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A dual-axis pointing mirror with moving-magnet actuation
View the table of contents for this issue, or go to the journal homepage for more
2013 J. Micromech. Microeng. 23 025002
(http://iopscience.iop.org/0960-1317/23/2/025002)
Home Search Collections Journals About Contact us My IOPscience
Received 18 July 2012, in final form 21 September 2012Published 21 December 2012Online at stacks.iop.org/JMM/23/025002
AbstractA large-aperture and large-angle MEMS-based 2D pointing mirror is presented. The device iselectromagnetically actuated by a moving-magnet/stationary-coil pair and potentially suitedfor high power laser beam shaping and beam pointing applications, such as LIDAR. The4 × 4 mm2 mirror, the radially symmetric compliant membrane, and the off-the-shelfpermanent magnet are manually assembled, with the planar coil kept at a well-defined verticaldistance from the permanent magnet by simple alignment pins. The mirror and the compliantmembrane structures are separately microfabricated on bulk silicon and SOI wafers,respectively. The hybrid integration of microfabricated and off-the-shelf components enablelow-risk/high-yield fabrication, while limiting the throughput. The device features minimuminter-axis cross coupling and good linearity and is highly immune to alignment and assemblyimperfections, thanks to the robust actuation principle. All the components including thebi-axial electromagnetic actuator provide a device footprint as small as the top mirror,allowing the design to be used in compact and high-fill-factor mirror arrays. With a drive coilof 400 mA and 5.12 W drive power, the total uniaxial dc rotation exceeds ±16◦ (optical) forboth axes with good decoupling. At maximum measured angle (biaxial 10◦ (mechanical)), aposition stability better than 0.05◦ over 7 h, and a position repeatability of 0.04◦ over 5000switching cycles is reported. Thermally, the simulated mirror temperature increases to 64 Kabove the heat sink temperature with a thermal in-flux of 1 kW m−2, under absolute vacuum.
(Some figures may appear in colour only in the online journal)
1. Introduction
Scanning and pointing micromirrors were among the earliestpurveyors of the now mature MEMS technology in commercialmarkets, proving their versatility and reliability as agentsof light manipulation. Despite their rather established placeon the MEMS landscape, novel applications of micromirrorscontinue to emerge, thanks to the perpetual interest they attractfrom the industry and academy alike. Some of the mostrecent research fields actively making use of micromirrorsin one form or another, include endoscopic medical imaging
2 Present address: Department of Microsystems Engineering—IMTEK,University of Freiburg, Georges-Kohler-Allee 102, 2. OG, D-79110 Freiburg,Germany.3 Present address: CSEM SA, Rue Jaquet-Droz 1, CH-2002 Neuchatel,Switzerland.4 Present address: COMET AG, Herrengasse 10, CH-3175 Flamatt,Switzerland.
(optical coherence tomography [1], two-photon microscopy[2], etc), 3D optical tracking [3], and projection displays[4]. The dominant trend in scanning mirrors favored small(usually smaller than 1.5 mm), but very fast devices that enablehigh resolution and high refresh rate operation. However,there is also potential for micromirrors to be employedin more conventional systems through precision assemblywith macro-machined components. Conventional precisionmachining techniques now offer precision levels down to10–20 μm; while being impressive, they are at least anorder of magnitude worse than those that microfabricationcan achieve. Some more advanced techniques, such as lasermachining, can offer micrometer level precision, but arecostly and slow. Thus, the combination of microfabricatedstructures with conventional assembly techniques has thepotential to significantly improve the performance and/orprecision of conventional electromechanical systems, whileoffering significant cost advantages. One of the fundamental
J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
motivations of ever shrinking MEMS devices has been theupscaling of force densities with decreasing size. Moreover,due to the brittle nature of single crystal silicon, largedevices with considerable stroke are mechanically fragile.Electromagnetic actuation, which is widely exploited formacroscopic actuation systems, has the combined advantagesof high force densities even at large size scales, and versatileintegration possibilities, due to the availability of off-the-shelfminiaturized magnetic components, such as micro-magnetsand micro-coils.
The precision, speed, power efficiency and costadvantages of hybrid devices can be very enabling for manyapplications that conventionally rely on precision mechanicsonly. One example of such applications is watchmaking, withnumerous commercial products employing microfabricatedcomponents in their movements [5]. Further application fields,which are primarily targeted in this work, include LIDAR,inter-satellite optical communication systems, object tracking,and laser pulse shaping, and require sufficiently fast steeringand/or pointing of high-power laser beams. Texas Instrumentsoffered a commercial mirror with 2D steering capability [6],but with limited rotation range and power handling capability.This device is no longer in production. Milanovic et al havedeveloped a very capable and versatile platform for 2D beamsteering, but again, the power handling is an issue [7]. Severalother electrostatic [8, 9], thermal [10], and electromagnetic[11] bi-axial mirrors were developed in array or singularform with exceptional mechanical specifications yet lackingthe thermal strength to handle high heat loads. There aretwo important prior works that need to be included in adiscussion as this. In 1992, an impressive set of moving-magnet type devices was proposed by Wagner, Engelmannet al, featuring microfabricated MEMS mirrors of one, twoand three degrees of freedom integrated with conventionalpermanent magnets [12]. The mirror is translated/rotated bythe force/torque induced on the magnet by the planar goldmicro-coils electroplated on the mirror wafer. In a similarfashion Iseki et al, have proposed a 2D mirror with moving-magnet actuation that uses vertical stacking of the mirror andconventional solenoids with ferromagnetic cores, enabling alarge DC pointing angle of ±30◦ with 80 mA drive current[13]. No information on the power consumption was provided.Despite the large rotation range, the total device size issignificantly larger than the mirror, which makes the approachunfeasible for array applications.
In this work, we present a biaxial electromagneticmoving-magnet actuated scanning mirror targeting high powerbeam steering applications. The hybrid manufacturing processfeatures precision assembly of microfabricated and of-the-shelf precision micromechanical components. With a 4 mm ×4 mm mirror size, the device is capable of ±8.6◦ mechanicalrotation in both axes with good rotation symmetry anddecoupling between the rotation axes, while maintaining aresonance frequency of 170 Hz. The mechanical suspendingstructure, which is a radially symmetric compliant membranecomprised of 25 S-shaped beam elements, is designed andoptimized for a maximum thermal conductivity to stiffnessratio. Although the electromagnetic actuation is inherently
more power hungry compared to rivaling methods such aselectrostatics, the heat generating elements are isolated fromthe mirror, and the dc actuation creates minuscule heat loadsdue to the Eddy currents on the mirror. This is enabled by themoving-magnet type actuation scheme. The electromagneticactuator footprint is smaller than the mirror size, making thedevice suitable for a high fill-factor array configuration. Thisproposed device uniquely combines a large mirror aperture,a small vertical form factor due to planar coils, identicalperformance around the two axes independent of fabricationtolerances, and the possibility to be extended into very largefill-factor arrays.
The design, actuation principle and modeling results forthe MEMS mirror are investigated in section 2. Details on thefabrication and assembly procedures are given in the followingsection. Detailed characterization results are presented insection 4.
2. Design and modeling
In a laser beam steering system, the diffraction spread, andtherefore the range of the beam is inversely proportionalto the initial aperture size, which is predominantly limitedby the size of the steering mirror. Thus, a large aperturesize is a critical requirement for a pointing mirror devicetargeting the state-of-the-art high-power laser applicationssuch as LIDAR or inter-satellite communication systems. Alarge mirror, on the other hand, means a large inertia andlowers the maximum operation speed of the mirror. Moreover,the suspensions and actuators that can handle the inertia ofsuch a large mirror is significantly area consuming, loweringthe process throughput and the fill-factor of the mirrorsin the array configuration. An additional concern overarchingthe ones listed is the thermal management. Particularly forsystems that operate under vacuum, the heat dissipationmechanism occurs only through the mechanical suspensions,whose thermal and mechanical compliance are antagonistic.This fundamental trade-off between the rotation range and thethermal conduction, together with fabrication limitations, setthe boundaries of the design space.
2.1. Device structure
An exploded view of the dual-axis pointing mirror is given infigure 1. The main structural elements are the mirror with anassembly post, the compliant membrane bearing an assemblypillar, and the actuator consisting of the micro-magnet andthe current carrying elements underneath. The mirror postsits on the post base located at the center of the compliantmembrane. The micro-magnet is attached to the membranepillar, forming a concentric tri-layer stack. The planar micro-coils are not part of the mirror assembly, but kept at a well-defined distance from the magnet using spacers, as explainedfurther in subsection 3.5. Via the vertical stacking of allthe device components including the bi-axial electromagneticactuator, the device footprint is kept smaller than the mirrorsize, rendering arrays with arbitrarily large fill-factor feasible.Since the moving-magnet actuator does not need any electrical
J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
Mirror
CompliantMembranePost Base
PermanentMagnetPlanar Coils
Figure 1. Exploded view of the device. The mirror, compliantmembrane and the magnet are permanently attached via manualassembly, and the planar coil is kept at a well-defined distanceunderneath the magnet with spacers.
access to the mirror assembly, all the wiring can be run on theplanar ceramic coil, facilitating the array integration and thedissipation of the heat generated by the actuator.
The fundamental geometrical parameters are depicted inthe cross section view of figure 2(a), with their numericalvalues in table 1. The square mirror has an edge length of 4 mm,but could also be chosen differently, according to the needs ofthe specific application. Due to the vertical offset between thecenter of rotation and the mirror surface, any rotation resultsin a parasitic lateral translation (figure 2(b)). This undesiredlateral translation stems from the fact that the axis of rotation islocated at the membrane center rather than the mirror surface,and is given by �x = rpi sin θ and �z = rpi sin θ tan θ for thex and z directions, respectively. In an array configuration, acertain clearance between the mirrors would be necessary toavoid mirror contact due to this undesired motion, reducingthe maximum attainable fill-factor. To minimize lateral mirror
Table 1. Parametric representation of device dimensions and theiractual values
drift, the post should be chosen as short as possible, whileavoiding contact between the mirror and compliant membraneat maximum rotation. This also reduces the mirror momentof inertia, which potentially increases the resonant frequencyof the device. Figure 2(b) also depicts the basic operationprinciple for the actuator, which consists of a double layerplanar micro-coil, and an axially magnetized micro-magnet.A current in the y-direction flowing through the top layerinduces a uniform magnetic field in the x-direction. The dipolesmagnetized in the vertical (z-) direction then are forced toalign themselves with the external field, creating a net forcein the field direction. The actuation is bidirectional; reversingthe current direction reverses the force, as well. The bottomcoil, running in the perpendicular direction, exerts a force onthe magnet along the y-direction, producing the force for thesecond axis. The membrane pillar, onto which the magnet isattached, acts as a torque arm to translate the torque on themagnet to mirror rotation with the axis of rotation verticallylocated in the middle of the membrane. The current runningthrough two coils can be independently controlled, thereforeallowing independent, bidirectional rotation.
Top Coil
dm
tmhp
hpo
dmc
rpo
tmb
rpi
hmg
rmg
Bottom coil
Mitop 0
ibot = 0
Δx Δz
Btop
Bbot
M
(a) (b)
Figure 2. Cross-section view of the device with major geometrical parameters (a) and current and field components involved in actuation(b). The uniform magnetic field induced by the current lines exerts a torque on the axially magnetized permanent magnet to align itsmagnetization vector with itself, which in turn rotates the mirror with the pillar acting as a torque arm.
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Torquex [ nN.m ]
Tor
que y [
nN
.m ]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
0.1
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0.6
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1
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7
8
Figure 3. Rotation angle amplitude (color coded) and orientation(arrows) as functions of bidirectional torque for the device. Thestiffness exhibits a very high level of rotational symmetry.
2.2. The compliant membrane
Cascaded frames with perpendicular springs is the mostcommon mechanical architecture for dual-axis MEMS mirrors[14, 15]. This configuration is area consuming, unless thesprings are hidden below the membrane by bonding/manualassembly of the mirror on the actuator/spring structure [8].Moreover, obtaining identical stiffness along the primaryrotation axes requires careful spring design, and processcontrol. The device presented in this work has a membrane typespring structure which is comprised of 25 S-shaped beams withradial symmetry. It is this radial symmetry of the compliantmembrane that provides identical torsional stiffness along anyrotation axis on the x − y plane, as demonstrated by the FEAsimulations in figure 3. In the rest of the discussion, the x-
and y-axes which lay along the two current directions will bereferred to as the primary axes, for simplicity.
The shape of individual beams forming the compliantmembrane is given by
y(x) = hb
⎛⎝ x
lb−
sin(
2πxlb
)
2π
⎞⎠ . (1)
This specific shape, plotted on figure 4(a), was previouslyemployed in several devices from the authors’ group and ischosen due to the uniform stress distribution it provides atthe beam ends [17]. In addition to the stiffness symmetry,the compliant membrane is also desired to provide mutualindependence of rotation axes and good spring linearity withinthe desired range for facilitating the device control. Figure 5(a)plots the principle (around the x-axis) and parasitic (aroundthe y-axis) rotation as a function of the torque (around thex-axis). The parasitic rotation is quadratically dependent onthe torque along the principle rotation axis and becomessignificant beyond 5◦, exceeding 0.25% of the principlerotation angle. FEA simulation results depicting the linearityof the compliant membrane as a function of the bi-axial torqueis given in figure 5(b). The simulations are performed withANSYS, using the beam4 type simplified beam elements.The position dependent torque from the actuator is omittedto analyze the mechanical linearity independently. The beamdimensions and the maximum torque values are chosen tobe the same as figure 3. As expected from a membrane typecompliant structure, and despite the curved beam shape, thesprings stiffen considerably with increasing rotation angle,reaching 25% at maximum rotation. Mechanically, there aretwo ways to improve linearity, by: (1) reducing the beamwidth and increasing their number (2) increasing the beamheight, and therefore the beam curvature. In this work, thedimensions are chosen as such to minimize fabrication risks,while demonstrating the potential of the pursued approach.
The thermal dissipation performance of the compliantmembrane is analyzed through FEA simulations using the
0 150 300 450 600 7500
100
200
300
400
500
600
700
X-Position (µm)
Y-P
osit
ion
(µm
)
-zmax —
0 —
zmax —
Beam Ends
MirrorBase
Side View
Top View
(a) (b)
Figure 4. (a) The compliant membrane is comprised of 25 individual beams defined by a simple mathematical expression. Due to thisshape, each individual beam has good compliance along all translational and rotational degrees-of-freedom [16]. (b) The contour plot of thez-displacement of the membrane when the mirror is rotated around the y-axis. The simulation is performed using ANSYS with 40 μm widebeams for visual clarity.
J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
0
2
4
6
8
x [ d
eg ]
0.0 0.2 0.4 0.6 0.8-0.04
-0.03
-0.02
-0.01
0.00
Torquex [ nN.m ]
y [ d
eg ]
0.250.50
0.75
0.250.50
0.75
1.000
1.150
1.300
Torquey [nN.m]
k / k
Torquex [nN.m]
θ θ
(a) (b)
Figure 5. Mechanical FEA simulation results on the linearity and cross-coupling characteristics of the compliant membrane. Angledependency of the actuator torque is omitted to isolate the effects of mechanical membrane nonlinearity. (a) Cross-coupling between theperpendicular rotation axes is a quadratic function of the torque and becomes significant beyond 5◦. The rotation along the torque directionexhibits cubic nonlinearity. (b) Spring stiffness as a function of bi-directional torque. At the extremity, the effective stiffness is 30% largerthan the initial value.
Figure 6. FEA simulation results for the steady-state temperaturedistribution with 1 kW m−2 heat load on the mirror surface with300 K heat-sink temperature using temperature dependent thermalconductivity for single-crystal silicon. The temperature gradient isalong the springs only with 64 K temperature rise on the mirrorsurface (see the inset).
temperature-dependent thermal conductivity for single-crystalsilicon features. Convection losses are ignored. In absolutevacuum and with 1 kW m−2 heat flux, the steady statetemperature on the mirror surface rises 64 K above the heat sinklevel of 300 K, and the temperature gradient occurs only alongthe compliant membrane beams (figure 6). At lower operatingtemperatures, the thermal conductivity of silicon is higher, thusthe thermal management is facilitated. Similarly, an increasein the ambient pressure also improves the thermal behaviordue to the additional heat dissipation through conduction.
2.3. Actuator characteristics
Electrostatic actuators perform poorly in the millimeterrange due to their shrinking force density with increasing
dimensions. Thermal actuators, on the other hand, can providesimilar, or even higher force densities at this range, buttheir response times are considerably slower than that ofelectromagnetic actuators. In addition, for a mirror targetinghigh-power laser beam guiding applications, where thermalconsiderations are of great importance, thermal actuationis not very suitable due to the large heat generation onthe device. Similar considerations render moving coil typeelectromagnetic actuation not suitable, as well. With a moving-magnet actuator, on the other hand, the heat dissipation on themirror can be kept at minimum, with Eddy currents on themagnet being the only heat dissipating process. Eddy currentscan be a significant factor in resonant/AC operation, but forsteering mirrors, which mostly operate in dc mode, the effectis negligible.
The electromagnetic torque on a permanent magnet in auniform magnetic field can be expressed as
�τ =∫
�M dV × �B (2)
where �τ is the magnetic torque, �M is the magnetic momentdensity of the permanent magnet, and �B is the external fieldintensity. Selection of the magnet material, and maximizing thecurrent density of the coil within a given thermal budget arethe cost-free ways to maximize the magnetic torque. However,increasing the magnet volume comes with the price of a largermoment of inertia.
The actuation scheme, summarized in figure 2, is immuneto pull-in instability, since the magnet moves parallel tothe coil unlike the common gap-closing actuators. A furtheradvantage is the weak dependency of the actuation force onthe rotation angle, as illustrated by the FEA simulation resultsgiven in figure 7(a). Even at 10◦ mechanical mirror rotation,the electromagnetic force decreases only by 5% compared tothe initial value. This small decrease stems from the slightlyshrinking normal component of the magnetization vector, asthe magnet rotates. The parasitic force in the perpendiculardirection is initially zero, but grows into 1.5% of the principle
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0 2 4 6 8 100.1
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Forcey
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For
cex [
mN
]
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cey [
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]
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Magnetic Force Lorentz Fit
Mag
neti
c F
orce
[m
N]
Coil-Magnet Spacing [ mm] (a) (b)
Figure 7. Actuation force as a function of device geometry. (a) Within the rotation range of interest, the actuator force is almost independentof the rotation angle. (b) The force has a Lorentzian dependency to the gap between the coil and the magnet.
(i)
(ii)
(iii)
(iv)Chip 1 Chip 2
(ii)
(iii)
(iv)
(i)
Silicon Oxide Aluminum
(a) (b)
Figure 8. (a) Main steps of the compliant membrane fabrication process based on an SOI wafer with 10 μm thick device layer and 390 μmthick handle layer. Chip singulation is accomplished via the HF vapor release. (b) Fabrication and dicing of the mirrors.
component at maximum rotation. In figure 7(b), the initialmagnetic field intensity is plotted as a function of coil–magnetseparation. As expected, the force on the magnet shrinkswith increasing magnet–coil separation; however, unlike anelectrostatic actuator, the decrease is not exponential, butresembles a flat-top Lorentz function, allowing a comfortablemagnet– coil separation and large tolerance to misalignmentswithout significantly compromising the force.
3. Fabrication and assembly
All the components of the mirror are microfabricated eitherin-house, or by a commercial supplier, with the exceptionof the permanent magnet. There have been numerous effortsto deposit permanent magnets on wafers with considerablesuccess. Electrodeposition, spin casting, sputtering and screenprinting [18] are the most common methods to integrate
permanent magnets with MEMS. However, the energy densityand the thickness of even the best microfabricated permanentmagnets is significantly lower than what is offered by theoff-the-shelf micro-magnets. Therefore, high energy-densityoff-the-shelf micro-magnets were chosen for this application.This choice substantially facilitated the fabrication process andreduced the main challenge to micro-assembly.
3.1. Compliant membrane and mirror fabrication
The fundamental steps of the microfabrication process for thecompliant membrane and the mirror are shown in figure 8. Themembrane process starts with an SOI wafer with 10 μm and390 μm thick device and handle layer thicknesses, respectively(figure 8(a.i)). The buried oxide layer between the device andhandle layers is 1 μm thick. The compliant beams are definedwith a single front side DRIE etch (figure 8(a.ii)). Followingthe front side silicon etch, the handle layer was patterned
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J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
Figure 9. SEM micrographs of the compliant membrane: front view of the post-base and the 10 μm diameter release holes on the baseperimeter (left) and the membrane pillar and the post base seen from the backside (right).
through another DRIE etching process to remove siliconunderneath the compliant beams (figure 8(a.iii)). In addition tothe spring and backside window, 6 mm × 6 mm square framesenclosing the compliant beams are also formed to be able torelease individual chips. The release of the membranes is donevia the HF-vapor release process (figure 8(a.iv)), which alsoreleases the individual chips eliminating need for mechanicaldicing. Details of this process can be found in Reference[19]. Separately, the mirrors are fabricated on 390 μm thickdouble-side polished bulk silicon wafers (figure 8(b.i)) usinga simple process. To enhance reflectivity, the front side is firstcoated with 200 nm thick evaporated aluminum (figure 8(b.ii)).Then the mirror posts are defined through a 290 μm deepDRIE silicon etching (figure 8(b.ii)). Finally, the mirrors areseparated by mechanical dicing. Depending on the application,the mirror thickness can be chosen thicker to minimize themirror curvature due to thermal stresses occurring underoperation. Furthermore, since no etching is performed on themirror surface, the initial flatness of the polished silicon surfacecan largely be retained during the fabrication and assemblysteps. Front and back view SEM micrographs of the compliantmembrane are given in figure 9.
3.2. Micro-magnets
The axially magnetized, 1 mm diameter and 365 μm thicksamarium–cobalt (Sm2Co17) rare earth magnets are off-the-shelf components from Audemars Microtec. With an energydensity of 215 Kj m−3 and a maximum operating temperatureof 350 ◦C samarium–cobalt is significantly weaker comparedto neodymium–iron–boron (NdFeB) (energy density up to511 Kj m−3), but possesses superior temperature stability.
3.3. Planar micro-coils
The schematic depiction of the printed circuit board and amagnified view of the coil pair is shown in figure 10. The planarcoil geometry offers several advantages over the conventionalsolenoids for this application. Vertical stacking of the two
BottomCoil
TopCoil
4 mm
4 m
m
22 mm
Magnet
Vias
Figure 10. The ceramic printed coil board and the layout of thetwo-layer electromagnetic coil pair. Within the central 2 mm ×2 mm area, the current flow is unidirectional and mutuallyperpendicular on both levels. The left and right (bottom and top)turns on the top (bottom) level are parts of a single line, which areconnected through the bottom (top)layer.
coils required for bi-axial actuation is achieved within a smallform-factor, and minimal dead-volume. Despite the large forceoutput in the order of a mN, the entire actuator could befitted within the mirror footprint. Also, the coil PCB can beput in direct contact with a heat-sink to facilitate the heatdissipation. The circuit boards were fabricated by HightecMC AG on 635 μm thick aluminum oxide (Al2O3) substrates.Each individual PCB has 9 coil pairs arranged in a 3 × 3array, designed to investigate the feasibility of arrays in termsof power handling and cross-talk between neighboring pixels.These issues will be addressed in a future work addressing theextension of the current concept into arrays.
The PCB has two metal layers, isolated by a polyimidelayer in between. Each metal layer consists of a 5.5 μm thickcopper layer with 1.5 μm/1.5 μm gold/nickel capping on top.All copper lines and spacings within the coil area are 20 μm.At room temperature, the nominal resistance of the top andbottom coils are 32 and 29 �, respectively. The maximumcurrent rating for the coils depends heavily on the operationtemperature and the heat sink below. Under ambient pressure,
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J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
(b)
(a)
Figure 11. Packaging of the MEMS mirror: assembly and packaging sequence (top), and the cross-section view of the final package (bottom)
no long term coil degradation was observed up to 500 mAcurrent per coil. No vacuum tests were performed.
3.4. MEMS assembly
The integration of the moving components (the mirror,the compliant membrane and the magnet) is performedusing a custom micro-assembly tool placed under an opticalmicroscope, as previously reported for similar multi-layerMEMS components [20]. As the base platform of the assemblytool, a hot-plate integrated with a temperature-controller isused. In the middle of the hot-plate, there is a hole of 1 mmdiameter which is connected to the house vacuum that canbe toggled on or off with a simple switch. With the magnetplaced on a perforated silicon piece and held firmly in place byvacuum, a tiny drop of Epotek H70S silver based epoxy glueis manually deposited on top with a probe tip. The compliantmembrane, held by a vacuum tip and precisely moved by athree-axis micrometer stage, is aligned with and then placedon the magnet. The outer edge of the magnet and center ofthe compliant membrane are used for alignment in this stage.A preliminary curing of the epoxy is required to maintain thealignment during the following mirror assembly step. This isachieved by increasing the hot-plate temperature to 80 ◦C, andcuring for 5 min. Then, another tiny drop of the same epoxyis manually deposited on the central part of the compliantmembrane using the same probe tip, and the mirror is placed onit in the same manner as the membrane. The outer square edgesof the membrane chip and the mirror are used for alignment.For both steps, the alignment is only visual, and the accuracyis estimated to be ±20 μm. Following the assembly, the stackis cured at 100 ◦C for 20 min.
3.5. Packaging
The main steps of the packaging process are depicted infigure 11. As the first step, the snap-in connectors are attachedto the micro-coils, followed by the placing and fixing of thefour brass pins with 500 μm thick elbows. These pins actboth as passive alignment features between the PCB and thehousing and a spacer of well-defined height between the mirrorand the coil. Next the mirror housing is assembled with thecoils. This housing consists of a 50 μm thick molybdenum
bottom layer, and a 390 μm thick silicon top layer. It bearspassive alignment features to center the mirror. In the finalpackaging step, the mirror is placed on the housing and fixedin place by epoxy. In the worst case, the assembly tolerancesare estimated to be limited to ±200 μm, which still keepsthe magnet within the area where the coils create a uniformmagnetic field, and therefore ensures tolerance independentmirror behavior. A photograph of all the device constituents,and a complete packaged device are shown in figures 12(a)and (b), respectively.
4. Test and characterization
4.1. Methodology
For static rotation characterization, a screen with a 2D gridof 1 mm divisions has been utilized. With a mirror-screenseparation of 1 m and assuming two resolvable spots perdivision, this configurations provides an angle resolution of0.5 mRad (29 millidegrees). While such a level of precisionmight be sufficient for a basic functionality test, it is far fromwhat is necessary for repeatability and stability measurements.Hence, an OnTrak 2L10SP position sensitive detector (PSD)based test setup was utilized for those measurements. Acalibrated OnTrak OT-301 amplifier was used for PSDsignal processing. A Keithley 2401 Sourcemeter in currentsource mode was used for driving the device. For dynamiccharacterization, the device was excited directly with an HP33120A function generator, while the mirror motion wasmonitored with a Polytec MSA400 laser Doppler vibrometer.
4.2. Mirror flatness
The static curvature of the mirror is measured by a Wykowhite light interferometer at room temperature, and the resultis depicted in figure 13. Since the mirror is fabricated froma polished bulk silicon wafer, the initial flatness is lessthan 70 nm, despite the large mirror aperture. At elevatedtemperatures, the mirror warp is expected to increase dueto the bimaterial mirror structure. The mirror curvature canbe reduced by using a thinner aluminum coating and/orthicker mirror, depending on the requirements of the particularapplication.
J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
Ceramic PCB Housing
Snap-in Pins
Magnet
Mirror
Membrane Spacers
(a) (b)
Figure 12. The electromagnetic 2D rotating mirror. (a) Constituent components (ceramic coil, silicon/molybdenum MEMS housing,membrane chip, mirror, magnet, alignment pins, side connectors) and (b) assembled device. The ceramic PCB was designed toaccommodate MEMS assemblies in a 3 × 3 array configuration to investigate certain issues related to the extension of this work into arrays,such as inter-mirror cross-talk, elevated heat generation, and uniformity. These aspects will be explored in a future work.
-33
76
0
40
-20
20
60
nm
Figure 13. Mirror flatness at room temperature measured with aWyko white light interferometer. The peak-to-valley heightdifference over the 4 mm clear aperture is less than 70 nm.
4.3. Coil current versus rotation angle
Mechanical rotation of the mirror as a function of dc drivecurrent is plotted for the two perpendicular axes along the coildirections in figure 14. With 400 mA drive current and 5.12 W(coil resistance is 32 �) power consumption, the mechanicalscan angle exceeds 8◦, which was the limit set by the clearancebetween the mirror and the membrane wafer underneath (hp).This mechanical limit can easily be extended by increasingthe height of the mirror post, or etching the area of membranewafer beneath the mirror to accommodate for the rotatedmirror. Despite the manual assembly and packaging, therotation profile around the principal axes are strikingly similar.This is a qualitative demonstration of both the circularlysymmetric membrane stiffness in combination with the robustactuation scheme. At the extremes, the horizontal and verticalscan angles differ only by 0.7%. The slight nonlinearitybetween the drive current and the rotation angle stems from thegeometrical nonlinearity of the springs, and the parasitic cross-
axis torque induced by the actuator. For a visual demonstrationof the 2D scanning capabilities of the devices, several Lissajouspatterns are formed by low-frequency (30–40 Hz) sinusoidalexcitation of both axes. Some of these patterns are shown infigure 15.
4.4. Eigenmode characteristics
Independent of the operation mode, the frequency behaviorof an electromechanical system defines some of the mostimportant performance parameters. For sufficient immunityagainst environmental vibrations, the lowest eigenmode shouldbe located outside of the bandwidth of undesired excitations.Although this bandwidth is strictly application dependent,100 Hz can be set as a reasonable lower limit on the firsteigenmode frequency for most cases. Furthermore, a goodspectral separation between different eigenmodes is alsocrucial to avoid the excitation of higher eigenmodes duringoperation. In figure 16 the first four simulated eigenmodesare depicted with their corresponding eigenfrequencies.Simulations are performed using ANSYS with the full 3Dgeometry. Due to the rotational symmetry of the membranestiffness, the two fundamental rotational modes, whichare desirably the first two modes, are located ideally at thesame frequency, effectively manifesting themselves as a singleeigenmode. The out-of-plane (z) translation mode and thein-plane rotation modes are located sufficiently away fromthe fundamental mode.
The experimental modal analysis results depicted infigure 17 illustrates the effect of the manual assemblyimperfections on the eigenmode characteristics. Due tothe assembly tolerances, the symmetry of the mirror isdestroyed, and the first two modes are split in frequency.The measurement was done with a Polytec MSA400 scanninglaser Doppler vibrometer pointing at the mirror corner, asshown in the figure inset. The mode shapes are confirmed
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J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
Electrical Current in Coil #1 I1 [ mA ]
Ele
ctri
cal C
urre
nt in
Coi
l #2
I 2
[ mA
]
-400 -200 0 200 400-400
-300
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[ mA ]
stat
ic-t
ilt a
ngle
th
eta x,
y [ d
eg ]
Static tilt angle (thetax)
Static tilt angle (thetay)
Cubic fit (thetax)
Cubic fit (thetay)
(a) (b)
Figure 14. (a) Experimental static mirror rotation as a function of biaxial coil current. Rotation angle amplitude (color coded) and orientation(arrows) as functions of biaxial coil current (as in figure 3). (b) Uniaxial current versus rotation angle behavior for the two axes of rotationdriven independently. With 400 mA of coil current, the uniaxial mirror rotation exceeds 8◦, with very similar behavior for the primary axes.
Figure 15. Various Lissajous patterns displayed by sinusoidalactuation of both axes with different frequency combinations. Theimages were converted to their negative for clarity.
by scanning the measurement point along the mirror area,as well. Both coils were connected in parallel for in-phaseexcitation of both axes. This splitting in modal frequency isnot due to the asymmetry of the spring stiffness along differentdirections, but by the difference of the moment of inertiaaround different axes, arising from the assembly tolerances.The actual dimensions of the fabricated devices were slightlydifferent than the initial designs, due to fabrication tolerances,
and device layer thickness tolerance of the SOI wafer. For bothdevices, there is about 9% difference between the simulatedand experimental values, which can be attributed to theadditional mass of the epoxy used for mirror assembly.
4.5. Position stability and repeatability
To analyze the angular position stability of the mirror, againthe Wyko optical profilometer was used. The measurementswere performed at 6 different angles (i.e. six different currentpairs on the coils), with a measurement performed about every1 minute and a total of 400 times over night. Figure 18summarizes the results of these measurements. The verticalscan (VSI) mode of the Wyko, with which these measurementsare performed, has an accuracy of about 0.5%, leading to a totalmeasurement error of 0.015◦ to 0.025◦. Figure 18(b) showshence the accuracy of the Wyko rather than the one of themirror position, demonstrating the excellent position stabilityof the device.
Top View Top View Top View Top View
MODE 1 MODE 2 MODE 3 MODE 4
OOP Rotation188.1 Hz
OOP Rotation188.1 Hz
OOP Translation288.3 Hz
IP Rotation994.9 Hz
x
y
z
Figure 16. Finite element analysis of the modal behavior of the mirror showing a fundamental eigenfrequency of 188 Hz, and a goodspectral separation between the fundamental and the higher eigenmodes. The color coded contour plots depict the displacement along thez-axis for the first three modes, and the displacement vector sum for the fourth mode.
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J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
162.4 Hz
171.3 Hz
150 155 160 165 170 175 1800.0
0.5
1.0
1.5
2.0
2.5
3.0A
mpl
itud
e [
a.u.
]
Frequency [ Hz ]
Figure 17. First two rotational eigenmodes of the magnetic mirror.Although the torsional membrane stiffness is circularly symmetric,the rotational mode is split due to the alignment tolerances duringthe assembly process, of which a non-symmetric moment of inertiais a manifestation.
For the position repeatability measurements, the PSDwas placed 0.6 m away from the mirror, where its centercorresponds to 2.9◦ at both axis. A HeNe laser was used forposition monitoring. The drive current was toggled on andoff with a frequency of 1 Hz, and the mirror position wasregistered after every positive switching over 5000 cycles. Theresults of this measurement are depicted in figure 19. Despitethe lack of any temperature or laser beam position control, themirror exhibited better than 0.1◦ of position repeatability.
5. Discussion
Despite the large power consumption, the thermo-mechanicalperformance of the proposed device is very competitive.
0 1000 2000 3000 4000 5000-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
Cycle Number
Ang
ular
dri
ft [
deg
]
Horizontal AxisVertical Axis
Figure 19. Open-loop position repeatability of the mirror over 5000switching cycles, expressed in angular drift from the set position of2.9◦ in both axes. The variation of the horizontal position is limitedto 0.02◦. The drift in the vertical position, on the other hand, is 0.1◦.
However, a number of improvements might push the deviceperformance even further. The choice of the spring materialhas a profound effect on the thermo-mechanical mirrorperformance. An ideal spring material should have a largethermal conductivity (W m−1 K−1) to Young’s modulus (GPa)ratio (k/E), low thermal expansion coefficient, and resistanceto thermal cycling. In addition to its excellent mechanicalproperties, single-crystal silicon has a relatively high thermalconductivity (145 W m−1 K−1), k/E ratio of around 1, andgood thermal stability. Therefore, it is a good starting point.However, the k/E ratio for some other materials, such ascopper (≈6), or diamond (≈2) are considerably higher, makingthem viable candidates for spring material, given that their
0 1 2 3 4 5 6 7
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[ h ]
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ude
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◦]
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+250 / 0 mA
+250 / −250 mA
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+250 / +400 mA
0 1 2 3 4 5 6 7−0.08
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0
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Time of measurement tm
[ h ]
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tion
ϕto
t−
ϕto
t[
◦]
0 / 0 mA
0 / −250 mA
+250 / 0 mA
+250 / −250 mA
0 / +400 mA
+250 / +400 mA
(a) (b)
Figure 18. (a) Open-loop position stability of the mirror at four different coil current combinations over 7 h with no drift in the set position.The experiment was performed with a Wyko optical profilometer. (b) Deviation of the angular position from the set point over theexperiment duration.
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J. Micromech. Microeng. 23 (2013) 025002 C Ataman et al
thermal properties are well controlled. A more straightforwardpath to improving the thermo-mechanical performance is toincrease the actuation force and use stiffer beams with higherthermal compliance, without compromising the rotation range.The device presented here can be modified in several ways toproduce larger actuation force:
• Given that the mirror temperature is kept well below theCurie temperature, the SmCo magnet can be replaced withan NdFeB one, roughly doubling the magnetic torquewithout any structural modification.
• Aluminum oxide has a thermal conductivity of10 W m−1 K−1, which is relatively low compared tothose of single-crystal silicon or Aluminum Nitride(170 W m−1 K−1). Implementing the planar coils on oneof these substrates would allow higher current densitieson the coil.
• The aspect ratio of the coils can be increased bymicrofabrication to increase the number of lines, andhence the current density, within the active area.
The mechanical nonlinearity of the springs and the slightdependence of the actuation force to the magnet position canbe problematic for a practical application by complicating thenecessary control and driving schemes. This issue of non-trivial drive versus position function, which is common inmicromechanical as well as conventional electromechanicalsystems, can be addressed in two major ways. Throughextensive characterization of the electromechanical behavior,comprehensive look-up tables can be established relatingthe bi-axial rotation space with the actuator behavior.Device addressing by look-up tables can potentially yield areasonable open-loop pointing precision given that, as it isfor the device presented in this work, it is coupled with goodposition stability and repeatability. For other applications withrequirements beyond the capabilities of open-loop operation,some form of angle monitoring is necessary. By detecting thegradient of the magnets, tri-axial Hall sensors can decode themirror rotation from the magnet position. Alternatively, the gapbetween the mirror and the membrane chip can be configuredto accommodate segmented capacitors to differentially detectthe mirror rotation through electrostatic means. On the otherhand, if volume constraints are less stringent, any conventionaloptical position tracking scheme (quad-detector, 2D positionsensitive devices, etc) can be employed for position feedbackin close-loop operation, as well.
6. Conclusions
An electromagnetically actuated two degree-of-freedommicromechanical tilting mirror for high thermal loadapplications is developed. The device consists of amicrofabricated compliant membrane bearing a single crystalsilicon mirror and a permanent micro-magnet and a 2-layerplanar coil for actuation. The two rotation axes exhibitexcellent decoupling and both provide more than ±8◦ ofmechanical rotation with 5.12 W power consumption. With1 KW m−2 continuous heat flux, the temperature increase onthe mirror surface is limited to 64 K, assuming zero ambient
pressure and a heat sink maintained at room temperature. Evenin open-loop operation, the device exhibits excellent positionstability and repeatability.
Acknowledgments
The authors would like to thank Dr Dara Bayat, Dr FabioJutzi, Dr Jonathan Masson for their help in the mirrorcharacterization, and Dr Terunobu Akiyama for his helpin the assembly process. The Division C of CSEM, andEuropean Space Agency are gratefully acknowledged forthe microfabrication facilities, and partial financial support,respectively.
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