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Sensors and Actuators A 159 (2010) 33–40

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

n situ MEMS gradiometer with nanometer-resolution optical detection system

umberto Campanellaa,∗, R.P. del Realb, Marta Ducha, Christophe Serrec, I. Lucasb, V. De Manuelb,éctor Guerrerob, Jaume Estevea, Marina Díaz-Michelenab, José A. Plazaa

Instituto de Microelectrónica de Barcelona IMB-CNM (CSIC), Campus UAB, 08193 Bellaterra (Barcelona), SpainInstituto Nacional de Técnica Aeroespacial Manuel Terradas (INTA), Carretera Torrejón-Ajalvir, km 4, 28850 Torrejón de Ardoz, SpainUniversitat de Barcelona, Dept. of Electronics, c/Martí i Franquès, 1, E-08028 Barcelona, Spain

r t i c l e i n f o

rticle history:eceived 18 November 2009eceived in revised form 3 February 2010

a b s t r a c t

Mechanically resonant ferromagnetic MEMS sensors intended for magnetic field gradient measurementsare presented. Suspended quad-beams with proof mass have been designed to improve their sensitivityand to simplify the detection. Fabricated devices exhibit the compact size of current MEMS technologies

ccepted 10 February 2010vailable online 19 February 2010

eywords:agnetic transducersradiometersagnetic devices

and are built within a simple deep-reactive-ion etching-based process. Nanometer-resolution detec-tion based on optical interferometry and signal processing techniques have been employed to find outdynamic-mode transformation factors of 6.25 × 10−3 T/m/Hz with 0.1-Hz resolution. The device per-forms in situ gradiometry with a single-sensor structure, which represents a technological advance tocurrent-art gradiometers.

© 2010 Elsevier B.V. All rights reserved.

EMS sensors

. Introduction

Magnetic sensors of many kinds have been designed andssisted different applications throughout more than a century.mong them, magnetometers and gradiometers based on micro-lectromechanical systems (MEMS) are considered in the rangef medium resolution devices [1–2]. Ferromagnetic MEMS sen-ors implement a magnetic material integrated somehow to itsicromachined mechanical structure. Whenever the MEMS sen-

or is under the influence of a magnetic perturbation, the MEMSeflects (static) or changes its state of motion (dynamic), and

t happens to move due to the ferromagnetic attraction arisingetween the magnetic material and the source of the magneticerturbation. The change in motion can be measured through sev-ral detection techniques, like optical means [3]. Current MEMSagnetometers have magnetic field sensitivities in the range

rom 1 �T to 1 mT [4], and they are a well-demonstrated tech-ology, largely based on torsional beam with magnetic plate5–7], and cantilever topologies [4,8]. The sizes of such struc-ures range from hundreds of micrometers to some millimeters.

ost recently, a nano-cantilever implementation has been demon-trated [9].

Nevertheless, MEMS gradiometers are not as developed as theiragnetometry-oriented counterparts. Magnetic gradiometry has

∗ Corresponding author. Tel.: +34 93 5947700; fax: +34 93 5801496.E-mail address: humberto.campanella@imb-cnm.csic.es (H. Campanella).

924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2010.02.007

gained a lot of interest for magnetic anomaly mapping and in situmagnetic material characterization properties for mining, magneticgeology and geological, archeological and geophysical prospectingresearch applications [10–13]. Although conventional magnetome-ters are more commonly deployed on satellites, interest is growingin the use of magnetic gradiometers to extract data that cannot beobtained from magnetic field measurements alone [14].

Gradiometers usually measure the difference in a magneticfield across a baseline. Although not an optimal approach, itis practical and has avoided implementing best-case, single-point gradient measurements. Under this assumption, they offerseveral advantages over magnetometers: short-time and small-baseline field variations, increased spatial resolution of the source,temporal-change discrimination in the magnetic field gradient, sig-nal enhancing from near sources, independent information aboutthe structure of the field, and improved sensitivity, among others[15–16]. New gradiometer concepts have been explored, like thedirect string magnetic gradiometer (DSMG) [17–19], although theyneed further development to fully demonstrate their potential. Onthe other hand, commercial gradiometers are based on the flux-gate sensor approach, like the Bartington product implementingtwo fluxgate sensors separated 1 m [20].

Previous developments of our group have begun exploring

single-sensor gradiometer concepts [21]. To overcome the limita-tions of current-art devices and other technologies, we contributeherein a quad-beam MEMS device able to measure the gradientvalue just at the place where it is wanted to be measured. Thisis done by implementing a single-sensor structure, instead of the

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wo or more magnetometers required in fluxgate-based gradiome-ry, for example. Miniature, single-sensor and in situ measurementepresent a step forward in the field of magnetic gradiometersevelopment. Additionally, it addresses two main issues of current-rt gradiometers: portability and power consumption. Quad-beamtructures are intended to perform future gradient and suscepti-ility measurements for magnetic particle detection applications.hey exhibit the compact size of current MEMS technologies andre built within a simple fabrication process, their layouts hav-ng been optimized to improve their magnetic transformationactor and to simplify the optical detection setup. Another fea-ure is its resonant operation, which we expect to augment theevice sensitivity. Section 2 highlights the impact of choosingard magnets with a high-aspect ratio in the transformation fac-or of the sensor, which clarifies and justifies their combinationith MEMS technologies over soft magnetic materials or thin-lm composites. The concept, main features and design of theagnetic sensor are discussed in this section as well. Sectiondescribes the fabrication technology and processes of quad-

eam structures. Sections 4 and 5 are devoted for modeling andxperimental characterization of the MEMS gradiometer, whereanometer-resolution optical detection and signal processing tech-iques have been implemented to estimate the transformation

actor of the system. Finally, in Section 6, we conclude with a workummary and a discussion on future MEMS gradiometry perspec-ives.

. Operation principle and sensor design

Our MEMS gradiometer features an NdFeB magnet as the activelement of the magnetic system, thus constituting a ferromagneticensor. MEMS quad-beams with proof masses have been devised toxplore sensitivity ranges of the gradiometer. The schema of Fig. 1epicts a conceptual view of the system which can detect the pres-nce of magnetic materials when they introduce an asymmetry onhe magnetic field generated by the coil (the B-field, in the figure).he main purpose of the coil is to drive the quad-beam into res-nance through magnetomotive actuation, although it also servesor DC system calibration.

Once in resonance, the operation principle of the MEMS gra-iometer relies on detecting in situ anomalies of the alternateagnetic field generated by the coil. Whenever such an anomaly

ccurs – for example, due to the proximity of a magnetic particlethe device deflects. Such deflection assumes an added stress to

ig. 1. Conceptual view of an apparatus for performing MEMS-based in situ gra-iometry: magnetic materials in the sensor vicinity introduce an asymmetry onhe magnetic field generated by the excitation coil. Whenever the magnetic fieldhanges, ferromagnetic attraction forces arising between magnetic particles, theagnet and the coil deflect the MEMS device. Nanometer-resolution optical detec-

ion systems measure the MEMS deflection and/or the vibration amplitude andrequency (static or resonant modes).

ctuators A 159 (2010) 33–40

the quad-beams, which will result in resonance frequency shifting.Therefore, deflection and vibration can superpose during operation.The AC + DC magnetic force FM arising on the magnet is given by

FM = Faccoil + FDC

part = m∂Bz

∂z(1)

where Faccoil is the excitation AC force provided by the coil, FDC

part isthe particle detection DC force, m is the magnetic moment of themagnet, and ∂Bz/∂z is the gradient of the magnetic field in the zdirection (assuming that the volume magnetic moment of the mag-net is oriented in the z direction). In magnetostatic equilibrium, themagnitudes of FDC

part and the MEMS restitutive force Fz have the samevalue, as a first approximation. The deflection is thus governed bythe Hooke’s law:

Fz = −k�z (2)

where k is the spring constant of the MEMS structure and �z is themaximum deflection in static operation due to FDC

part. The value ofdeflection �z can be determined by optical means. If we calibratethe MEMS in order to determine the value of k [22] and detect thedeflection with enough accuracy, then we can calculate the mag-nitude of FDC

part through Fz. According to (1), the value of ∂Bz/∂z canbe found for a given magnet with known magnetization:

∂Bz

∂z= −k�z

m(3)

Previous coil calibration at given and known magnet-to-coil dis-tances allow determining the value of ∂Bz/∂z when FDC

part = 0.Effective amplitude values are considered in dynamic, resonantoperation of the gradiometer. The presence of magnetic parti-cles adding stress to the beams will result in frequency shifting.Thus, the magnitude of frequency shifting indicates the magni-tude of magnetic momentum or amount of detected particles. Inthis scheme, the gradient-measurement task is performed by asingle-sensor device at an arbitrary small distance between thegradiometer and the analyzed sample, without knowing the actualmagnetic field magnitude (which is an added advantage of thisapproach).

Why to use such approach and macroscopic NdFeB magnets?The answer relies on both performance and technological issues.Size and aspect ratio do matter when determining the magneticproperties of the magnet, these aspects being in close relation withthe transformation factor of the magnetic sensor. The magneticmoment m (A × m2) of the magnet immersed in a homogeneousmagnetic field B (Tesla) is the product of the magnetization M (A/m)and the magnet volume V (m3) (sometimes M is expressed as mag-netic polarization J (Tesla)):

m = MV (4)

Reducing the magnet dimensions does not alter the magni-tude of B, although m is reduced in a linear fashion with volume.Additionally, the aspect ratio of the magnet strongly influencesthe moment: high-aspect ratio magnets increase the moment, thetorque and the magnetic force of the system, again improvingtransformation factor in comparison to low-aspect ratio magnetsor magnetic materials with low magneto-crystalline anisotropy,like most of deposited thin-film magnets. Nowadays, NdFeB andSmCo magnets exhibit the best performance in micro-magnetapplications, although they suffer from a lack of integrability andbatch fabrication. Many methods and magnetic materials havebeen developed to challenge such performance, yet they need to

overcome drawbacks arising from difficulties of the fabricationtechnique, poor magnetic properties, or both [23–26]. For exam-ple, magnetic thin-films are very suitable for micro-fabrication –which solutions practical matters like passivation of the magnet –despite their low-aspect ratio and small magnetic moment [27–28].

H. Campanella et al. / Sensors and A

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ricated chips after completion of the whole process. With a proper

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Fig. 2. Layout and dimensions of the quad-beam MEMS gradiometers.

recisely, one of the biggest technological challenges is how toroduce and to place small magnets on another structure. Previ-us works have made use of soft magnetic materials like FePt ori compounds deposited on microcantilevers for torque magne-

ometry study [29] and micromechanical oscillators for magneticetection [30]. We agree with Cugat et al. in their opinion that therexists at present no perfect candidate for cheap and fast fabricationnd integration of good-quality permanent micro-magnets with aigh-aspect ratio, which are still compatible with MEMS technol-gy [31]. However, technologies in the field are evolving on the wayf great advances. At the nanometer-scale, recent developments ofur group support high-aspect ratio NdFeB micro and nanomag-

et fabrication as a key technology towards future high-sensitivityanomagnetic applications [32].

Quad-beam devices hold up the NdFeB mini-magnet, whose lay-ut and dimensions are depicted in the schematic drawings of Fig. 2.

ig. 3. Fabrication process of the quad-beam MEMS gradiometer in SOI wafers: (a) therml/Cu and oxide patterning; (d) cured photo-resist deposition and patterning, and 200-�ollow and Al/Cu removal; (g) front-side anisotropic RIE of the 15-�m-thick DEV Si layer

ctuators A 159 (2010) 33–40 35

From the design point of view, the main features of our MEMSgradiometers are:

(a) Mini-magnet insertion hollow to align the device Si mass andthe magnet, which maximizes the magnetic interaction.

(b) Target alignment patterns micromachined on the Si to alignthe magnet with the optical detection and magnetic actuationsystems.

(c) Thick Si mass for providing the structure with mechanical sta-bility, robustness, and reduced stress due to magnet coupling.

(d) Break-off tabs to extract the chips from the wafer by manualcleaving.

The magnet is attached to the Si mass from the backside ofthe device, and the optical detection system is placed above thesensor. Such configuration allows the confocal microscope illumi-nation from the front side of the sensor, whose alignment is aided bythe target patterns. The hollow ensures good mechanical couplingand alignment between the magnet and the MEMS. Thus, we max-imize the mechanical response of the MEMS for a given magneticexcitation.

Designed devices make use of micromachined hollows with adepth of 300 �m and a diameter of 3 mm at the backside of theSi mass. Suspended beams are 2-mm-long, 15-�m-thick, and 1-mm-wide. The proof mass has a length of 5 mm, and a thickness of500 �m. Break-off tabs facilitate manual cleaving extraction of fab-

design, these tabs avoid dicing of the fragile, micromachined struc-tures on the wafer. Commercial magnets of diameter equal to 3 mmand thickness of 1.5 mm have been inserted in the hollow to com-plete the device.

al oxidation of the wafer; (b) oxide patterning; (c) backside Al/Cu film deposition,m-deep etching of Si (DRIE); (e) resist removal; (f) patterning of the 300-�m-deep; and (h) thermal oxide removal.

3 and Actuators A 159 (2010) 33–40

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. Fabrication technology

The main technology behind the fabrication process of theEMS gradiometer is based on deep-reactive-ion etching (DRIE) of

ilicon. Micromachining is performed at both the front and back-ides of the wafer and it has three main purposes, namely: (a)eleasing the proof mass and quad-beams, (b) defining the opti-al detection alignment patterns, and (c) providing the hollow tonsert the magnet. Silicon-on-insulator (SOI) wafers of thicknessqual to 500 �m, buried oxide (BOX) layer of 2 �m, and Si deviceayer of 15 �m were processed inside the clean room facilities athe IMB-CNM (CSIC). Dry etching steps have been carried out bysing Alcatel A601 DRIE equipment [33]. The sequence of Fig. 3ummarizes the fabrication process:

(a) Thermal oxidation of the wafer with thickness of 8000 Å.(b) Patterning of the front-side silicon oxide through dry etching.

This layer is used as etching mask to shape the device and pat-tern the alignment markers.

(c) Backside deposition and patterning of a thin Al/Cu film of5000 Å. The Al/Cu layer serves as high-selectivity mask materialto perform deep etching at the backside of the wafer. After thisprocess, we open an etching window to the Si substrate. Thebackside oxide is also patterned.

d) Backside deposition and patterning of a thick, cured photo-resist, and deep Si etching through DRIE process up to 200 �m.

(e) Resist removal to discover the central part of the mass wherethe hollow will be dug.

(f) Backside deep Si etching of 300 �m to dig the hollow and thebackside device shaping. Once finished the Si etching, the BOXis etched too. The break-off tabs are also patterned around theMEMS chip in this step. Finally, the Al/Cu mask is removed andthe backside SiO2 is uncovered.

(g) Alignment markers can now be patterned and the quad-beamreleased through the front-side anisotropic dry etching of the15-�m-thick Si device layer of the SOI process.

h) Lastly, thermal oxide at both sides is completely removed andthe process is finished.

Fabrication results can be seen in the picture of Fig. 4. An exam-le quad-beam device is observed from both the back and frontides of diced chips with close-up views of a magnet attached tone of them. The hollows were designed with a diameter slightly

ig. 4. Quad-beam fabrication results: (a) front-side chip, (b) backside chip, and (c)etail of magnet attached to the sensor.

Fig. 5. Modal analysis of the suspended quad-beam MEMS gradiometer: DOF solu-tion and mode shaping of the 278 Hz resonance frequency (deformation setup inarbitrary units).

bigger to that of the magnets to allow proper insertion of theminside the cavity. Despite the high anisotropy of the DRIE process,few microns of lateral etching still increase the hollow’s diameter,which ensures that the magnet will be inserted into the hollow andproperly glued to the Si by using a compatible bonder. Due to itssimplicity, the process exhibits a high yield and repeatability.

4. Finite-element modeling

We build two-dimensional (2D) and three-dimensional (3D)finite-element models (FEM) of the gradiometer in ANSYS® (ANSYSInc., Canonsburg, PA, U.S.A.) to predict the structural, modal, andmagnetic response of the quad-beam. Static analysis allows calcu-lation of the spring constant k of the structures, which is usefulto quantify the Hooke’s force and the magnetic counter-force.Magnetic analysis gives us an idea of the magnetic perturbationintroduced by particles near the resonator. Modal analysis aids uspredicting the fundamental mode shapes and frequencies wherethe device operates. SOLID95 elements were employed for the 3Dstructural analysis, whereas the 2D magnetic model was meshedwith PLANE53 elements.

First, we extracted the spring constant of the quad-beam withmagnet after meshing and structural analysis calculations with anapplied force due to gravity (F = mg, g = 9.8 m/s2). Deflection in thevertical axis is about 3.22 �m, which gives us a spring constantkquad = 331 N/m. With this at hand, we extract the Hooke’s forceand the equivalent magnetic force (at equilibrium). This shows thatthe structure is not as soft as a cantilever of similar dimensions(which can reach spring constants in the units of N/m or less). Inresonant operation, the clamped–clamped quad-beam will be moresensitive to add stress and its frequency shift will be higher than in asingle-clamping cantilever (although second-order effects may beobserved). Therefore, we have centered our study on fully clamped,suspended quad-beams.

Modal analysis of the structures was also carried out to pre-dict the resonance frequencies. The prediction intends reducingthe span where the frequency sweep is to be performed during thedynamic characterization of the sensor. Fig. 5 shows the shape ofthe 278-Hz fundamental mode of the quad-beam resonator, which

is calculated without considering added stresses to the structure.As we study in Section 5, the first resonance modes shift when themagnetic force FDC

part due to magnetic particles adds stress to theresonator, superposing to the Fac

coil component of FM.

H. Campanella et al. / Sensors and A

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26-dB amplitude sensitivity gain, in comparison to the non-filteredsignal. The slight variations in the FEM and experimental frequencyvalues are explained by material property differences and, proba-bly, over-etching of silicon.

ig. 6. Magnetic FEM analysis of the MEMS gradiometer: (a) meshed model com-

rises MEMS device, air, infinite and magnetic particle elements; (b) flux-line plotllustrating the magnetic field anomaly introduced due to the magnetic particle.

We performed 2D FEM magnetic analysis to illustrate the effectf introducing magnetic particles near the resonator. The modelncludes the quad-beam, magnet, air, infinite, and magnetic par-icle elements, as it shows the mesh of Fig. 6(a) (because of axialymmetry, only half of the device is observed). We analyzed theagnetic field intensity, the magnetic-flux lines and the force.NSYS found the magnetic force to be in the 10−3 to 10−2 N range

or magnet-particle distances in the few millimeters to few cen-imeters (particle size: 300 �m), which is in the magnitude order oforces we are expecting for. The flux-line plot of Fig. 6(b) illustrateshe anomaly introduced in the magnetic field due to the magneticarticle.

. Dynamic-mode characterization

Dynamic-mode characterization examines the frequency trans-ormation factor of the quad-beam gradiometer to external

agnetic field perturbations. Magnetic particles and other DCeld sources close to the device are expected to modify its res-nance frequency. Dynamic characterization also explores the

ctuators A 159 (2010) 33–40 37

minimum detectable resonance peak above the system’s noisefloor.

Dynamic measurements are performed through optical inter-ferometry by using the sub-nanometer-resolution Zoomsurf 3D®

system of Fogale Nanotech (Nîmes, France). In the setup, the res-onator with the magnet is placed under the optical objective of theZoomsurf 3D system, which illuminates the sample with either vis-ible or infrared light (option selected by the user depending on theamplitude range). Before scanning the sample, the user focuses thesample until interference bands on the device surface are sharp.As in the static characterization case, the coil is placed under theMEMS-magnet ensemble. Now, the signal generator connected tothe coil provides AC or AC/DC signals to drive the MEMS into reso-nance.

First, we evaluate the main resonance modes and the minimumdetectable peak amplitude. To do that, we carried out frequencysweeping of the generator in the span where the fundamentalmodes are expected. The AC signal amplitude is adjusted to pro-viding enough motional energy to the MEMS. At the same time,this amplitude should be kept as low as possible to avoid the excita-tion of spurious modes, which saturate the frequency response andmask the fundamental mode peaks. On the other hand, expectedmodes are found in the audio range, circumstance that makes theMEMS structure extremely sensitive to external vibrations andsound. Thus, we discriminate the MEMS response from environ-mental noise by reducing the input signal amplitude and filteringand averaging the response of independently acquired samples. Theminimum input signal required to excite the MEMS is obtained afterseveral iterations. The amplitude sensitivity value is fixed defin-ing an observation criterion: discrimination of the full width athalf maximum (FWHM) of the resonance peak with at least a 3-dB value above the noise floor. Benefits of filtering and averagingcan be appreciated in Fig. 7, which shows filtered and non-filteredversions of the MEMS response. The RMS AC current feeding thecoil is 1 �A at a coil-to-magnet distance d1 = 1500 �m. The lightgray curve has a 3-dB amplitude variance, making difficult to dis-criminate the resonance peak. However, after 20 accumulations theresonance at 260 Hz is clearly differentiated from noise, the peakamplitude of vibration being 6 nm. Equivalently, this represents a

Fig. 7. Sensitivity of dynamic-mode performance of the quad-beam MEMS gra-diometer: the fundamental 260-Hz mode has effective amplitude of about 6 nm,which can be clearly discriminated from background audio noise after 20 accu-mulations (light gray represents single-acquisition data, where peaks of similaramplitudes mask the resonance peak).

38 H. Campanella et al. / Sensors and Actuators A 159 (2010) 33–40

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ig. 8. DC-current tuning of the MEMS gradiometer frequency response: non-odulated (0 mA) response is compared against modulated signals. Frequency

ransformation factor is 4 Hz/mA. According to coil calibration data, this is equivalento 160 Hz/T/m (or 6.25 × 10−3 T/m/Hz).

Next, we explored the DC-current-modulated frequencyesponse of the sensor. The information extracted from this exper-ment is useful to calibrate the frequency transformation factor

hen the sensor detects magnetic particles. DC-currents super-osed to AC signals in the frequency span of 250–290 Hz, thusbtaining the responses shown in Fig. 8. The black thick curve is theesponse when no DC signal modulates the resonance frequencyf the MEMS-magnet ensemble. Positive and negative DC-currentsn the range between −2.5 and +2.5 mA modulated the AC sig-al to achieve linear tuning with transformation factor equal toHz/mA. According to coil calibration curves, this is equivalent to60 Hz/T/m (or 6.25 × 10−3 T/m/Hz). Results suggest static deflec-ion superposing to vibration. The amount of uncertainty presentn the experiment makes it necessary to consider this value justs a reference. Accuracy and uncertainty of measurement parame-ers like magnet-to-coil distance and inhomogeneity of magnet’s

agnetization are subjects to be faced in future stages of theork.

On the other hand, the frequency response to gradient variationss linear, as it shows Fig. 9. This is evidence of the usefulness of theevice as a magnetic sensor. After calibration, frequency shifting

ndicates the gradient magnitude. Linearity in the tuning responselso suggests that the explored range is small in comparison to theotential of frequency tuning of the MEMS structure (the tuningange was limited by the excitation setup).

ig. 9. Linear response of the sensor: frequency shift against detected gradient.

Fig. 10. In situ magnetite detection: the resonance frequency of the MEMS gra-diometer is modulated according to the position and presence of magnetite nearthe sensor (no magnetite is present in the straight black curve—“No”).

Previous results gave us confidence to explore magneticparticle-based tuning of the MEMS frequency response. There-fore, the MEMS-magnet ensemble is used as a gradiometer whosefrequency transformation factor to magnetic field variations canbe evaluated. To demonstrate the concept, we introduced mag-netic particles close to the resonator, we measured the resonancefrequency, and then we modified the position of particles. The exci-tation setup involves only AC driving signal, and the detection isperformed as described for the previous experiments. Two mag-netic materials were employed to test the gradiometer: magnetiteand NdFeB dust embedded in resist. First, we carried out qualita-tive analysis of a piece of magnetite located at different sides ofthe MEMS quad-beam. Side location breaks the symmetry of thesystem, which should introduce an anomaly in the magnetic field(the coil, the magnet and the MEMS are in the same vertical axis).Therefore, a gradient can be in situ detected as resonance frequencyshifting. Fig. 10 compares frequency responses of the gradiome-ter before and after magnetite detection. Four magnetite positionsmodulated the detection: in front, back, right, and left with respectto the MEMS center. Vertically, the magnetite is on the same planeof the quad-beam surface. Borrowing from previous experimentresults, observed linearity, and coil calibration, we can magnifythe detected gradients. For example, and referring again to Fig. 10,the frequency shifting �f when particles are placed back of thesensor is 11.6 Hz (�f = fNO − fBACK = 285.6–274 Hz). This differencecorresponds to a detected gradient of 72.5 × 10−3 T/m. Note thatafter calibration we can estimate the gradient without the need ofknowing the sample-to-sensor distance.

6. Conclusions and future applications

Quad-beam structures for the measurement of magnetic fieldgradients have been reported. The fabrication process was dis-cussed and experimental and modeling results presented. Themain features of the technology demonstrated their effective-ness in detecting magnetic field gradients. By using interferometrytechniques, we detected nanometer-scale deflection and effectivevibration with an amplitude resolution of 6 nm. DC-current tuningand magnetic particle detection changing the MEMS gradiometerresonance frequency were demonstrated through a simple but stillsensitive excitation and detection setup. Thus, we succeeded on

demonstrating single-sensor in situ gradiometry with a transfor-mation factor of 6.25 × 10−3 T/m/Hz with 0.1-Hz resolution. Basedon the device linearity, magnetic particle detection has been carriedout.

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Future development of this work will involve optimization ofhe MEMS design to maximize its spring constant and transforma-ion factor. Refined estimations of actual magnetic field gradientsill enable us to determine the ultimate MEMS gradiometer res-

lution. The reduced size and weight of our MEMS gradiometershould enable their future use in portable instruments. Develop-ents may be of interest in mining and geological prospectionshere magnetic particles are to be detected. Conditions required

or this application are to be further explored in the future.

cknowledgments

This work was funded by the INTA, under industrial researchollaboration in the context of MICINN Project OPTOMAG-MANTISESP-2005-05278). Partial support was provided by the MINAHE 2MEC-TEC2005-07996-CO2-01) and MINAHE 3 (TEC2008-06883-03-01) projects.

eferences

[1] J. Lenz, S. Edelstein, Magnetic sensors and their applications, IEEE Sens. J. 6(2006) 631–649.

[2] M.D. Michelena, R.P. del Real, H. Guerrero, Magnetic technologies for space:COTS sensors for flight applications and magnetic testing facilities for payloads,Sensor Lett. 5 (2007) 207–211.

[3] D. Ciudad, C. Aroca, M.C. Sánchez, E. Lopez, P. Sánchez, Modeling and fabricationof a MEMS magnetostatic magnetic sensor, Sens. Actuators A: Phys. 115 (2004)408–416.

[4] S. Brugger, O. Paul, Resonant magnetic microsensor with �T resolution, Proc.IEEE Int. Conf. MEMS (2008) 944–947.

[5] H.H. Yang, N.V. Myung, J. Yee, D.-Y. Park, B.-Y. Yoo, M. Schwartz, K. Nobe, J.W.Judy, Ferromagnetic micromechanical magnetometer, Sens. Actuators A: Phys.97–98 (2002) 88–97.

[6] D.J. Vasquez, J.W. Judy, Optically interrogated zero-power MEMS magnetome-ter, IEEE/ASME J. Microelectromech. Syst. 16 (2007) 336–343.

[7] H.J. Cho, Ch.H. Ahn, Magnetically driven bi-directional optical microscanner, J.Micromech. Microeng. 13 (2003) 383–389.

[8] S.M. Goedeke, S.W. Allison, P.G. Datskos, Non-contact current measurementwith cobalt-coated microcantilevers, Sens. Actuators A: Phys. 112 (2004)32–35.

[9] D.J. Vasquez, J.W. Judy, Flexure-based nanomagnetic actuators and their ulti-mate scaling limits, Proc. IEEE Int. Conf. MEMS (2008) 737–741.

10] M.H. Acuna, Space-based magnetometers, Rev. Sci. Instrum. 73 (2002)3717–3736.

11] W. Magnes, D. Pierce, A. Valavanoglou, J. Means, W. Baumjohann, C.T. Russell,K. Schwingenschuh, G. Graber, A sigma-delta fluxgate magnetometer for spaceapplications, Meas. Sci. Technol. 14 (2003) 1003–1012.

12] S. Vijendran, H. Sykulska, W.T. Pike, AFM investigation of Martian soil simulantson micromachined Si substrates, J. Microsc. 227 (2007) 236–245.

13] M. Díaz-Michelena, Small magnetic sensors for space applications, Sensors 9(2009) 2271–2288.

14] A. Sunderland, A.V. Veryaskin, W. McRae, L. Ju, D.G. Blair, Direct string mag-netic gradiometer for space applications, Sens. Actuators A: Phys. 147 (2008)529–535.

15] R. Hastings, R.P.S. Mahler, R. Schneider Jr., J.H. Eraker, Cryogenic magneticgradiometers for Space applications, IEEE Trans. Geosci. Remote Sens. GE-23(1985) 552–561.

16] M. Díaz-Michelena, P. Sánchez, E. López, M.C. Sánchez, C. Aroca, Opticalvibrating-sample magnetometer, J. Magn. Magn. Mater. 215–216 (2000) 677–679.

17] A.V. Veryaskin, Magnetic gradiometry: a new method for magnetic gradientmeasurements, Sens. Actuators A: Phys. 91 (2001) 233–235.

18] W. McRae, A.V. Veryaskin, L. Ju, D.G. Blair, E. Chin, J. Dumas, B. Lee, String mag-netic gradiometer system: recent airborne trials, SEG Expand. Abstr. 23 (2004)790–793.

19] H. Golden, W. McRae, A. Sunderland, A.V. Veryaskin, D.G. Blair, L. Ju, A novelmagnetic gradiometer: description, design issues, and trial results, in: Aus-tralian Institute of Physics 17th National Congress, Brisbane, 2006.

20] Bartington Instruments. Grad601 Single-axis Magnetic Gra-diometer. Available on-line: http://www.bartington.com/products/Grad601singleaxisgradiometer.cfm (accessed October 2009).

21] I. Lucas, M.D. Michelena, R.P. del Real, V. de Manuel, J.A. Plaza, M. Duch, J. Esteve,H. Guerrero, A new single-sensor magnetic field gradiomenter, Sensor Lett. 7(2009) 1–8.

22] C.T. Gibson, D. Alastair Smith, C.J. Roberts, Calibration of silicon atomic forcemicroscope cantilevers, Nanotechnology 16 (2005) 234–238.

23] S. Yamashita, J. Yamasaki, M. Ikeda, N. Iwabuchi, Anisotropic Nd–Fe–B thin-filmmagnets for milli-size motor, J. Appl. Phys. 70 (1991) 6627–6629.

24] F.J. Cadieu, R. Rani, X.R. Qian, L. Chen, High coercivity Sm–Co based films madeby pulsed laser deposition, J. Appl. Phys. 83 (1998) 6247–6249.

ctuators A 159 (2010) 33–40 39

25] G. Rieger, et al., Nd-Fe-B permanent magnet (thick films) produced by avacuum-plasma-spraying process, J. Appl. Phys. 87 (2000) 5329.

26] F. Yamashita, et al., Preparation of thick-film Nd–Fe–B magnets by direct Jouleheating, in: Proceedings of the REM XVII, 2002, pp. 668–674.

27] H.J. Cho, C.H. Ahn, Electroplated Co–Ni–Mn–P-based hard magnetic arrays andtheir applications to microactuators, Proc. Electrochem. Soc. (2000) 586.

28] G. Zangari, P. Bucher, N. Lecis, P.L. Cavallotti, L. Callegaro, E. Puppin, Magneticproperties of electroplated Co–Pt films, J. Magn. Magn. Mater. 157/158 (1996)256–257.

29] L. Yuan, L. Gao, R. Sabirianov, S.H. Liou, M.D. Chabot, D.H. Min, J. Moreland,B. Han, Microcantilever torque magnetometery study of patterned magneticfilms, Proc. IEEE Int. Magnetics Conference INTERMAG (2006) 583.

30] M.D. Chabot, J.M. Moreland, G. Lan, L. Sy-Hwang, C.W. Miller, Novel fabricationof micromechanical oscillators with nanoscale sensitivity at room temperature,J. Microelectromech. Syst. 14 (2005) 1118–1126.

31] O. Cugat, J. Delamare, G. Reyne, Magnetic micro-actuators and systems (MAG-MAS), IEEE Trans. Magn. 39 (2003) 3607–3612.

32] H. Campanella, R. Pérez, M. Díaz, M. Duch, H. Guerrero, J. Esteve, J.A.Plaza, Focused-ion-beam-assisted magnet fabrication and manipulation formagnetic-field-detection applications, ACS Appl. Mater. Interfaces 1 (2009)527–531.

33] Alcatel Micro-Machining Systems. Available on-line: http://www.alcatelmicromachining.com (accessed October 2009).

Biographies

Humberto Campanella is a post-doctoral research fellow at the Consejo Superiorde Investigaciones Científicas (CSIC) assigned to the Instituto de Microelectrónica deBarcelona IMB-CNM, Spain, and associate professor in the Department of Telecom-munications and System Engineering at the Universitat Autònoma de Barcelona. Heholds a BSc in electronics engineering from the Pontificia Universidad Javeriana,Bogotá, Colombia (1995); a MSc in telecommunication systems from the Universi-dad Politécnica de Madrid, Spain (1999); a PhD in microelectronics and automatedsystems from the Université de Montpellier, France (2008); and another PhD in elec-tronics engineering from the Universitat Autonoma de Barcelona, Spain (2008). Dr.Campanella has more than 15 years of industry and academic experience in research,development, and engineering of integrated circuits, micro-electromechanical sys-tems, telecommunications, and signal processing, and he has served as a referee forscientific journals and public-funded projects in the U.S., Europe, and South America.

Rafael Pérez del Real (Madrid, Spain, 1966), MSc (1989) in Physical Sciences andPhD (1993) by the Universidad Complutense de Madrid (UCM). His research hasbeen focused on soft magnetic materials, magneto-optics, magnetic materials forcancer treatment, and magnetic sensors.

Marta Duch was born in Barcelona, Spain in 1967. She received her Technical Engi-neer degree in Chemistry in the Universitat Politècnica de Catalunya, Barcelona,Spain (1992). She is currently working in the Micro- and Nanosystems Departmentat the Instituto de Microelectrónica de Barcelona IMB-CNM (CSIC). Her main areaof activity is focused on bulk and surface silicon micromachining, glass etching andchemical and electrochemical deposition of metals.

Christophe Serre was born in Tarbes, France, in 1963. He received the PhD in micro-electronics from the University of Grenoble, France, in November 1992. During 1993,he was an assistant professor at the same university and worked on the lumines-cence of porous silicon at the Laboratory of Physical Spectrometry. He joined theElectronic Materials and Engineering (EME) laboratory of the Department of AppliedPhysics and Electronics at the University of Barcelona in November 1993 as a post-doctoral fellow. In February 2002, he was contracted as a researcher (within theSpanish “Ramon y Cajal” program) in the same laboratory, where he obtained a fullprofessor permanent position in November 2007. His research activities focus onthe fabrication and characterization of SiC and SiGeC thin-films for micromechan-ics, as well as micromachining technologies and processes and their application tointegrated sensors and actuators.

Irene Lucas (Ciudad Real, Spain, 1978), MSc (2003) in Physical Sciences and PhD(2009) by the Universidad Complutense de Madrid (UCM). During the PhD, herresearch was focused on growth and characterization of soft and hard magneticmaterials, and magnetic sensors. She is currently working as a Marie Curie post-doctoral researcher in the Leibniz Institute for Solid State and Materials Research(IFW) in Dresden, Germany. Her research is currently focused in high temperaturesuperconductors.

V. De Manuel (Madrid, Spain, 1981), MSc (2004) in Physical Sciences by the Univer-sidad Autónoma de Madrid (UAM). PhD student in soft magnetic amorphous andnanocrystalline materials and magnetic sensors.

Héctor Guerrero (Madrid, Spain, 1965), BSc (1988) and PhD (1992) in Physical Sci-ences by the Universidad Complutense de Madrid (UCM). He joined in 2000 theNational Aerospace Institute of Spain (INTA), where he leads the Optoelectronics

Laboratory in the Payloads and Instrumentation Area. He is involved in the R&D inseveral Space missions (INTA, ESA and Roskosmos), and in the development of plat-form technologies for optical wireless intra-spacecraft communications (OWLS). Atpresent is the Principal Investigator of MEIGA-MetNet Precursor, a Finnish, Russian-Spanish mission to deploy on Mars a meteorological station that involves magneticsensors. Dr. Guerrero has leaded more than 15 projects related to Telecommuni-

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tems Department at the Instituto de Microelectrónica de Barcelona IMB-CNM (CSIC).

0 H. Campanella et al. / Sensors

ations and Space Technologies, in the framework of the Spanish National Spacerogram and the European Space Agency. His current areas of publishing are relatedith Applied Physics (Optoelectronics and Magnetism), Knowledge Management,

nd Space Technology.

aume Esteve was born in Parets del Vallés, Barcelona, Spain, in 1961. He receivedhe BSc and the PhD degrees in physical electronics from the Universitat de Barcelonan 1984 and 1988, respectively. In 1990, he joined the Department of Silicon Technol-gy and Microsystems, at the Instituto de Microelectrónica de Barcelona IMB-CNMCSIC), as a senior research scientist. His areas of interest include silicon microma-hining technologies and their application to integrated sensors and actuators. Pr.steve holds six patents and has published more than 70 research papers, and he

as served as a reviewer of several international scientific journals, and as a referee

or public-funded projects.

arina Díaz-Michelena (Madrid, Spain, 1975) MSc (1998) in Physical Sciences byhe Universidad Complutense de Madrid (UCM) and PhD (2004) in Applied Physics inhe Universidad Poletécnica de Madrid (UPM). She is researcher in magnetic sensors

ctuators A 159 (2010) 33–40

COTS-Commercial Off-The-Shelf based for Space applications at INTA, the SpanishNational Institute of Aerospace Technology. Associate Lecturer in the Departmentof Physics of the Materials of the Faculty of Physics of UCM. Dr. Díaz-Michelena iscollaborator in the evaluation of research and industrial projects of Xunta de Galicia,local government of Galicia, Spain.

José A. Plaza was born in Cerdanyola del Vallés (Barcelona), Spain in 1968. Hereceived his physicist degree and his PhD degree in Electronics engineering from theUniversitat Autònoma de Barcelona, Spain (1992, 1997). He has the degree of Spe-cialist in simulation by the Finite-Element Method from the Universidad Nacional deEducación a Distancia since 1995. He is currently working in the Micro and Nanosys-

He has been focused on technology development, design, simulation and charac-terization of microsystems and nanosystems. Now, his research is focused on thefabrication of MEMS/NEMS to study living cells. Dr. Plaza has served as a reviewer ofseveral international scientific journals, and as a referee for public-funded projectsin Spain.