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Sensors and Actuators A 172 (2011) 462– 470

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

j ourna l h o me pa ge: www.elsev ier .com/ locate /sna

icron-scale polymer–metal cantilever actuators fabricated by focused ion beam

.C. Leea, G. Alicib, G.M. Spinksb, G. Prousta,c, J.M. Cairneya,∗

Australian Centre for Microscopy & Microanalysis, University of Sydney, AustraliaSchool of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, AustraliaSchool of Civil Engineering, University of Sydney, Australia

r t i c l e i n f o

rticle history:eceived 17 May 2011eceived in revised form 29 August 2011ccepted 29 August 2011vailable online 7 September 2011

a b s t r a c t

Polymer–metal microcantilever actuators have been fabricated using an innovative approach based onfocused ion beam micromachining technology. The fabrication involves depositing a thin metal filmonto the surface of the polymer and machining using the ion beam. The microcantilever created is thenextracted and transferred to a desirable support using a micromanipulator. This approach demonstratesthe potential for maskless and resistless prototyping of cantilevers that can be evaluated for use as

eywords:olystyrene–platinum microcantileverocused ion beamhermal microactuatornalytical modelinite element model

MEMS/NEMS actuators. Nanometer-scale displacement of the resulting polystyrene–platinum bimorphmicroactuator with respect to temperature change is demonstrated via visual monitoring in a scanningelectron microscope with a heating stage. The performance of the bimorph cantilever microactuators isverified using both analytical and finite element modeling.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

Cantilevers, also known as single-clamped suspended struc-ures, are of interest due to their simplicity and the fact that theseimple structures are readily used as micro/nanoelectromechanicalystems (MEMS/NEMS) sensors and actuators. Micrometer-sizedantilevers, such as atomic force microscopy (AFM) probes, are theimplest MEMS, and can also be considered as basic building blocksor a variety of more complex MEMS devices. Polymer MEMS [1,2],ncluding cantilever MEMS/NEMS sensors and actuators [3,4] haveeen widely investigated over the past decades.

The modes of operation of these cantilevers are extensive, rang-ng from electrostatic to thermal, and are accompanied by a varietyf displacement read-out schemes [4,5]. Thermal actuation is oneechnique that exhibits significant potential in MEMS/NEMS tech-ology [5]. This method can be realized using a bi-layer structurealso known as a bimorph) with different coefficients of thermal

xpansion. If the bimorph is exposed to a change in temperature,he cantilever will bend. The tip deflection can be predicted basedn a classical bimetallic model by Timoshenko [6] in which the tipeflection of a bimorph cantilever is given by:

∗ Corresponding author.E-mail address: julie.cairney@sydney.edu.au (J.M. Cairney).

924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2011.08.022

ı = L2

�

= L2

2

[6E1E2b1b2h1h2(h1+h2)(˛2−˛1)�T

4E1E2b1b2h31h2+4E1E2b1b2h1h3

2 + 6E1E2b1b2h21h2

1+E21b1

2h41 + E2

2b22h4

2

]

(1)

where � is the radius of curvature, L is the length, h is the thickness,b is the width, E is the Young’s modulus, ̨ is the thermal expansioncoefficient and �T is the change in temperature. The subscripts 1and 2 refer to each layer.

Assuming that the width ratio of the bimorph cantilever is equalto unity, the expression in Eq. (1) can be simplified to a term givenby:

ı = L2

2�

= 3L2�˛�T(1 + (h1/h2))2

h[(h1/h2)3(E1/E2)+4(h1/h2)2+6(h1/h2) + (h2/h1)(E2/E1) + 4)](2)

The displacement amplitude of a bimorph cantilever can beoptimized by choosing appropriate cantilever lengths and layerthicknesses as well as relatively large differences in ther-

mal expansion coefficient. The combination of polymer–metalor polymer–ceramic bimorphs can dramatically enhance thethermally induced bending compared to that of conventionalmetal–ceramic bimorphs [7–9].

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raop

C.C. Lee et al. / Sensors and

Microcantilevers are traditionally fabricated using a combina-ion of conventional lithography and dry/wet etch techniques [10].hese methods provide the possibility of mass production; how-ver, they do pose some drawbacks such as complicated fabricationteps and poor dimensional control. Two critical issues are thenability to scale to a much smaller size and the limit to the typesf materials that can be fabricated. For example, photolithographys restricted by the light source, the availability of suitable photo-

asks and the necessary multi-step photo-resist etching processes.lthough this technique is commonly used in integrated circuitnd MEMS/NEMS, the resolution is limited by the wavelength ofhe light source to a minimum of around 50 nm and it requiresxtremely clean operating conditions and a flat substrate.

Focused ion beam (FIB) micro/nanomachining is an alterna-ive technique that can be used to fabricate structures of sizess small as 10 nm [11]. A FIB is a powerful and versatile tool foricro/nanofabrication prototyping [12–14], materials characteri-

ation [15,16], microscopy specimen preparation [17–19], deviceodification [20] and ion lithography [21]. This maskless tech-

ology has emerged as a practical technique for machining andepositing a wide range of materials with nanoscale precision.ased on accelerated and focused ions with a beam diameter wellelow 10 nm, current commercial FIBs are capable of milling virtu-lly any hard or soft material.

An additional deposition capability is achieved by ion-assistedecomposition of a gas precursor containing the specific materialo be deposited, be it electrically conducting or insulating. Com-ining SEM and FIB columns on one platform further expands theicro/nanofabrication capabilities [22,23]. The electron beam can

e used for both imaging and electron-beam-assisted deposition.eposition with the electron beam creates less damage but is much

lower than the ion beam-induced deposition at a given beam cur-ent. In practice, the FIB is generally used for processing tasks suchs milling and deposition, while the SEM is used for imaging.

In this study, deposition and milling were used to create free-tanding polymer–metal cantilevers for use as thermal actuators.

lift-out approach was employed to release the cantilevers. Thisechnique, which requires the use of an in situ micromanipula-or system, is classically used for atom probe tomography andransmission electron microscopy specimen preparation, but isncommon in micro/nanofabrication. The cantilever is extractedrom the bulk specimen and transferred to a new support. This

ethod eliminates the need of a sacrificial layer, hence abolishesdditional resist coating and long undercut etching processes. Ourpproach is different from previous work where FIB technologyas been used to fabricate inorganic microcantilevers in which theantilevers were still attached to the bulk specimen [24,25].

Most FIB applications involve metals and semiconductors, whilets application to soft materials such as polymers is an emergingrea. The understanding of ion-induced damage is very importantor the application of FIB technology to polymers. Some criticalssues that need to be addressed for polymer micro/nanofabricationnclude chemical changes due to ion–polymer interaction, ionmplantation and charge build-up due to their insulating nature,ocal heating due to their low thermal conductivity, and changes inhe surface morphology under the beam. However, these problemsan potentially be reduced using several supplementary techniquesuch as milling the polymer at cryogenic temperature to reduceocal heating, applying an electrically conductive coating or usingn electron flood gun against ion charging and removing the poly-er at a faster rate using water vapour [26–29].Here, we report an innovative approach to the fabrication of

eleased polymer–metal microcantilevers as thermal microactu-tors. We test the viability of FIB as a tool for the fabricationf polymer–metal bimorph microactuators composed of spin-castolystyrene and ion-induced platinum. We also assess the effect of

ors A 172 (2011) 462– 470 463

ion-induced artifacts such as ion implantation and structural dam-age with respect to ion beam characteristics. An SEM equipped witha heating stage is used to obtain the displacement of the microac-tuator and the actuation is compared with the predicted results.

2. Methodology

A cantilever structure designed for thermally actuated deviceswas fabricated using a combination of spin coating (Laurel Technolo-gies WS400B-6NPP/LITE), sputter coating (EmiTech K550X) and theuse of a dual-beam FIB-SEM workstation (FEI Quanta 200). For poly-mer film preparation, a drop of polystyrene solution is first cast ontothe surface of cleaned glass slide while being rotated at 7000 rpmfor 120 s. The film is then annealed on a hot plate at 75 ◦C for 2 hto fully remove the solvent. The film is peeled from the glass witha sharp razor and mounted onto an aluminium stub using carbonadhesive tape. In order to suppress charging during ion/electronexposure, the electrically insulating polystyrene film is sputter-coated with a thin layer of platinum (nominally 30 nm thick). Theresulting film has a thickness of ca. 5 �m, as measured using SEM.This polymer is found to be stable, with minimal outgassing undervacuum conditions.

By using a lift-out method, the cantilever structure can beextracted from any site of interest in the bulk film. Once in themicroscope, the specimen is tilted to 52◦, so that the ion beam,which is then at an angle of 52◦ with respect to the electron beam,is at normal incidence upon the specimen. Fig. 1a is a CCD cam-era image of the interior of the microscope chamber showing theposition of the specimen relative to the two columns. The stage ismoved so that both beams are aligned.

To locally deposit platinum onto the polystyrene film, anorganometallic gas precursor i.e. C9H16Pt, is introduced in the vicin-ity of the surface of the specimen (within ∼100 �m) through anozzle from a gas injection system (labeled ‘gas injector’ in Fig. 1b).The gas molecules, adsorbed on the film surface, are decomposed bythe secondary electrons/ions produced during ion rastering, leav-ing a layer of platinum on the surface, while volatile componentsare pumped away by the vacuum system. The correct choice of ionbeam current density for platinum deposition is ca. 2–6 pA/�m2

[22]. The thin sputtered platinum film already present acts as aprotective layer to avoid direct ion impingement onto the poly-mer. Fig. 2 shows SEM micrographs taken during the fabrication todemonstrate the procedure. In Fig. 2a, a rectangular platinum stripof 25 �m long, 2 �m wide and 0.5 �m thick is deposited using abeam voltage of 30 kV, beam current of 30 pA, dwell time of 0.2 �sand overlap of 0%. The time taken for the ion-induced platinumdeposition is 1670 s (∼28 min).

A series of milling steps is performed to produce a releasedcantilever structure. In the first step (Fig. 2b), trenches of27 �m × 4 �m × 5 �m are milled on both sides along the length ofthe platinum strip using a beam voltage of 30 kV, beam current of0.5 nA, dwell time of 0.1 �s and overlap of 50% with a total millingtime of approximately 54 min. Once desired dimensions have beenachieved, the milling is terminated. A higher beam current is cho-sen to mill the trenches in a reasonable time. These trenches serveas an avenue for the sputtered material to escape, avoiding rede-position. In the next step (Fig. 2c), the stage is tilted to 22◦. A trenchof 27 �m × 2 �m × 5 �m is milled on one side of the platinum stripusing similar beam conditions except for a lower beam current of0.3 nA. The stage is then rotated 180◦ and the milling is repeated onthe opposite side. The total stated milling time is 4532 s (∼75 min).

Following this trench cut, a bridge structure is formed, fixed at bothends.

To cut free one end of the bridge structure, the stage is tiltedto 52◦ and a 4 �m × 4 �m × 4 �m trench is milled using the same

464 C.C. Lee et al. / Sensors and Actuators A 172 (2011) 462– 470

F positi um.

bO(p

Fiatt

ig. 1. CCD camera image of the interior of the microscope chamber: (a) shows thenjector nozzle that is used to provide the precursor gas for the deposition of platin

eam conditions, requiring a total milling time of 321 s (∼5 min).nce the microcantilever is partially freed from the bulk film

Fig. 2d), the stage is positioned back to 0◦, and a tungsten micro-robe, controlled by a manipulator control panel, is inserted. The

ig. 2. Micrographs showing the fabrication sequence of the polystyrene–platinum bimormages except (g) and (h) which are ion beam-induced secondary electron images): (a) plat an angle to create a triangular wedge, (d) one end is then cut free, (e) a micromanipulao the tip and it is cut free and lifted from the surface, (f–g) the needle is then transferredo the structure before cutting it free with the ion beam.

ion of the specimen relative to the electron and ion columns and (b) shows the gas

tungsten microprobe is carefully brought into place by viewingwith both ion and electron beams. Imaging with ion beams isminimized or where absolutely necessary, carried out at low cur-rent (less than 30 pA) to avoid undesired sputtering. Once the

ph microcantilever (all micrographs are electron beam-induced secondary electrontinum is deposited on the surface, (b–c) the ion beam is used to cut into the sampletor needle brought to the surface, platinum deposition is used to ‘weld’ the needle

to a support structure, (h–i) platinum deposition is again used to weld the sample

C.C. Lee et al. / Sensors and Actuators A 172 (2011) 462– 470 465

of the

mtmt

pcTbTadttfft

fwgmsttio

3

metstdtri

nbcmoT

Fig. 3. (a) Schematic diagram and (b) SEM image

icroprobe makes contact with the microcantilever, the gas injec-or is once again used to deposit platinum at the contact point. The

icrocantilever is then completely cut free from the bulk film usinghe ion beam.

Upon lifting the microcantilever (Fig. 2e), the tungsten micro-robe is retracted from the field of view. A new support (a sectionedopper TEM grid) is inserted, mounted upright in the chamber.he microcantilever, still attached to the tungsten microprobe, isrought in and slowly brought into contact with the grid (Fig. 2f–g).he free end of the microcantilever, which eventually serves as annchor point, is attached to the grid using ion-induced platinumeposition. The microprobe is then cut free from the microcan-ilever with the ion beam (Fig. 2h). The stage is rotated to 180◦

o deposit platinum on the other side of the anchor. The time takenor the lift-out step is about 30 min. The entire fabrication process,rom the initial ion-induced platinum deposition as structural layero the final platinum deposition at the anchor takes around 3 h.

The finished microcantilever (Fig. 2i) can now readily be testedor thermal actuation in an SEM equipped with a heating stage,ith a chamber pressure in the range of 10−5 mbar. For this, the

rid is placed onto a 45◦ tilt aluminium stub using single-sided alu-inium tape and the stub is mounted onto a custom-built heating

tage, made from molybdenum. An electrical potential is passedhrough the resistive metal to generate desired temperatures. Aime of 2–5 min is required to achieve a stable temperature read-ng at a given voltage. A K-type thermocouple is carefully placednto the grid to measure the operating temperature.

. Results and discussion

The FIB-based fabrication approach described here possessesany important aspects of rapid prototyping. Cantilevers of differ-

nt dimensions (length, width and thickness) can be fabricated andailored to fit specific MEMS/NEMS applications. The cantilevershown here are about 20 �m long × 2 �m wide × 1.5 �m thick, areriangular and rectangular in cross-sections and have very sharplyefined flat edges (Fig. 3). FIB offers the possibility of scaling downhese dimensions to smaller sizes still. In fact, it would be quiteealistic to prepare cantilevers of just a few hundred nanometersn thickness.

The micrometer-sized bimorph cantilever did not exhibit anyoticeable vertical bending upon impingement of high intensity ioneams at any stage. The application of the initial sputtered platinum

oating is sufficient to overcome ion charging issues; hence theilled trenches are sharply defined and the patterns did not smear

ut in x- or y-direction (an issue that can arise from charging [27]).he microcantilever thickness was consistent along the length. If

polystyrene–platinum bimorph microcantilever.

necessary, geometrical and dimensional modifications can easily bemade to the finished product using the ion beam. With this method,it is also straightforward to fabricate unusual shapes and structuresif required. Ultimately, the cantilever can be fabricated with a veryhigh degree of geometrical and dimensional precision.

FIB micromachining does also have some drawbacks. Cuttingwith a high energy ion beam will inevitably result in ion-inducedartifacts. These artifacts, such as ion implantation and damage tothe structure of the surface layer, have been well-characterised inhard materials [30] but may have more complex effects in organicmaterials. Soft, electrically insulating polymers are extremely sen-sitive to ion irradiation and hence, could easily be damaged. Localheating and ion charging effects could potentially lead to severephysical and chemical changes such as melting, chain scission,chain cross-linking, swelling or decomposition [26–29].

The polystyrene surface that has been cut with 0.3 nA ionbeam was not perfectly smooth (Fig. 4a), in contrast to pristinepolystyrene film that is usually flat and feature-less. The sur-face roughness, which appears to have a nodular structure ofnanometer-scale, is greater when a higher beam current of 1 nAwas used (see inset image). The presence of a gallium peak in theEDX spectrum taken from the milled area of the sample (Fig. 4b)showed that the amount of gallium implanted in the polymer wasnot insignificant.

To determine the depth of the layer affected by the galliumion beam, Monte Carlo simulations were performed using a soft-ware package called TRIM/SRIM [31] which is able to estimate thestopping range of ions and the damage distribution in the targetmaterial for a given ion energy and incidence angle. Fig. 5a showsthe simulated results of 1000 gallium ions implanted at an acceler-ating voltage of 30 kV at an incidence angle of 0◦, impinging into a100 nm thick polystyrene film. Fig. 5b shows the stimulated resultsfor the same ions implanted into pure platinum. The ion–polymerinteraction is deeper than ion–metal, with the 30 keV gallium ionspenetrating about 80 nm into the polystyrene, as opposed to about30 nm in the pure platinum.

However, during the preparation of the cantilever, the ion beamdoes not impact the sample in a direction normal to the specimensurface. Instead, it impacts the surface at close to parallel to thecut created. Fig. 5c shows the interaction path of 1000 × 30 keVgallium ions in the polystyrene where the incidence angle is 85◦.The mean projected range is less than 30 nm, but with much moresevere damage near the specimen surface. The damage can, how-

ever, be reduced using a lower beam voltage [22]. Fig. 5d showsthe interaction path of 1000 × 10 keV gallium ions in polystyreneat an incidence angle of 0◦. The mean projected range is now sig-nificantly reduced to less than 40 nm (compared to ca. 80 nm for

466 C.C. Lee et al. / Sensors and Actuators A 172 (2011) 462– 470

Fig. 4. (a) SEM images showing the surface after FIB milling for a beam current of 0.3 nA and (inset) after milling with a beam current of 1 nA. (b) EDX spectrum of theFIB-fabricated polystyrene-platinum bimorph microcantilever. Aluminium (Al) and copper (Cu) are attributed to the substrates and the gallium (Ga) peak that arises fromion implantation is indicated by an arrow.

Fig. 5. SRIM plots of gallium ion impingement in polystyrene (a, c, d) and platinum (b) for beam voltages of 10 kV and 30 kV and incidence angles of 0◦ and 85◦ , as indicatedin figure labels.

C.C. Lee et al. / Sensors and Actuators A 172 (2011) 462– 470 467

Fig. 6. (a) Superimposed SEM images of the polystyrene–platinum bimorph thermal microactuator (room temperature and 65 ◦C). (b) A plot showing the tip displacementof the microactuator with response to temperature change. The dashed line shows a linear fit used to calculate thermal sensitivity. (c) Plot showing the tip displacement ofthe microactuator with response to temperature change compared to the displacement from an analytical approach in which a conventional rectangular bimorph (Rec–Rec,d ed for

3aw

auotmiptt

c[o

ashed line) and current triangular–rectangular bimorph (Tri–Rec, solid line) are us

0 keV ions implanted under the same conditions). However, using lower beam voltage normally means a poorer beam resolution,hich can affect the precision of the cuts.

The simulation results have implications for the minimumttainable micro/nanoactuator size that could possibly be preparedsing the FIB technique presented in this paper. The minimum sizef the actuator is not only governed by the ultimate resolution ofhe FIB, but also the interaction of the ion beam with the target

aterial. Considering the ion beam interactions shown in Fig. 5,t is therefore likely that the minimum dimension of a workableolystyrene-based actuator is likely to be around 100 nm. In addi-ion, we also need to consider the effect of local beam heating onhe polymers due to their low thermal conductivity [32].

For thermal quantification of the bimorph actuators, a majorhallenge is the efficiency of delivering the heat to the structure5]. Heat transfer poses an even greater challenge in the casef micro/nanometer-sized actuators. Several feasible approaches

the calculation.

have been employed as the heating source, for instance, resistivemetal, laser pulsing and high-energy electron beam [5]. In addition,the testing of the performance of the actuators is far from straight-forward. Common methods such as visualization using a lightmicroscope are not appropriate as the displacement amplitudesare too small to be detected. Other read-out options such as opticalleverage using atomic force microscopy, piezoelectric, piezoresis-tive and capacitive methods have shown promising results but aremore complicated to assemble [4,5].

Here, we use a scanning electron microscope with a heatingstage to observe the actuator movement. Molybdenum is usedas the heating element to supply a narrow temperature range(below 80 ◦C) to the polystyrene–platinum bimorph microactua-

tor. Upon gradually heated up to a maximum temperature of 65 ◦C,the bimorph exhibits normal actuation behaviour, bending upwardsince the polystyrene exhibits higher thermal expansion than theplatinum, and reaching a maximum displacement of about 750 nm.

4 Actuators A 172 (2011) 462– 470

TFa

pasmm

rve(bcabc

ı

T

ı

wr

admp7

upbbsts

hsbd(

st99Teoce±Sfi

Fig. 7. (a) ANSYS simulation of the tip displacement of the bimorph microcantileverfor a fixed temperature of 65 ◦C. The origin of the referential system denotes theinitial position of the bottom part of the cantilever. (b) Plot showing the tip displace-

68 C.C. Lee et al. / Sensors and

he thermal sensitivity of the polymer–metal reaches ca. 20 nm/◦C.ig. 6a is an image composed of two superimposed images of thectuator before heating (at room temperature) and at 65 ◦C.

If the actuator is heated above the softening temperature ofolystyrene (ca. 90 ◦C), it will not return to its original positionnd is left permanently bent or deformed in the direction oppo-ite to that of normal actuation. Once ‘overheated’, the actuatoray undergo localised material deformation such as plastic defor-ation, creep, melting and/or oxidation [8,9].Fig. 6b is a graph showing the movement of the actuator in

esponse to temperature change. The plot of tip displacementersus temperature change is linear, and it is predicted by anxpression very similar to Eq. (2). The expression shown in Eq.2) has been modified to accommodate the current shape of theimorph actuator (a combined isosceles triangular and rectangularross-section). The main difference is the second moment of areand the location of the neutral axis, which is not at the centre of thei-layer cross-section. The expression for the tip displacement forurrent bimorph can be derived as:

= L2

2�

= 18L2�˛�T(1 + h1/h2)2

2h[(h1/h2)3(E1/E2)+10(h1/h2)2 + 18(h1/h2)+3(h2/h1)(E2/E1) + 12)](3)

he expression in Eq. (3) can be simplified to a term given by:

= L2

2�= 9L2�˛�T(1 + m)2

h(m3n + 10m2 + 18m + (3/mn) + 12)(4)

here m and n are the bi-layer thickness and Young’s modulusatios of polystyrene to platinum, respectively.

Since FIB micromachining provides a high degree of geometricalnd dimensional precision, we can have a great deal of confi-ence in the thickness ratio. The actual dimensions of the bimorphicroactuators are 19 �m long and 1.6 �m thick (1.4 �m thick for

olystyrene and 200 nm thick for platinum, with an aspect ratio of), which deviates less than 5% from the desired dimension.

However, there are some uncertainties about the Young’s mod-li and the coefficients of thermal expansion of polystyrene andlatinum that may affect the thermo-mechanical properties of theimorph microcantilever. The uncertainty arises due to the possi-le differences in the properties of the thin films compared to bulkamples, to a wide variation in values reported in literature, and inhe case of platinum, due to the differences in the composition andtructure between FIB-deposited and bulk platinum.

The Young’s modulus ratio of polystyrene and platinum does notave a substantial effect on the displacement amplitude [6]. Even aignificant change in the E ratio (multiplying or dividing the E ratioy two with reference to the values intended for bulk) results in aifference in the tip displacement of only 3%. Therefore, bulk values168 GPa for platinum and 3 GPa for polystyrene) were used.

As for the thermal expansion coefficient, polystyrene can pos-ess a wide range of reported values, ranging from 80 × 10−6 K−1

o 280 × 10−6 K−1 and ̨ for bulk platinum is reported to be × 10−6 K−1 [8]. For our calculations, values of 120 × 10−6 K−1 and × 10−6 K−1 were used for polystyrene and platinum, respectively.he value for polystyrene was consistent (within the range ofrror) with rough measurements made of the thermal expansionf polystyrene with our SEM heating stage. The resulting analyti-al results match closely with the experimental data (Fig. 6b). Therror bars in figure correspond to an uncertainty in measurement of

20 nm, which is the smallest scale that could be measured in theEM images, using appropriate software, based on the SEM scale baror the magnification at which the images were taken. For compar-son, Fig. 6(c) shows the analytical calculations of tip displacement

ment of the bimorph microactuator in response to temperature change includingexperimental data and data from analytical and numerical simulations.

versus operating temperature for a conventional rectangular cross-section (based on Eq. (2)) and the bimorph used in this work (basedon Eq. (4)).

A 3D analysis of the thermally actuated polystyrene–platinumbimorph microcantilever was developed using finite element soft-ware ANSYS 11.0 and a coupled-field element, Solid98 is chosenwhich takes into account the interaction between thermal andmechanical fields. The materials properties are assumed to behomogenous, isotropic, and temperature-independent. The inputloading is the operating temperature which is uniformly distributedacross the cantilever and the output is the bimorph deflection. Theboundary condition stipulates that one extremity of the cantileveris fixed at the anchor point. Fig. 7(a) shows an ANSYS simulation ofthe bimorph microcantilever deflection upon heating up to 65 ◦C.The induced displacement of the bimorph is predominantly out-of-plane, bending upward to the platinum side. Maximum deflectionis observed at the tip of the cantilever.

Fig. 7(b) shows the comparison of tip displacement with

response to temperature change obtained experimentally as well asfrom analytical and numerical simulations. The experimental dataand analytical results based on the Timoshenko model are in good

C.C. Lee et al. / Sensors and Actuators A 172 (2011) 462– 470 469

F ctuatot ent ofs

abFmq

meifeisih

ig. 8. (a) ANSYS simulation to determine the blocking force of the bimorph microao temperature change. (c) Plot showing the blocking force against tip displacemtiffness.

greement with the FEA simulation results. The close correlationetween the analytical and numerical results using Timoshenko andEA models, respectively demonstrates that the tip displacementodel employed in this study is accurate enough to estimate the

uasi-static deflection of a bimorph cantilever microactuator.With reference to the results shown in Fig. 7, we can also esti-

ate the tip blocking force of the cantilever microactuator usingither an analytical blocking force model or a finite element block-ng force model, given that it was not possible to measure thisorce directly with the current experimental setup. Here, we havemployed the finite element approach to estimate the tip block-

ng force of polystyrene-platinum bimorph microactuator. Fig. 8(a)hows an ANSYS simulation to determine the blocking force. A forces applied at the apex of the cantilever to constraint its motion uponeating.

r. (b) Plot showing the tip blocking force of the bimorph microactuator in response the bimorph microactuator. The dashed line shows a linear fit used to calculate

For an operating temperature of 65 ◦C, the blocking force atthe tip of the microactuator is estimated to be ca. 40 nN, which isconsiderably lower than that generated by metal–ceramic microac-tuators [8]. Despite this low blocking force, these microactuatorscould be useful in MEMS/NEMS applications where a low force andan adequate displacement are needed at relatively low operatingtemperature. Fig. 8(b) is a plot showing the variation of blockingforce in response to operating temperature. The tip blocking force isdirectly proportional to the operating temperature. Fig. 8(c) showsthe relationship between the blocking force and the tip displace-ment, which is also linear. In addition, the stiffness of the bimorph

microactuator can be obtained using Hooke’s Law, F = kı, where k isthe spring constant, F is the blocking force and � is the tip deflection.The calculated spring constant, k of the bimorph microactuator isca. 50 mN/m.

4 Actua

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70 C.C. Lee et al. / Sensors and

. Conclusions

The fabrication approach described here shows the applicationf focused ion beam micro/nanofabrication technology for the pro-uction of polymer–metal microcantilevers. The capability of FIBilling, deposition and lift-out techniques provides an alterna-

ive avenue for maskless, rapid prototyping. To the best of ournowledge, there has been no investigation on the production ofolymer-based thermal microactuators using FIB technology. Ion-

nduced damage, in our case, did not impede the performance of thehermally induced polystyrene–platinum microactuators, in whichhe thermal sensitivity reaches ca. 20 nm/◦C. The analytical andumerical results agree well with the experimental data. Finite ele-ent model is used to estimate the blocking force and stiffness of

he bimorph thermal microactuators.

cknowledgements

This work is funded by the Australian Research Council (ARC)iscovery Project (DP0878931). The authors gratefully acknowl-dge the scientific and technical input and support from theustralian Microscopy & Microanalysis Research Facility (AMMRF)ode at the University of Sydney and the Faculty of Engineering athe University of Wollongong.

eferences

[1] M. Ataka, A. Omodaka, N. Takeshima, H. Fujita, Fabrication and operation ofpolyimide bimorph actuators for a ciliary motion system, Journal of Microelec-tromechanical Systems 2 (4) (1993) 146–150.

[2] H.Y. Chan, W.J. Li, Thermally actuated polymer micro robotic gripper for manip-ulation of biological cells, in: IEEE International Conference on Robotics andAutomation, 2003.

[3] N.J. Lavrik, M.J. Sepaniak, P.G. Datskos, Cantilever transducers as a platform forchemical and biological sensors, Review of Scientific Instruments 5 (7) (2004)2229–2253.

[4] A. Boisen, S. Dohn, S. Keller, S. Schmid, M. Tenje, Cantilever-like micromechan-ical sensors, Reports on Progress in Physics 74 (2011), 036101 (30 pp).

[5] K.L. Ekinci, Electromechanical transducers at the nanoscale: actuation and sens-ing of motion in nanoelectromechanical systems (NEMS), Small 1 (8–9) (2005)786–797.

[6] S. Timoshenko, Journal of the Optical Society of America 11 (3) (1925) 233.[7] M.C. LeMieux, M.E. McConney, Y.H. Lin, S. Singamaneni, H. Jiang, T.J. Bun-

ning, V.V. Tsukruk, Polymeric nanolayers as actuators for ultrasensitive thermalbimorph, NanoLetters 6 (4) (2006) 730–734.

[8] S. Wilson, C. Bowen, New Materials for Micro-scale Sensors and Actuators:An Engineering Review, Paper Published in Materials Science and EngineeringReports 56 (2007).

[9] C. Liu, Recent developments in polymer MEMS, Advanced Materials 19 (2007).10] M.J. Madou, Fundamentals of Microfabrication, CRC Press, 1997.11] Z. Cui, Nanofabrication: Principles, Capabilities and Limits, Springer, 2008.12] P.D. Prewett, Focused ion beam: microfabrication methods and applications,

Vacuum 44 (1993).13] A.A. Tseng, Recent developments in nanofabrication using focused ion beams,

Small 1 (10) (2005).14] D. Vick, V. Sauer, A.E. Fraser, M.R. Freeman, W.K. Hiebert, Bulk focused ion beam

fabrication with three-dimensional shape control of nanoelectromechanicalsystems, Journal of Micromechanics and Microengineering 20 (2010), 105005(9pp).

15] J.M. Cairney, P.R. Munroe, J.H. Schneibel, Examination of fracture surfaces usingfocused ion beam milling, Scripta Materialia 2 (5) (2000) 473–478.

16] L.W. Ma, J.M. Cairney, D. McGrouther, M. Hoffman, P.R. Munroe, Three dimen-sional imaging of deformation modes in TiN-based thin film coatings, Thin SolidFilms 15 (6) (2007) 3190–3195.

17] L.A. Giannuzzi, F.A. Stevie, A review of focused ion beam milling techniques forTEM specimen preparation, Micron 30 (1999) 197–204.

18] J.M. Cairney, P.R. Munroe, D.J. Sordelet, Microstructural analysis of a FeAl

quasicrystal-based composite prepared using a focused ion miller, Journal ofMicroscopy 201 (2001) 201–211.

19] D.W. Saxey, J.M. Cairney, D. McGrouther, T. Honma, S.P. Ringer, Atom probespecimen fabrication methods using a dual FIB/SEM, Ultramicroscopy 107(2007) 756–760.

tors A 172 (2011) 462– 470

20] J.F. Walker, D.F. Moore, J.T. Whitney, Focused ion beam processing formicroscale fabrication, Microelectronic Engineering 30 (1996) 517–522.

21] A.A. Tseng, Recent developments in nanofabrication using ion projection lithog-raphy, Small 1 (6) (2005) 594–608.

22] L.A. Giannuzzi, Introduction to Focused Ion Beams, Springer, 2005.23] A. Romano-Rodrıguez, F. Hernandez-Ramırez, Dual-beam focused ion beam

(FIB): a prototyping tool for micro and nanofabrication, Microelectronic Engi-neering 84 (2007) 789–792.

24] J. Teng, P.D. Prewett, Focused ion beam fabrication of thermally-actuatedbimorph cantilevers, Sensors and Actuators A 123–124 (2005) 608–613.

25] B.Z. Kupfer, R. Ahmad, A. Zainal, R.B. Jackman, Fabrication and characterisa-tion of triangle-faced single crystal diamond microcantilevers, Diamond andRelated Materials 19 (2010) 742–747.

26] K. Niihara, T. Kaneko, T. Suzuki, Y. Sato, H. Nishioka, Y. Nishikawa, T. Nishi, H.Jinnai, Nanoprocessing and nanofabrication of a structured polymer film by thefocused-ion-beam technique, Macromolecules 8 (8) (2005) 3048–3050.

27] D.J. Stokes, T. Vystave, F. Morrissey, Focused ion beam (FIB) milling of electri-cally insulating specimens using simultaneous primary electron and ion beamirradiation, Journal of Physics D: Applied Physics 40 (2007) 874–877.

28] J.J. Kochumalayil, A. Meiser, F. Soldera, W. Possarta, Focused ion beam irradi-ation: morphological and chemical evolution in PMMA, Surface and InterfaceAnalysis 41 (2009) 412–420.

29] M. Sezen, H. Plank, P.M. Nellen, S. Meier, B. Chernev, W. Grogger, E. Fisslthaler,E.J.W. List, U. Scherfe, P. Poelta, Ion beam degradation analysis of poly(3-hexylthiophene) (P3HT): can cryo-FIB minimize irradiation damage, PhysicalChemistry Chemical Physics 11 (2009) 5130–5133.

30] S. Rubanov, P.R. Munroe, FIB-induced damage in silicon, Journal of Microscopy214 (2004) 213–221.

31] http://www.srim.org (accessed 15.12.10).32] C.A. Volkert, A.M. Minor, Focused ion beam microscopy and micromachining,

MRS Bulletin 32 (2007) 389–399.

Biographies

Cheng Lee (Nikki) completed her BSc (with Hons) in Physics in 2004 and MSc (withDistinction) in Advanced Materials in 2007 at the University of Malaya, Malaysia.She is presently undertaking a PhD under the supervision of Julie M. Cairney at theUniversity of Sydney, in collaboration with, and under the associate supervision of,Gursel Alici and Geoff M. Spinks at the University of Wollongong on the develop-ment of micro/nanofabrication tool, and synthesis and characterisation of polymermicro/nanoactuators.

Gursel Alici received a PhD degree in Robotics from the Department of Engi-neering Science, Oxford University, UK, in 1994. He is currently a Professor atthe University of Wollongong, NSW, Australia, where he is the head of School ofMechanical, Materials and Mechatronic Engineering. His current research interestsinclude intelligent mechatronic systems involving mechanisms/serial/parallel robotmanipulators, micro/nano robotic systems for medical applications, and model-ing, analysis, characterisation, control of conducting polymers as macro/micro/nanosized actuators and sensors for robotic and bio-inspired applications. He is a tech-nical editor of IEEE/ASME Transactions on Mechatronics, and a member of theMechatronics National Panel formed by the Institution of Engineers, Australia.

Geoffrey M. Spinks obtained his PhD from the University of Melbourne in 1990for his work on the fracture behaviour of thermosetting polyesters. He has sinceworked at the University of Wollongong and currently holds an Australian ResearchCouncil Professorial Fellowship. His main research interest is in the developmentof polymer actuators based on conducting polymers, hydrogels, carbon nanotubesand graphene.

Gwénaëlle Proust obtained an undergraduate engineering degree in Nantes, Franceand graduate MS (2002) and PhD (2005) degrees from Drexel University in Philadel-phia, USA in the department of Materials Science and Engineering. She is currentlyworking as a lecturer at the University of Sydney. Her current research interestsinclude the study and modeling of elastic and plastic deformation of materi-als, including the study of twinning in hexagonal metals. She also works onmicrostructure characterisation by scanning electron microscopy (SEM) and elec-tron backscattered diffraction (EBSD).

Julie M. Cairney received her PhD (Physical Metallurgy) in 2002 from Univer-sity of New South Wales. She is currently working as an Associate Professorat the University of Sydney. Julie’s research interests focus on the relationshipbetween microstructure and properties of materials, with a particular empha-sis on the application and development of new characterisation technologies in

electron microscopy, focused ion beam (FIB) technology and atom probe tomog-raphy. Current materials of interest include steels, non-ferrous engineering alloys(such as Ni-based superalloys and Ti alloys), nanocrystalline metals, hard coat-ings (including nanocomposites and thermal barrier coatings), and thin films(including ferroelectrics).