EXPERIMENTAL CHARACTERIZATION OF MICROMACHINED ELECTROMAGNETICPROBES USING SCANNING HALL PROBE MICROSCOPY

M. K. Yapici , A. E. Ozmetin2, J. Zou and D. G. Naugle2Department of Electrical and Computer Engineering, 2Department of Physics

Texas A&M University, College Station, TX 77843, USA(Tel: + 1-979-862-1640; E-mail: junzougece.tamu.edu)

Abstract: We present the experimental characterization of micromachined electromagnetic probe usingscanning Hall probe microscopy. The tested electromagnetic probe consists of a protruding sharpPermalloy needle embedded into a three-dimensional gold conducting coil, which is fabricated with asimple and straightforward micromachining process. By using a scanning Hall probe microscope, acomprehensive high-spatial resolution characterization of the probe performance (e.g. peak magneticintensity and spatial field distribution) is achieved for the first time.

Keywords: micro electromagnetic probe, magnetic field measurement, scanning Hall probe.

1. INTRODUCTION

t In the past few years, there has been a

significant interest in developing micrc;' c electromagnetic needles or probes for localizec- n manipulation of micro particles and cells [1-8]m z Typically, the micro electromagnetic probem consists of a 3D conducting coil for magnetic field

9generation and a protruding magnetic core (with asharp tip) for magnetic field concentration. Tcachieve optimal performance, the intensity ancdistribution of the magnetic field generated by theelectromagnetic probe needs to be wellcharacterized. However, due to the small rangeand large gradient of the magnetic field, it iextremely difficult to achieve a good experimentacharacterization. As a result, the design of micrcelectromagnetic probes largely has to rely ornumerical simulation and modeling, which mighlincur errors due to the variation in materialproperties and structure dimensions caused by theimperfection in probe fabrication. Recentlyscanning techniques coupled with Hall magneticsensing devices were employed for thecharacterization of stray magnetic field emanatingfrom magnetic force microscope tips [9]Equipped with a Hall probe (with a small sensingaperture) and a high resolution piezoelectric stagea scanning Hall probe microscope (SHPM) systemcan achieve the needed sensitivity and spatialresolution for micro scale magnetic fielcmeasurement, and thus could be adapted to the

experimental characterization of microelectromagnetic probes. In this paper, we present

i a comprehensive experimental characterization ofmicro electromagnetic probes using SHPM. To

I our knowledge, this is the first one of suchexperiments conducted up to date.

1 2. TEST PROBE FABRICATION

To conduct the experimental characterization,I micro electromagnetic probes were fabricated

using a hybrid process consisting of surfaceI micromachining and guided assembly which

consists of the following three steps: (1)3 fabrication of the probe substrates with bottomI conductors of the conducting coil; (2) fabrication

of the magnetic cores; and (3) assembly of theI magnetic core and top conductors of thet conducting coil (Fig. 1).I (1) To fabricate the probe substrate with bottom

conductors, a layer of chromium (10 nm thick)and gold (300 nm thick) was deposited onto anitride coated silicon wafer. Goldelectroplating with AZ4620 photoresist moldand a subsequent etching of thechromium/gold layer were conducted to formthe bottom conductors (10 pm thick) of theconducting coil. An SU-8 resist layer (10 ptmthick) was patterned to provide electricalisolation between the bottom conductor andthe magnetic core (to be assembled in Step 3).A second SU-8 layer (50 pm thick) was

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patterned to form the guiding structures for theassembly of the magnetic core (Fig. la).

(2) To fabricate the magnetic core, a layer ofchromium (10 nm thick) and copper (300 nmthick) was deposited on a separate siliconwafer. Permalloy (Ni8oFe2o) electroplatingwith AZ4620 photoresist mold was thenconducted to form the magnetic core (10 pmthick) of the probe. After the electroplating,the AZ4620 mold was completely removed,which was followed by the patterning of anAZ5214 photoresist layer onto the Permalloycore as an insulating layer between the topconductors and the core. Next, the Permalloycore was released by sacrificial etching of thechromium /copper layer.

(3) To assemble the entire probe, an electroplatedPermalloy core was placed and bonded ontothe fabricated silicon chip. The topconductors were placed by conducting goldwire bonding to form a complete 3Dconducting coil (Fig. lb and Ic).

(a)

Fig. 2 Scanning electron micrograph of afabricated micro electromagnetic probe.

3. EXPERIMENTAL CHARACTERIZATION

The experimental characterization of thefabricated micro electromagnetic probe wasconducted using an SHPM system(NanoMagnetics Instruments Ltd.) (Fig. 3). TheSHPM is equipped with a Hall sensor probe with avery small sensing aperture (less than 1 x 1 m2) tosignificantly reduce averaging effect in themeasurement of the magnetic field. The scanningof the Hall sensor probe is controlled by apiezoelectric crystal with a step size less than100nm. The small sensing aperture coupled withthe fine scanning step size ensures a high spatialresolution necessary for the probecharacterization.

(b)

(c)

of the microexperimental

A scanning electron micrograph of a fabricatedtest probe is shown in Fig. 2. The goldconducting coil consists of 13 turns which spans a

region of 1.85 mm. The magnetic core has a

width of 400 pm, total shank length of 4.75 mmand a protruding extension of 1.4 mm. The probetip has a taper angle of 15.20.

Fig. 3 Experimental setup for micro electro-magnetic probe characterization using SHPM.

To avoid possible collision damage, a small gap

( 20 pm) was maintained between the Hall sensor

element and the probe tip. Under thisexperimental setup, the Hall sensor probe mainlypicks up the magnetic field component parallel theprobe axis. However, this should not significantlyaffect the accuracy of magnetic field measurementsince almost all the magnetic flux at the probe tip

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Fig. ] Fabrication process flowelectromagnetic probe for thecharacterization.

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is aligned to the probe axis due to the taperedshape of the probe tip. Before the measurement,the micro electromagnetic probe tip was firstdemagnetized using the built-in coil of the SHPMby applying an exponentially decaying sinusoidalmagnetic field with alternating polarity to the axisof the Permalloy core. After the demagnetization,the tip of the micro electromagnetic probe wasaligned to the Hall sensor and the peak outputmagnetic field density (B) as a function of theinput current (I) was measured. As shown in Fig.4, the output magnetic field density (B) firstincreases linearly as a function of the input current(I) and then saturates at around 300 Gauss. Next,the B-H curve of the Permalloy core wascharacterized, which reveals a characteristichysteresis behavior of Permalloy (Fig. 5).

350

300

CD 250

7, 200

. 150a)r_ 100

! 50

00 0.05 0.1 0.15 0.2

Current [A]0.25 0.3 0.35

Fig. 4 Measured magnetic field density (B) at the,P probe tip as afunction ofinput current (I).

Tesla) widely reported in literature [10]. This isbecause the Hall sensor is positioned around 20pm away from the probe tip. Due to its largegradient, the magnetic field of the probe willquickly diminish at locations farther away fromthe probe tip. In many real applications, thesamples usually have to be placed at a smalldistance away from the probe tip. Therefore, ourmeasurement setup and results are valid anduseful in assessing the actual magnetic field thatwill be experienced by the samples since theyreflect the real working condition of theelectromagnetic probes.To obtain the spatial distribution of the

magnetic field, the Hall sensor probe was scannedacross an area of 25x25 tm2 around the probe tip,while maintaining the gap between the probe tipand the Hall sensor. Fig. 6a shows the spatialdistribution of the magnetic field with an inputcurrent of 300 mA. Fig. 6b and 6c show themagnetic field distribution at the two diagonalcross sections of the probe tip, respectively. Thefield drops rapidly to half of its peak value (297.2Gauss) within a distance of 4 pm and to a fewgauss within a distance of about 12 pm, whichclearly indicates a highly focused magnetic field.

297. _

40

30

20 -

10/

m -4000 -2000 10

-30

Anr

/ 2000(a)

4000

-44u

H [A/m]

Fig. 5 The B-H curve of the Permalloy magneticcore characterized by the scanning Hall probemicroscope.

It should be noted that the measured saturationintensity of 300 Gauss is much lower than thesaturation magnetization of Permalloy ( 0.9

n

._E

10 11 1 2 2 2 (b)0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

Distance [pm]

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A) f

r-9

.8.'a4)

LL

28026024022020018016014012010080604020

)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 (CDistance [rn]

Fig. 6 Spatial distribution of the magnetic fielc(the component parallel to the axis of the probe)with an input current of300 mA. (a) Surface Plot,(b) and (c) Axialfieldplots along the diagonals o)the probe tip obtained from the cross-sectionafield distribution.

4. CONCLUSION

In our experimental characterization, it is founcout that the spatial distribution of the magnetic

t field is extremely sensitive to the actual profile o1z m the probe tip, which is always somewhat differeni

from the design due to imperfection in probe> t fabrication. This indicates that in applications

Z where an accurate mapping of field distribution icritical, a good experimental characterization i

S: indispensable. A comprehensive experimental< t characterization is critical for the design and

optimization of micro electromagnetic probesSHPM provides the needed high spatial resolutiorfor micro magnetic field measurement, and thuscan be a useful tool for the characterization oi

MEMS-based magnetic devices.

ACKNOWLEDGEMENT$

This work is partially supported by the WelcdFoundation and National Science Foundation.

REFERENCES[1] C. S. Lee, H. Lee, and R. M. Westervelt

"Microelectromagnets for the control ol

magnetic nanoparticles," Applied Physic6Letters, vol. 79, no. 20, pp. 3308-3310.2001.

[2] T. Deng, G. M. Whitesides, MRadakrishnan, G. Zabow, and M. Prentiss"Manipulation of magnetic microbeads ir

suspension using micromagnetic systemsfabricated with soft lithography," AppliedPhysics Letters, vol. 78, no. 12, pp. 1775-1777, 2001.

[3] C. H. Ahn, M. G. Allen, W. Trimmer, Y.-N.Jun, and S. Erramilli, "A fully integratedmicromachined magnetic particleseparator," Journal of Microelectro-mechanical Systems, vol. 5, no. 3, pp. 151-158, 1996.

[4] H. Lee, A. M. Purdon, and R. M.Westervelt, "Manipulation of biologicalcells using a microelectromagnet matrix,"Applied Physics Letters, vol. 85, no. 6, pp.1063-1065, 2004.

[5] M. Barbic, J. J. Mock, A. P. Gray, and S.Schultz, "Scanning probe electromagnetictweezers," Applied Physics Letters, vol. 79,no. 12, pp. 1897-1899, 2001.

[6] B. D. Matthews, D. A. LaVan, D. R.Overby, J. Karavitis, and D. E. Ingber,

f "Electromagnetic needles with submicront pole tip radii for nanomanipulation of

biomolecules and living cells," AppliedPhysics Letters, vol. 85, no. 14, pp. 2968-2970, 2004.

i [7] C.-H. Chiou, Y.-Y. Huang, M.-H. Chiang,I H.-H. Lee, and G.-B. Lee, "New magneticI tweezers for investigation of the mechanical

properties of single DNA molecules,"Nanotechnology, vol. 17, pp. 1217-1224,2006.

f [8] R. Rong, J.-W. Choi, and C. H. Ahn, "Anon-chip magnetic bead separator for biocellsorting," Journal of Micromechanics &.Microengineering, vol. 16, pp. 2783-2790,2006.

[9] A. Thiaville, L. Belliard, D. Majer, E.Zeldov, and J. Miltat, "Measurement of thestray field emanating from magnetic forcemicroscope tips by Hall effectmicrosensors," Journal of Applied Physics,

f vol. 82, pp 3182-91, 1997.[10] J. Y. Park and M. G. Allen, "Development

of magnetic materials and processingtechniques applicable to integratedmicromagnetic devices," Journal ofMicromechanics & Microengineering., vol.

1 8, pp. 307-316, 1998.

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