OCT images of human finger tissue by usinga Fourier domain fiber interferometer with a newly devel-oped MEMS mirror. © 2011 Wiley Periodicals, Inc. Elec-tron Comm Jpn, 94(11): 24–31, 2011; Published online inWiley Online Library (wileyonlinelibrary.com). DOI10.1002/ecj.10377

Key words: electromagnetic; two-axis beam steer-ing; MEMS mirror; Fourier domain; OCT; planar coil.

1. Introduction

OCT (Optical Coherence Tomography) imaging isused for noninvasive tomographic observation of the skin,eyeballs, and other body tissues [1, 2]. OCT is an imagingtechnology that can be implemented by an optical inter-ferometer with a light source providing a wavelength inter-val, such as an SLD (Super Luminescent Diode) or atunable laser. The central wavelength of the light sourcemust be selected with consideration of optical transmissionand the scattering properties of the observed object. In thenear-infrared range, the 830-nm band, with low water ab-sorption, is used mainly for the fundus of the eye, while the1.3-µm band, with low light scattering, is used for theepithelium and visceral tissues [2]. In order to acquire 2D(one-axis scan × depth) or 3D (two-axis scan × depth) OCTimages, we may scan the object or the measuring probe. Inpractice, however, the process is accelerated by one- ortwo-axis optical steering by means of a polygonal mirror orgalvanometer mirror; in addition, MEMS mirrors, with theadvantages of compact size and low power consumption,are also employed. MEMS mirrors can be designed asmultimirrors (mirror arrays) for projectors and opticalcross-connects [3, 4], or as single mirrors for projection andprecise measurement by portable terminals. In the case ofsingle mirrors, the drive schemes can be divided into pie-zoelectric, electrostatic, and electromagnetic. Piezoelectricdrives offer a simple structure (for example, with a piezo-electric film formed on movable elements) [5] and compactdesign; however, the generated force is small, and a rela-tively large voltage of several tens of volts must be applied.As regards electrostatic drive, there are reports of the use ofelectrostatic forces between vertical comb-tooth electrodesformed by DRIE [6, 7]. However, this design involves anumber of problems, including nonlinearity of the scanangle with respect to the applied voltage. Electromagneticdrive [8, 9] requires magnet circuits and magnets, and isinferior to other designs in terms of compactness; on the

© 2011 Wiley Periodicals, Inc.

Electronics and Communications in Japan, Vol. 94, No. 11, 2011Translated from Denki Gakkai Ronbunshi, Vol. 130-E, No. 4, April 2010, pp. 107–112

24

other hand, electromagnetic drive offers large forces at lowvoltages. Many electromagnetically driven MEMS mirrorsuse a Lorentz force produced by current flowing perpen-dicular to a magnetic field applied along the device surface.A magnetic circuit is arranged on the device periphery,which may hinder size reduction. Aiming at application ofOCT to low-coherence optical interferometry, we studiedMEMS mirrors provided with two-axis beam steering andmicromodulation of the optical path by longitudinal vibra-tion. However, such applications require operation at anarbitrary nonresonant frequency in the low-frequencyrange, including static tilting. Thus, we developed an elec-tromagnetically driven MEMS mirror [10, 11]. These mir-rors are operated by electromagnetic interaction betweenplanar coils formed on a MEMS device and a permanentmagnet placed immediately beneath the device. This designseems advantageous in compactness and efficiency com-pared with the previous electromagnetic drives using atransverse magnetic field [8, 9]. In this study, we focus onthe effects of the magnet shape, the distance between themagnet and the device, and other parameters of the mag-netic field that have not yet been clarified. In particular,aiming at compact size and highly efficient drive, we exam-ine the shape and size of the magnet installed on a two-axisoptical beam steering MEMS mirror, and the dependenceof the tilting characteristics on the gap between the magnetand the drive coils. Here we report tomographic observa-tions obtained by a Fourier domain fiber interferometerwith a MEMS mirror.

2. Structure and Fabrication of MEMS Mirror

2.1 Device structure

The structure of the MEMS mirror is shown in Fig.1. The device comprises a silicon layer (thickness 200 µm)with a movable structure, a printed circuit board (thickness1.0 mm), and a magnet and magnet holder (typical thick-ness 5.0 mm). An Au mirror and Y scan coils are formed onthe Y frame in the middle part of the silicon layer. Thefolded Y scan beams are supported by an X frame. X scancoils are formed on the X frame. The folded X scan beamsare supported by an external fixed frame. When current ispassed through the two Y scan coils on the Y frame, themirror tilts in the Y direction (around the X axis); whencurrent is passed through the four X scan coils on the Xframe, the mirror tilts in the X direction (around the Y axis).Thus, light incident on the mirror can be steered in twodimensions. When current flows in the coils, the X frameand Y frame tilt inside the print board, not interfering withthe magnet.

The shape and dimensions of the device and thetwo-axis scan beams are shown in Fig. 2. The whole device

measures 10 × 10 mm2. The outer X frame for X scanmeasures 6.8 × 6.8 mm2, and the inner Y frame for Y scanincluding the Au reflective mirror (1.8 × 1.8 mm2) measures6.2 × 2.2 mm2 [Fig. 2(a)]. The X scan beam has a twofold

Fig. 1. Device structure. [Color figure can be viewed inthe online issue, which is available at

wileyonlinelibrary.com.]

Fig. 2. 2D view of device and two-axis scan beams.[Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

25

configuration within 3.4 × 0.6 mm2 [Fig. 2(b)], and the Yscan beam has an eightfold configuration within 1.5 × 1.7mm2 [Fig. 2(c)]. Both are designed 100 µm wide. The drivecoils include four coils for X scan and two coils for Y scan.All the coils have the same design, with outer dimensionsof 2.0 × 2.0 mm2, 2 layers, and 30 turns.

2.2 Fabrication process and cross-sectionalstructure

The fabrication process of the device is illustrated inFig. 3.

(a) Silicon with a crystal orientation of (100) and athickness of 200 µm is etched anisotropically on the bottomside. The thickness of the beams and frames is set to 20 µm.On the face side, the first layer of Au/Cr coils (thickness 1µm × width 40 µm) is formed by sputtering and wet etching.After that, a polyimide film (Photoneece PW-1200, Toray)is deposited for insulation between the first and secondlayers.

(b) The second layer of Au/Cr coils (thickness 1 µm× width 40 µm) is formed by sputtering and wet etching,and then an Au/Cr mirror (1.8 × 1.8 mm2) is formed in thecentral part by lift-off.

(c) The silicon is dry-etched on the face side, using aphotoresist mask to form the beams and frames.

(d) The photoresist mask is removed, and the chips(10 × 10 mm2) are cut by blade dicing.

(e) The device is fixed on the printed circuit board,and wire bonding is performed. The magnet and holder arethen attached.

A photograph of the finished module is shown in Fig.4. The thickness of the entire module is typically 6 mm. Thedevice is controlled by flexible wiring connected to theprinted circuit board.

3. Principle of Electromagnetic Operation

The proposed device is operated by interaction be-tween the current flowing in the coils and the magnetic fieldproduced by the magnet. When considering operation inqualitative terms using a one-dimensional model with themagnetic field distributed only in the Z direction, the verti-cal component F(z) of the generated force can be expressedin terms of the spatial gradient of the magnetic potentialenergy U(z) as follows:

wherez is the gap between the magnet surface and the coil;mm is the magnetic moment of the magnet;mc is the magnetic moment of the coil;Hm is the field strength of the magnet;Hc is the field strength of the coil;t is the thickness of the magnet.

Fig. 3. Fabrication processes and cross-sectional view.[Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

Fig. 4. Fabricated device and module. [Color figurecan be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

(1)

(2)

26

The following can be derived from Eqs. (1) and (2):

Figure 5 shows how the MEMS mirror tilts whencurrent flows in the X or Y scan coils. When current flowsin two coils in opposite directions, upward and downwardelectromagnetic forces are generated, and the magnitude ofthe forces is determined by the gap between the magnet andthe coils as follows:

wherez0 is the initial gap between the magnet surface and

the coils;∆z is the gap variation at the coil center.

The tilt spring constant and resonance frequency arerelated as shown in Eq. (5), and the tilt angle is determinedby the rotating torque of the electromagnetic force and thetilt spring constant as shown in Eq. (6):

Herefreso(i) is the resonant frequency in the x or y direction;kθi is the tilt spring constant in the x or y direction;Ii is the moment of inertia of the x or y frame;Ti is the rotating torque in the x or y direction;

θi is the tilt angle in the x or y direction;Li is the distance from the center of rotation to the

center of the x or y scan coils.

4. Estimation of Magnetic Field Dependence of TiltAngle

At a fixed coil shape, the size of the magnet affectsmm in the first term and Hm(z) in the second term on the rightside of Eq. (3). The second term pertains to the spatialgradient of the magnetic field produced by the magnet(magnetic gradient); the higher this value is, the greater theelectromagnetic force that can theoretically be obtained.

Thus, we assumed the use of neodymium magnetswith cubic, cylindrical, and spherical shapes, as shown inTable 1, and investigated how the X and Y tilt angles varywith the magnet size and the magnet-coil gap.

For every magnet, we measured the magnetic fluxdensity in the longitudinal direction as a function of thedistance from the magnet surface. The results are presentedin Fig. 6. Diagram (b) also shows the theoretical values forcylindrical magnets with diameters of 6 and 8 mm foundfrom the Biot–Savart law for a one-dimensional model(equivalent solenoid). In the case of cubic magnets, the fluxdensity on the outermost surface and the magnetic gradientincreased with decreasing field generation surface. A simi-lar trend was also observed for the cylindrical magnets; inthis case, both the flux density on the outermost surface andthe magnetic gradient were greater than for the cubic mag-net. In the case of the spherical magnets, the flux densityon the outermost surface was of the same order as that ofthe cylindrical magnets, and the magnetic gradient was thelargest among all magnets. For cylindrical magnets withdiameters of 6 and 8 mm, the measured magnetic gradientwas somewhat higher than the theoretical value, which canbe attributed to nonuniform in-plane distribution of themagnetic field and other factors.

Using the three types of magnets, we measured thestatic tilt angle of the MEMS mirror for various magnetsizes and magnet-coil gaps. The tilt angle was determinedby measuring the vertical displacements of the four edges

(3)

(4)

(5)

(6)

Table 1. Magnet shape, size, and magnet-coil gap

Fig. 5. Illustration of tilted mirror. [Color figure can beviewed in the online issue, which is available at

wileyonlinelibrary.com.]

27

of the square mirror (1.8 × 1.8 mm2) while passing a DCcurrent through the X and Y scan coils. The same deviceswere used in all measurements so as to exclude variation ofcharacteristics among individual devices.

The results are presented in Fig. 7. Diagram (b) alsoshows the theoretical values for a cylindrical magnet witha diameter of 8 mm found with the one-dimensional modeldescribed by Eqs. (3) to (6). The theoretical values of theelectromagnetic force were found assuming values of mc

and Hc for a rectangular coil in Eq. (3), and mm and Hm werefound from the Biot–Savart law for a one-dimensionalmodel (equivalent solenoid) [12, 13]. The tilt spring con-stant kθi was obtained by Eq. (6) from the measured valuesof the resonant frequency [freso(x) = 70 Hz, freso(y) = 90 Hz]and the calculated moment of inertia Ii. The theoreticalvalue of tilt angle θi was found from Eq. (6). In the experi-ments, the mechanical tilt angle per unit current for cubicmagnets [Fig. 7(a)] was smallest for the magnets 15 mmsquare; the angle tended to increase with decreasing magnetsize. In addition, the tilt angle increased with reduction ofthe gap between the magnet and coils, and this dependencebecame more pronounced for smaller magnets.

In the case of cylindrical magnets [Fig. 7(b)], the tiltangle was smaller for a diameter of 6 mm than for adiameter of 8 mm, especially in the case of the X tilt angle.This may be attributed to the overlap area between the Xscan coil and the magnet: this area is smaller for a diameter

of 6 mm, and hence the decrease in the tilt angle. Comparedwith the theoretical values for a cylindrical magnet with adiameter of 8 mm, the measured tilt angle decreased withincreasing initial gap size. The narrower the gap is, thesmaller the measured results compared to the theoreticalvalues. This may be attributed to errors caused by assuminga one-dimensional model of the electromagnetic force [12];FEM simulation using a 3D model will be needed forquantitative design. Comparing the magnets 8 mm squareand 8 mm in diameter, the tilt angle was larger for thecylindrical magnet, with a smaller area. This, too, suggeststhat a larger tilt angle can be obtained with smaller magnetsin the case of overlap between the coils and the magnet.

In the case of the spherical magnet, one could expecta large tilt angle because the coils may approach the magnetvery closely. However, no advantage in the tilt angle wasobtained compared to other magnet shapes, as shown in Fig.7(b). However, the maximum tilt angle is limited by theinitial gap in the case of cubic and cylindrical magnets. Onthe other hand, the initial gap can be set fairly arbitrarily inthe case of a spherical magnet shape, a fact which requiresfurther consideration.

Fig. 7. Dependence of DC tilting angle on magnetshape, size, and coil-magnet gap. [Color figure can be

viewed in the online issue, which is available atwileyonlinelibrary.com.]

Fig. 6. Dependence of magnetic flux density ondistance. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

28

As may be concluded from the above results, largertilt angles can be obtained by using magnets with a smallsurface area at a fixed magnet thickness if there is overlapbetween the coils and magnet. This means that the magneticgradient Hm(z) in the second term is predominant in Eq. (3),which agrees qualitatively with the measured data shownin Fig. 6. In the experimental results, the relationship be-tween the gap length and the tilt angle is not always clear,and the gap dependence is weak in the case of the cubicmagnet 15 mm square. However, the magnetic gradientcalculated from Fig. 6 agrees with the tilt angle, whichindicates that the electromagnetic force depends stronglyon the magnetic gradient.

Thus, we may conclude that the tilt angle varies withthe magnet shape and size, and that a larger tilt angledepends more strongly on the gap length, so that errors inthe initial gap setting will have a considerable effect onperformance. Therefore, devices should be designed forspecific applications with regard to the tilt angle and its gapdependence.

5. OCT Measurement Using MEMS Mirror

We installed the fabricated two-axis MEMS mirror inthe optical probe of a Fourier domain fiber interferometer,and performed tomographic observation of human fingers.In particular, we used a cubic magnet of 8 mm square,considering drive efficiency and linearity as well as smallvertical displacement. The coil-magnet gap was fixed at 1mm. The measuring system is shown in Fig. 8(a). An SLDwith a central wavelength of 1.55 µm was employed as thelight source. The reference arm and sample arm werebranched using an optical coupler. The sample arm used anoptical fiber about 2 m long so as to provide freedom in theselection of the measuring point. The optical probe head onthe end of the fiber [Fig. 8(a)] was a small module about 30mm square. The light output of a collimator lens wasdirected onto the measured object by a MEMS mirror andan objective lens, while two-axis beam steering was per-formed. The reflected light was then linked to the opticalfiber via the MEMS mirror. The light interfering with thereference light was observed by a spectrometer using anInGaAs line sensor, and OCT images were obtained bywavenumber analysis. OCT images of human fingers areshown in Fig. 8(b). As can be seen from the diagram, theuneven shape of the crista cutis, the epidermis includingstratum corneum, and the internal tissues can be observedclearly. The measurement area was 3 × 3 mm2 (512 × 512pixels). Tomographic imaging and data processing took 30µs per pixel; the total measurement time for 3D imagingwas 8 s. These results confirm that the MEMS mirror couldbe used efficiently as a two-axis optical beam steeringdevice in OCT fiber interferometers and other systems.

6. Conclusions

We examined the magnetic field dependence of theperformance of an electromagnetically driven two-axis op-tical beam steering MEMS mirror. We found that when themagnet overlaps the coils, the magnetic gradient increasesas the magnet surface decreases, providing a large tilt angleand efficient drive. In addition, we installed the developedMEMS mirror in a Fourier domain fiber interferometer andobtained clear OCT images of human fingers.

REFERENCES

1. Tanno N. Optical coherence tomography. Jpn J Op-tics 2002;31:106–108. (in Japanese)

2. Haruna M, Ohmi M. Sensing of biological functionsby using low coherence interferometry. J Soc InstrumControl Eng 2006;45:915–921. (in Japanese)

3. http://www.dlp.com: Texas Instruments website onDLP (digital light processing).

4. Kazawa A, Itou Y, Horino M, Fukuda K, KanamaruM, Akashi T, Ishikawa T, Harada T, Okada R. Low-insertion-loss compact 3D-MEMS optical matrixswitch with 18 input/output ports. Digest of Techni-cal Papers of Transducers’05, Vol. 1, p 972–975.

Fig. 8. OCT imaging by Fourier domain fiberinterferometer utilizing two-axis optical beam steering

MEMS mirror device. [Color figure can be viewed in theonline issue, which is available at

wileyonlinelibrary.com.]

29

5. Tani M, Akamatsu M, Yasuda Y, Fujita H, ToshiyoshiH. A combination of fast resonant mode and slowstatic deflection of SOI-PZT actuators for MEMSimage projection display. 2006 IEEE/LEOS Interna-tional Conference on Optical MEMS and their Appli-cations, p 25–26.

6. Mizoguchi Y, Esashi M. Design and fabrication of apure-rotation microscanner with self-aligned electro-static vertical combdrives in double SOI wafer. Di-gest of Technical Papers of Transducers’05, Vol. 1, p65–68.

7. Kawano K, Hagihara Y, Ueda H, Noge H. Two-di-mensional optical MEMS scanner with novel electri-cal insulation structure. Tech Rep Matsushita Denko2008;56:85–91.

8. Asada N, Matsuki H, Minami K, Esashi M. Siliconmicromachined two-dimensional galvano-opticalscanner. IEEE Trans Magn 1994;30:4647–4649.

9. Ahn SH, Kim YK. Silicon scanning mirror of twoDOF with compensation current routing. J Mi-cromech Microeng 2006;14:1455–1461.

10. Mitsui T, Takahashi Y, Watanabe Y. A 2-axis opticalscanner driven nonresonantly by electromagneticforce for OCT imaging. J Micromech Microeng2006;16:2482–2487.

11. Watanabe Y, Abe Y, Iwamatsu S, Mitsui T, TakahashiY, Sato T. Development of an electromagneticallydriven optical MEMS mirror with functions of Z-axisvibration and X, Y bi-directional tilting, and an ap-plication to low coherent optical interferometer. IEEJTrans SM 2008;128:70–74.

12. Suzuki H, Shima H, Kasagi N, Suzuki Y, ShimoyamaI, Miura H. Development of magnetic flap actuatorsfor active turbulence control. Mechanical Engineer-ing Congress 75th, No. 98-1(III), p 259–260, 1998.(in Japanese)

13. Suzuki H, Kasagi N, Suzuki Y. Active control ofaxisymmetric jet by using an array of electro-mag-netic flap actuators. 11th Dynamics Symposium onElectromagnetics, No. 98-1(III), p 287–290, 1999.(in Japanese)

AUTHORS (from left to right)

Yoshiyuki Watanabe (member) received a bachelor’s degree in physics from Niigata University (Faculty of Science) in1991 and joined the Yamagata Research Institute of Technology. He is engaged in the development of motion sensors and otherinertial sensors, medical sensors, and optical MEMS devices. He holds a Ph.D. degree.

Yutaka Abe (member) received a bachelor’s degree from the University of Tokyo (Faculty of Pharmaceutical Science) in2004 and joined the Yamagata Research Institute of Technology. He is engaged in the development of chemical MEMS, medicalsensors, and optical MEMS devices.

Shinnosuke Iwamatsu (member) completed the M.E. program at Yamagata University (Graduate School of Science andEngineering) in 2001 and joined the Yamagata Research Institute of Technology. He is engaged in the development of chemicalsensors, medical sensors, and biosensors.

Seiya Kobayashi (member) completed the M.E. program in mechanical engineering at Tokyo University of Science(Graduate School of Engineering) in 1984 and joined the Yamagata Research Institute of Technology. He is engaged in thedevelopment of acceleration sensors and other inertial sensors, medical sensors, and optical MEMS devices.

30

AUTHORS (continued) (from left to right)

Yoshiyuki Takahashi (nonmember) graduated from Yamagata University (Graduate School of Science and Engineering)in 2008. He is now affiliated with the Yamagata Research Institute of Technology and is engaged in the development of imageprocessing systems. He holds a Ph.D. degree.

Toshiyuki Sato (member) became a researcher at the Yamagata Research Institute of Technology in 1980. He completedthe doctoral program at Yamagata University in 1999. He is engaged in technological support for small-medium enterprises,and the development of electron spin resonance devices, industrial image processing systems, and airborne optical measurementsystems. Since 2008 he has been head of the Electronic and Information Department, Advanced Technology DevelopmentDivision, Yamagata Research Institute of Technology.

31