development of surface micromachined magnetic actuators using electroplated permalloy

\PERGAMON Mechatronics 7 "0887# 502Ð522

9846Ð3047:87 ,*see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved[PII] S 9 8 4 6 Ð 3 0 4 7 " 8 7 # 9 9 9 0 5 Ð 5

Development of surface micromachinedmagnetic actuators using electroplated

permalloyChang Liu

Microelectronics Laboratory\ University of Illinois at Urbana!Champaign\ 202 MicroelectronicsLaboratory\ 197 North Wri`ht Street\ Urbana\ IL 50790\ U[S[A[

Received 02 September 0886^ accepted 01 January 0887

Abstract

Results on design\ fabrication and testing of silicon micromachined magnetic actuators arepresented[ Magnetic actuators are capable of providing large force "in the order of mN# andlarge displacement "micrometer to millimeter range# within micro electromechanical devicesand systems[ Electroplated permalloy "Ni79Fe19# material is the media for magnetic interactionand force generation[ The permalloy piece is supported by a structural plate\ which consists ofpolycrystalline silicon thin _lm[ Applications of such magnetic actuators in massive parallelassembly and high!yield surface!structure release are discussed[ Þ 0887 Elsevier Science Ltd[All rights reserved[

0[ Introduction

Micromachining technology and micro electromechanical systems "MEMS# havebeen undergoing dynamic development during the past 04 years ð0\ 1Ł[ MEMS o}ersunique advantages including miniaturization\ mass fabrication and monolithic inte!gration with microelectronics[ It has enabled successful demonstration of novelsensors\ actuators and systems in many diverse application areas such as opticsð2Ł\ ~uid mechanics ð08Ł\ biomedical engineering\ communication and informationstorage[ The characteristic length scale of micromachined devices ranges frommicrometer to millimeter[

� Corresponding author[ Tel[] 990 106 222 3940^ fax] 990 106 133 5264^ e!mail] changliuÝuiuc[edu

C[ Liu:Mechatronics 7 "0887# 502Ð522503

Actuators transform various forms of energy into the domain of mechanical energy[Force\ torque\ displacement or strain can be generated using a number of energy!transformation mechanisms[ These include electrostatic interaction\ magnetostaticinteraction\ ~uidic momentum transfer\ thermal expansion\ thermally!induced phasechange and piezoelectric e}ects[ The relative merits of these mechanisms are sum!marized below[ "0# Electrostatic interaction is widely used in micromachined sensorsand actuators\ including accelerometers ð3Ł\ rotation!rate gyro and micro opticalcomponents[ Comprehensive understanding of materials\ processing and mechanismshave been established[ Implementation of electrostatic mechanisms in MEMS isgenerally compatible with integrated circuit processes[ However\ the magnitude ofelectrostatic forces is known to decrease rapidly as the spacing between electrodesincreases[ This can be illustrated with the example of a parallel!plate capacitor\ whichrepresents a fundamental con_guration for electrostatic sensors and actuators[ For aparallel!plate capacitor with an overlapping area of A and a distance of d\ themagnitude of the attractive electrostatic force is linearly proportional to A andinversely proportional to d1[ As a consequence\ an actuator based on the parallel!plate capacitor con_guration can not simultaneously satisfy requirements for largeforce "in the order of mN# and large displacement "in the order of tens of mm#[Likewise\ most electrostatic!actuation mechanisms with various electrode con!_gurations su}er from the same limitations "1# Actuation based on thermal processes\on the other hand\ typically has the following disadvantages] high level of energyconsumption and a slow time response[ "2# With regard to piezoelectric actuation\ thepiezoelectric co!e.cient of typical active materials is low and large material dimensionis required if a large displacement is required[ The compatibility of high!qualitypiezoelectric _lms with conventional IC and MEMS processes remains an active andchallenging research topic[

Magnetic actuation is potentially capable of realizing both large force and largedisplacement in an energy!e.cient manner[ This performance advantage is derivedfrom fundamental di}erences of static magnetic and electrical _elds[ Many materials"e[g[ silicon and silicon dioxide# typically encountered in MEMS have low magneticsusceptibility and do not develop appreciably internal magnetization[ These materialsare therefore transparent to a static magnetic _eld^ it is feasible to active integratedmicro devices using a global\ external magnetic _eld[ On the other hand\ the electricalsusceptibility "xe# of most materials is greater than zero "e[g[ xe of silicon is 09[6#^therefore\ globally!applied electric _eld lines usually cannot penetrate material layerseasily[ High electrical _eld is also known to create material damages "e[g[ dielectricbreakdown#\ whereas high magnetic _eld is often not associated with material defects[Magnetic force is a body force^ the magnitude of magnetic interaction can be increasedby simply increasing the volume of the magnetic material[ Such a process involvesminimum process complexity[ The process required for accomplishing the volumeexpansion is rather straightforward[ In contrast\ the electrostatic force is a surfaceforce and a large electrode area must be provided if large forces are desired[ Theprocess for increasing the electrode areas\ in both vertical and horizontal con_gur!ations\ requires more complex processing procedures[

Several authors have published results on micromachined actuators in the past[

C[ Liu:Mechatronics 7 "0887# 502Ð522 504

Wagner et al[ ð4Ł manually attached precision!machined permanent magnet pieces onsuspended plates[ Integrated in!plane coils on the same chip generate an externalmagnetic _eld[ Due to the relatively large volume of the permanent magnet\ largeforce can be demonstrated[ However\ such a permanent!magnet piece can only beachieved through precision assembly of discrete units and is not suited for integratedfabrication and packaging[ Liu et al[ developed an integrated coil!type magneticactuator capable of achieving out!of!plane vertical displacement of several hundredmicrometers and magnetic forces of 09 s of mN ð5\ 6Ł[ A torque is generated via theinteraction between an external magnetic _eld and the magnetic moment of a planarelectric coil[ Unfortunately\ coil!type actuators typically require large biasing electriccurrent "½ 49 mA#[ This current level\ coupled with a long wire length and largenumber of turns\ potentially causes signi_cant thermal heating problems[ Judy et al[demonstrated in!plane motion of a suspended polycrystalline silicon structure withan electroplated magnetic piece ð8Ł[ The plate was driven by an external magnetic_eld[ The actuator achieved large de~ection angle "over 079># under a torque ofapproximately 9[074 nNm[ Recently\ Miller et al[ ð7Ł and Judy et al[ ð09Ł have dem!onstrated permalloy magnetic actuators capable of individual addressing[ In additionto unit actuators\ complex electromagnetic sub!systems\ including a planar elec!tromagnet ð00Ł and magnetic micromotors ð01Ł\ have also been developed[

In this paper results will be presented on the design\ fabrication and testing ofsurface micromachined magnetic actuators based on electroplated permalloymaterials[ Two applications of these actuators will also be reviewed[

1[ Theories of operation

Schematic diagrams of three types of developed actuators are shown in Fig[ 0[ Acommon component of these actuators is a thin!_lm structure plate that supports anelectroplated permalloy piece[ The permalloy piece generates mechanical force andtorque when it is placed within a magnetic _eld[ These actuators are distinguished bythe nature of their mechanical supports\ which are based on cantilever beams "Type!0 actuator\ Fig[ 0a#\ torsion beams "Type!1 actuator\ Fig[ 0b# or mechanical hinges

Fig[ 0[ Schematic diagram of surface!micromachined magnetic actuators] "a# with cantilever!beam supports"Type!0 actuator#^ "b# with torsion beam supports "Type!1 actuator#^ "c# with a plain hinge support^ "d# ahinge support in conjunction with cantilever!spring loading "Type!2 actuators#[


"Type!2 actuator\ Fig[ 0c\d#[ The structure plate and support beams all use poly!crystalline silicon thin _lms[

The mechanism of actuation is illustrated using the example of a Type!0 actuator"Fig[ 0a and Fig[ 1#[ Key physical dimensions of the actuator are identi_ed in Fig[ 1[When the external magnetic _eld is zero\ the structural plate is parallel to the substrateplane "Fig[ 2a#[ When an external magnetic _eld \ Hext\ is applied normal to the planeof the structure plate\ a magnetization Ms develops within the permalloy piece andsubsequently interacts with Hext "Fig[ 2b#[ The interaction creates a torque "Mmag# anda small force "F#\ which causes the beam to bend out!of!plane "Fig[ 2c#[

An analysis of the quasi!static characteristics of these actuators is provided in thefollowing two sections[ The torque Mmag and force F due to magnetic interaction will_rst be analyzed[ The overall displacement of the actuator is then derived from thebalance of torque and force[

1[0[ Torques and forces due to ma`netic interaction

The magnetization vector "M# within the permalloy material is in~uenced both bythe externally applied magnetic _eld and by the angular position of the magneticpiece[ The direction of M is governed by the geometric shape of the permalloy piece[

Fig[ 1[ Schematic of a Type!0 permalloy magnetic actuator with two cantilever!beam supports] "a# topview and "b# side view[


Fig[ 2[ Schematic illustration of the mechanism of Type!0 magnetic actuator biased using an externalelectromagnet] "a# rest position of the actuator when Hext � 9^ "b# out!of!plane actuation when Hext � 9 isprovided by an external electromagnet*F0 and F1 are the induced magnetic forces on the upper and loweredges of the plate^ "c# a simpli_ed force system "containing Mmag and F# acting at the free!ends of thecantilever beams[

In the actuator shown in Fig[ 1\ the lateral dimension of the permalloy piece is muchgreater compared with the thickness^ the direction of M is within the plane andperpendicular to the direction of the thickness[ The magnitude of the magnetizationcan be obtained from the BÐH curve "Fig[ 6# of the permalloy material[ "For high!permeability magnetic material such as permalloy\ the magnetization\ M\ and theinternal magnetic ~ux density\ B\ are almost identical in magnitude#[ Since the remnantmagnetization is low\ the permalloy piece is considered non!magnetized when theexternal magnetic _eld\ H\ is zero[ As H increases\ =M= grows linearly until thesaturation magnetization Ms is reached[ Since the permeability of the material is high"mr ½ 3499#\ saturation of magnetization occurs at a relatively low H\ which is denotedas Hk[

The current actuator design focuses on the large!displacement regime and doesnot involve comprehensive modeling of actuator behavior at low _eld levels "belowsaturation#[ When an external bias is applied\ the permalloy material is treated ashaving a _xed in!plane magnetization with its magnitude equal to the saturationmagnetization\ Ms[

The force and torque acting on the permalloy piece is estimated using models ofe}ective magnetic charges[ It is assumed that two magnetic charges of oppositepolarities emerge along the upper and lower edges of the permalloy plate when thematerial is magnetized[ These two charges generate F0 "acting at the upper edge# and


F1 "acting at the lower edge#"Fig[ 2b#[ The magnitudes of these two force componentsare

F0 �Ms =W =T =H0

F1 �Ms =W =T =H1 "0#

Where H0 and H1 are the magnetic _eld strengths at the top and bottom edges of theplate "H1 ×H0 in the current con_guration#[ The magnitudes of H0 and H1 are linearlydependent on the respective distance to the surface of the electromagnetic core[

The structure plate\ along with the permalloy piece\ has a thickness of t¦T[ Itsmoment of inertia\ I\ is proportional to "t¦T#2 and is much greater compared withthat of the cantilever beam\ which has a thickness of t[ The combined structure plateand the permalloy piece is thus considered as a rigid body[ Based on this assumption\the force system is simpli_ed by translating F0 to coincide with F1[ The result is acounter!clockwise torque Mmag and a point force F "Fig[ 2# acting on the bottom edgeof the structural plate[ These are expressed as

Mma` �F0L cos u

F�F1−F0 "1#

The torque always tends to minimize the overall energy in an actuator system byaligning the magnetization with the magnetic _eld lines[

1[1[ Displacement analysis

The pro_le of the actuator is solved by coupling the magnetic torque and force tothe supports[ The analysis for three types of actuators is presented in the following]

1[1[0[ Type!0 actuator with cantilever!beam supportsThe angular as well as vertical de~ections due to Mmag and F are solved independentlyand the results are linearly super!imposed[ This simpli_cation is justi_ed because themagnitude of de~ection due to F is estimated to be at least one order of magnitudesmaller compared with the de~ection caused by Mmag[

Beam displacement under the Mmag is solved _rst[ The magnitude of the verticaldisplacement at the free!end of the beam is much more than the thickness of thebeam^ linearized\ small!displacement assumptions are no longer valid[ The de~ectionof the cantilever beam is determined by a universal governing equation ð03Ł thatrelates the radius of curvature with Mmag "Fig[ 3#

0r�−

−yýx

"0¦y?x1#2:1

�Mmag

EI"2#

Here\ x and y are the horizontal and vertical coordinates of a point along the cantileverbeam at an arc length of s\ E and I are the Young|s modulus and the moment ofinertia of the cantilever beam\ respectively[


Fig[ 3[ Schematic diagram of a ~exure cantilever beam under the magnetic torque Mmag[

The cantilever beam assumes the shape of a circular arc\ with the radius of curvaturebeing r[ The angular de~ection is the greatest at the free!end of the cantilever beam"s� l# and can be written as

utorque �lr

"3#

The x and y coordinates at s� l are therefore

x"s�l# �r sin 0lr1

y"s�l# �r $0 cos 0lr1% "4#

Beam bending due to the force is solved by applying F at the free end of a pre!curvedbeam under the in~uence of Mmag[ The maximum angular and vertical de~ectionsoccur at the end of the cantilever beam[ These de~ections can be expressed as ð03Ł

uforce �"p:1−0#FR1

EI

yforce �"2p:3−1#FR1

EI"5#

The overall angular de~ection of the beam "at s� l# is obtained by combining of eqn3 and eqn 5\


u"s�l# � utorque−uforce[ "7#

The maximum vertical de~ection at the end of the rigid structural plate is

ymax �y"s�l#−yforce¦L = sin u"s�l# "8#

1[1[1[ Type!1 actuators with torsion!beam supportsThe above analysis for a Type!0 actuator considers only the pure bending mode underideal loading conditions[ In the pure bending mode\ cantilever beams can withstand079> bending without fracture[ However\ non!ideal external loading conditions occurfrequently in certain applications such as ~uid!dynamical control[ Non!ideal loadinggenerates non!desirable mode of displacements[ One example of an undesired modeoccurring in Type!0 actuators is twisting along an axis that is parallel with cantileverbeams[ It has been experimentally observed that this twisting motion introduces aprevalent failure mode that causes a large percentage of devices to fracture[

Type!1 actuators with torsion!beam supports successfully suppress the twistingmotion[ These are more robust compared with Type!0 actuators[ For a Type!1actuator\ the angular displacement is related to the torque by the following expression\

Mmag �1u

lKG "09#

where u is the angular displacement experienced by each torsion beam\ l is the lengthof each torsion beam and G is the torsion modulus of elasticity of the material[ K isa constant determined by the speci_c cross!sectional geometry of the beams^ for atorsion beam with a rectangular cross!section and an area of w×t\

k�ab2 $052

−2[25ab 00−

b3

01a31% "00#

where a�w:1 and b� t:1[ The force F also creates bending within the support beamsthat contribute to an out!of!plane translation[ However\ this displacement is small"due to the _xed!_xed beam boundary conditions# and is typically ignored in ouranalysis[

1[1[2[ Type!2 actuators with hin`e supportsThere are two variations within Type!2 actuators] one is supported by a plain hingeand another is supported by a hinge and an add!on spring!loading mechanism[ Aplain hinge does not provide any restoring torque that can balance Mmag[ A Type!2actuator with a plain hinge "Fig[ 0c# would experience a net magnetic torque until a89> displacement is reached\ at which point Mmag is equal to zero[ Such an actuatorhas only two possible positions\ with the angular displacement being either 9 or 89>[However\ if the objective of a hinged magnetic actuator is to provide controllabledisplacement\ a cantilever or a torsion beam must be used to provide a counteractingtorque "Fig[ 0d#[ A spring!loading mechanism will generate such a torque and allowthe hinged actuator to achieve continuously variable positions[ The analysis can bepursued in a similar fashion as Type!0 actuators[


1[2[ Summary of actuator desi`n

A number of geometric parameters are _xed within our design[ The rigid structureplate has a _xed area of 0×0 mm1[ The thickness of the magnetic material is 4mm[For Type!0 actuators\ the typical length and width of the cantilever beam are 399 and099 mm respectively[ For Type!1 actuators\ the width and length of the torsion beamare 1 and 49 mm\ respectively[

2[ Fabrication

The fabrication process for a typical magnetic actuator is summarized in Fig[ 4[The entire process is divided into _ve steps and discussed accordingly[

2[0[ Process

2[0[0[ Step 0 "Fi`[ 4a#[ A 2!mm!thick phosphosilicate glass "PSG# thin _lm is _rstdeposited on top of the silicon substrate at 349>C[ The thin _lm functions as asacri_cial material[ The PSG layer is patterned using photolithography and thenetched using bu}ered hydro~uoric acid "BHF# to form separated mesas on top ofwhich individual actuators will be located[ These mesas isolate individual actuatorsand limit the total amount of lateral dimension expansion that will result fromundercut during the sacri_cial!layer etching processing[ This feature therefore pro!vides robust process control and results in high structural yield even when over!etching is applied[ It also increases the potential area density of actuators by allowingactuators to be placed closer to one another[

After removal of the photoresist layer\ the wafer is annealed in a nitrogen ambientat 0999>C for one h[ This step serves two purposes[ First it activates the phosphorousdopant "5 wt)# within the PSG layer and increases its etch rate by BHF[ Secondly\the PSG material re~ows slightly at the temperature of the oxidation\ creating roun!ded\ smooth pro_le along the perimeter of PSG mesas[ The wafer is then covered bya highly conformal deposition of thin LPCVD polycrystalline silicon[ The improvedpro_le of the mesas translates directly into rounded corners in structural layers[ Therounding alleviates stress concentration and enhances the reliability of actuators[

A 9[4!mm!thick PSG layer is then deposited on top of the polysilicon[ It serves as acomplimentary doping source[ During a 0 h\ 849>C stress!relief anneal in nitrogenambient\ the polysilicon is doped symmetrically from both sides[ The symmetricdoping reduces the intrinsic!stress gradient across the thickness of the polysilicon andminimizes residue beam bending[ In contrast\ a doping from only one side wouldgenerate a non!symmetric doping concentration and a stress gradient[ The top PSGlayer is later removed by using BHF[

2[0[1[ Step 1 "Fi`[ 4b#Before performing the electroplating\ an electrically conductive seedlayer must beapplied to the front surface of the wafer[ The seedlayer contains 199 _!thick Cr and


Fig[ 4[ Major fabrication steps of a Type!0 actuator[

0799 _!thick Cu thin _lms[ Both layers are thermally evaporated[ The Cr layerenhances adhesion between the Cu and the polysilicon layers[ For seedlayer evap!oration\ it is critical to maintain electrical continuity throughout the entire wafer[ Thecontinuity can potentially be broken because coverage of thermally evaporated metalis not conformal "compared with LPCVD polysilicon# and the deposition thicknesson vertical sidewalls is much reduced compared to the nominal value[ If an insu.cientamount of seedlayer material is deposited\ the active region on the top of the mesacould be electrically isolated from the seed!layer region on the bottom[ Electroplatingwill not occur on top of each mesa[


The front surface of the wafer must be protected after the wafer is removed fromthe vacuum ambient of the evaporation chamber[ Copper thin _lm oxidizes readilyin air and the resulted copper oxide layer will hinder the electroplating process[ Caremust be taken to ensure that the copper layer is not exposed to air and water forlong periods of time[ In the current development a 4!mm!thick photoresist layer isimmediately applied to the wafer to insulate the copper _lm[

The photoresist is patterned and developed only immediately before electroplating[Patterned photoresist form narrow frames[ In regions that are not covered by thephotoresist\ the seedlayer is exposed and permalloy "Ni79Fe19# electroplating will takeplace "Fig[ 4#[

2[0[2[ Step 2 "Fi`[ 4c#The recipe and technique for permalloy electroplating was originally developed in thethin _lm read:write!head industry[ During the plating process\ the wafer is a.xed tothe cathode and a pure Ni piece serves as the anode[ An external biasing magnet "349Oe# is applied with the _eld lines being parallel to the wafer substrate[ This biasestablishes directions of preferred magnetization "easy axis# within the permalloypiece[ Electroplating takes place at a rate of 4 mm:h under a bias!current density of 7to 01 mA:cm−1[ Two di}erent plating techniques are available] mold plating andframe plating[ In the mode!plating technique\ photoresist covers all area of the waferexcept where permalloy is intended[ In the frame!plating technique\ which is appliedin this study\ plating occurs over a majority portion of the wafer area[ Narrowphotoresist frames isolate regions on a wafer[ In certain regions\ the plated magneticmaterial is not used and it is selectively after the is completed[ Frame plating techniqueallows more uniform electroplating and the plating parameter is not varied when thegeometry is changed[

2[0[3[ Step 3 "Fi`[ 4d#After electroplating\ the wafer is ~ood!exposed with ultraviolet radiation and thephotoresist is removed with a standard photoresist developer[ The wafer is furthercleaned using acetone and then isopropanal alcohol solutions[

2[0[4[ Step 4 "Fi`[ 4e#The seedlayer that is currently exposed will be removed by using Cu etchant "099 ] 4 ] 4wt water ] acetic acid ] hydrogen peroxide# and then a Cr!mask etchant[ The Cr!layerremoval can be accomplished using either a commercial etchant ð06Ł or diluted HCL"Cr etchant ] 09 water ] 0 HCL#[ It typically requires 09 s to remove the Cr layer usingdiluted HCl[ Occasionally\ Cr etch does not occur spontaneously^ the Cr layer staysintact even when the wafers are immersed in Cr etchants for up to 099 times thenominal etching time[ This phenomenon is believed to result from non!favorableelectro!chemical potential on the wafer[ To solve this issue\ the electrochemical poten!tial of the seedlayer is modi_ed by contacting the Cr layer with a piece of purealuminum[

Actuators are then released by 38) HF within 19 minutes[ To facilitate the sac!ri_cial release process\ etch holes "29×29 mm in area\ and 149 mm apart# are opened


on the plate[ The permalloy material sustains HF etching without any structural orchemical damage[

Since the structure plates have large surface areas and the supporting beams aresoft "spring constant ½099 mN:0 mm�9[0 N:m for cantilever beams#\ these can beeasily pulled down by surface tension to the substrate and form permanent bonds ð13\15Ł if conventional drying techniques are used[ To ensure high yield\ the structuralplate is levitated away from the substrate surface through magnetic interactions[ Thismethod e}ectively prevents the actuators from coming into contact with the substrate\therefore guaranteeing that 099) yield is routinely achieved[ More details of therelease technique will be reviewed in section 4[0[

Shown in Fig[ 5 are top and perspective views of fabricated actuators\ which exhibitno intrinsic bending[

2[1[ Ma`netic properties of the permalloy material

The electroplated permalloy has a composition of 79) Nickel and 19) Iron ð05Ł[Thin!_lm permalloy is a preferred soft magnetic material for two main reasons] _rst\the material exhibits a near!zero magnetostriction e}ect and stress!free _lms can berealized^ secondly\ the magnetic switching speed is fast "in the order of fsÐms#[

The permalloy material has a poly!crystalline structure and contains a large numberof magnetic domains[ Each magnetic domain has 0909Ð0904 atoms and is spontaneouslymagnetized in one direction at room temperature[ The directions of magnetization ofdi}erent domains are randomly organized[ Despite this\ there are directions of easy

Fig[ 5[ SEM micrograph of] "a# top view and "b# perspective view of a Type!0 actuator[


Fig[ 6[ Calibrated BÐH loop "in both easy and hard axes# of the permalloy thin _lm[

and hard magnetization\ a phenomena called crystalline anisotropy[ Hk is de_ned asthe magnetic _eld intensity needed to saturate a soft magnetic material in a speci_cdirection "Fig[ 6#[ In the direction of each magnetization\ the easy axis Hk\easy of thehysteresis is small[ In the direction of di.cult magnetization\ the hard axis\ Hk\hard isgreater[ Experimental BÐH hysteresis curves along the easy axis and the hard axis areshown in Fig[ 6[ During the NiFe electroplating\ the direction of the biasing magnetic_eld dictates the orientation of the easy axis ð11\ 12Ł^ domain walls move in such amanner that domains favorably oriented with the magnetic _eld grow at the expenseof unfavorably oriented domains[ "It should be noted that the shape of the permalloyplate also favors shape!anisotropy with the plate plant[# Therefore\ the directionparallel with the external magnetic _eld during the plating process is the easy axiswhile the orthogonal in!plane direction is the hard axis[ This phenomenon has beenstudied by Takahashi ð11Ł^ it has been found that an external _eld of H− 29 Oe issu.cient to induce the direction order[ Other methods for establishing the easy axisinclude critical cold working and annealing[ The permalloy material used in thecurrent study show only a minor di}erence in magnetic behavior between easy andhard axes[ The properties of the electroplated magnetic material are summarized inTable 0[

3[ Testing

Actuation is quantitatively characterized by using a microscope monitoring system"Fig[ 7#[ An electromagnet provides an external magnetic _eld[ The magnetic core hasa large cross!sectional area "2×2 cm1# and therefore provides uniform _eld density[


Table 0Measured properties of permalloy magnetic thin _lm

Property Value

Saturation magnetization Ms 0Ð0[4 TeslaCoercive force Hc 9[5 Oe "36 A:m#Relative permeability mr 3499

Fig[ 7[ Video!microscopy inspection setup[

The variation of H with respect to the vertical spacing "d# to surface of the magneticcore is calibrated experimentally[ In a region near the surface of the core\ H is nearlylinear with respect to the spacing d^ it can be expressed as

H� 03×093−1[7×093d "01#

The units of d and H are mm and A:m\ respectively[ In their resting positions\actuators are separated from the electromagnet by a distance of 9[4 mm\ which is thethickness of the silicon substrate[

Figure 8 contains a sequence of video images showing the position of a Type!0actuator at the resting position and activated positions[ The angular and verticaldisplacement of the actuator is measured directly from the side pro_le of the actuator[Figure 09 shows the measured de~ection magnitude\ together with theoretical pre!dictions obtained using eqns 7 and 8[ Results from theory and experiments matchwell\ especially in the saturation regime[


Fig[ 8[ Sequential video images of a Type!0 magnetic actuator] "a# before applying the magnetic _eld^ "b#when Hext � 2[23×093 A:m^ "c# at the maximum displacement[

The behavior of magnetic actuators in low _eld situations is also experimentallycharacterized "Fig[ 00#[ An actuator is _rst activated with the magnetic _eld increasingin one polarity[ The polarity is then reversed and the magnitude of the magnetic biasis increased again[ The angular!de~ection curves for both cases highlight a peculiarbend at the low _eld level "H below 01×092 A:m#[ This lowered angular displacementis contributed by the switching of the magnetic domain[

The maximum response speed of magnetic actuator is studied[ The time constantis currently not limited by the domain switching process\ which dominates the estab!lishment of magnetization and has a typical time constant in the order of one ps[Rather\ the maximum response speed is limited by the electromagnet that is modulatedusing mechanical switches[ the relatively large size and inductance of the electromagnetincreases the time\ which has been measured to be between 0 and 09 ms[ The time!constant measurement was conducted by using a miniature secondary coil placed onthe front surface of the core[ If higher response speed is intended\ magnets withsmaller inductance must be developed[

to measure the vertical force!loading capacity of actuators under the current exper!imental conditions\ a 4×4 array of actuators is used to hold controlled weights\ inthe form of precision cut silicon chips[ The strongest magnetic _eld achievable using


Fig[ 09[ Angular and vertical displacement of the structure plate with respect to the biasing magnetic _eldintensityH] "a# theoretical and experimental rotation angle u^ "b# vertical de~ection ymax[ The size of theplate is 0×0 mm1\ the beam length and width are 399 and 099 mm\ respectively[ The beam thickness is 0mm[

Fig[ 00[ Hysteresis behavior of a Type!0 actuator under forward and reverse biasing conditions[

the current setup con_guration is applied to activate the entire array[ Silicon chips ofknown weights are sequentially stacked on top of raised actuators\ until the actuatorarray can no longer hold against the stack weight[ The amount of weight that can beheld by an array of actuators in their fully raised position is de_ned as the maximum


vertical loading capacity "MVLC#[ The measured MVLC weighs 111 mg\ or 1[07 mN[This translates into roughly a maximum loading force of 76 mN "or 7[77 mg# for eachactuator\ which has a mass of only 33[4 mg itself[ It should be noted that the loadingcapacity is a function of the angular position[ At lower displacement angles\ the sameweight would produce more torque[ The MVLC can not be lifted by the actuatorsfrom their rest positions[ In pure bending modes\ actuators with cantilever!beam andtorsion!beam supports can achieve more than 079> displacement without fracture[Fracture strain equals 9[82) according to an early report by Y[ C[ Tai et al[ ð04Ł[This unique characteristic is due to the reduced thickness of the cantilever beam[

4[ Applications

Developed magnetic actuators have been used in magnetic!assisted levitation "forsurface structure release#\ massively parallel assembly of array MEMS\ ~uid dynamiccontrol ð08Ł and active robotics surfaces[ The _rst two applications are described indetail in sections 4[0[ and 4[1[

4[0[ Ma`netic assisted levitation

Successful use of array MEMS devices demands a robust\ e.cient and high!yieldfabrication process[ Surface micromachined devices are typically developed usingsacri_cial!layer etching techniques[ Devices are freed by removing the underlyingsacri_cial layer in a wet chemical ambient "e[g[ concentrated HF solution for removingPSG#[ A drying process follows the wet chemical etching[ This step is known to causesigni_cant yield losses due to stiction "sticking and friction#[ The mechanism forstiction is the following] during the drying process\ the liquid on top of a free!standingstructure will evaporate _rst^ liquid that is trapped underneath structures remains[The trapped liquid exerts a pull!down force due to surface tension[ Because surfacemicrostructures are located close to the substrate "with spacing of only several mm#and are typically compliant "with spring constant ³0 N:m#\ the pull!down force iscapable of drawing the structure into intimate contact with the substrate[ IN manycases\ this contact produces permanently bonding and irreversible sticking ð13Ð16Ł[The probability of stiction damage increases with decreasing structural sti}ness andincreasing surface!contact area[

Previously published results on anti!stiction techniques focus on the followingmethodologies[ First\ certain post!release chemicals modify the surface!layer com!position ð15\ 17Ł and prevent sticking by chemical bonds[ Secondly\ the liquidÐvaporphase transformation\ which is the cause of the surface!tension force\ can be replacedby a freezeÐsublimation procedure ð17\ 20Ł[ Third microstructures can be kept awayfrom the substrate during release:drying by solid organic polymer columns\ which arethen removed by plasma dry etching ð20Ð21Ł[ Other novel techniques include usingspecial anti!stiction geometry ð16Ł\ applying pulsed magnetic forces to relieve stuckstructures ð22Ł\ roughening contact surfaces and reducing contact areas ð23Ł and usinggas phase etchant for sacri_cial!layer removal ð24Ł[


In practice\ many microstructures have relatively large surface areas "e[g[ ×0×0mm1#[ The probability of stiction!induced damages is higher\ as demonstrated by aninitial low yield "09)# for Type!0 actuators when no speci_c drying technique wasapplied[ To achieve high!yield drying for large!area structures\ a novel drying processhas been developed[ During the liquid removal process\ freestanding structures areactively levitated out!of!plane^ the liquid is removed\ after which suspended structuresare returned to the substrate plane[ This method prevents stiction because levitationforce counteracts the surface tension force so microstructures and the substrate willnever come into contact[ This process requires that an additional patch of permalloymaterial be integrated with individual micro devices[

E}ectiveness of this method is demonstrated using developed Type!0 magneticactuators[ Fabricated test structures "so called dies# were _rst immersed in HF solution"39)# for 09 minutes to remove the sacri_cial layer[ These wet dies are then transferreddirectly to de!ionized "DI# water and immersed for a period of time to completelyremove the HF contents[ Dies are then immersed in a _nal rinse solution\ which isone of the following chemicals] isopropyl alcohol\ methyl alcohol\ acetone or water[Wet dies are placed within the magnetic _eld of an electromagnet and air!dried[ Aninfrared lamp is used to provide heat and accelerate the evaporation process[ Theresult shows that 099) yield is routinely achieved[

4[1[ Parallel assembly of MEMS

MEMS technology is inherently characterized by mass production and three!dimen!sional structures[ Arrayed MEMS devices will undoubtedly o}er unique advantagesunavailable in macro!scopic\ conventional systems and singular micro devices[ Very!large!scale!integrated circuits "VLSI# exempli_es the tremendous bene_ts o}ered byarray operation of modular components "e[g[ transistors#[ Nowadays\ many MEMSdevices are developed based on fundamentally two!dimensional fabrication tech!niques[ In order to realize true 2!D structures\ a fabrication process for developingthree!dimensional devices from as!fabricated\ two!dimensional layers is required[Such a process must o}er high yield as well as global "instead of local# addressability[

One notable example is a hinged surface microstructure that has been applied formany applications[ However\ the wide use of this technology depends on an e.cient\robust and high!yield parallel assembly technology[ In this study\ Type!2 actuatorsare used to demonstrate the massive parallel assembly of these hinged surface micro!structures[ Type!2 actuators with hinges and spring loading provide a means ofsequentially levitation\ which allows for inter!locking mechanism and assembledmicrostructures "Fig[ 01b#[ Array devices can be assembled simultaneously by aglobally!applied magnetic _eld[ The mechanism is demonstrated in Fig[ 01a[ Twoactuators\ structure!0 and !1\ are attached with di}erent spring loading[ Both struc!tures are in their rest position when the magnetic _eld is zero[ As the external magnetic_eld reaches H0\ structure!0 is fully activated "de~ection angle�89># _rst whilestructure!1 has a much smaller angular displacement[ Structure!1 is then actuatedand locks structure!0 in place when the external magnetic _eld is increased to H1[


Fig[ 01[ "a# Schematic diagram of the assembly process^ "b# SEM micrograph of an assembled array of 2!D microstructures[

5[ Conclusions

Design\ fabrication and testing results of surface micromachined actuator has beenpresented[ Electroplated permalloy material is used to provide magnetic interactionand generate force:torque[ The magnetic actuators are mechanically supported by~exural cantilever beam\ torsion beams or hinges[ The advantage of magnetic actu!ators is to satisfy requirements for large force and large displacement simultaneously[This aspect has been demonstrated in the developed actuators[ Applications of mag!netic actuators in high!yield release:drying of surface microstructures and parallelassembly of array MEMS has been discussed in detail[

Acknowledgements

The author wishes to thank Prof[ Yu!Chong Tai\ Prof[ Chih!Ming Ho\ ThomasTsao\ Dr Weilong Tang and Dr Denny K[ Miu for their helpful discussions

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