T3P.057

3D HIGH ASPECT RATIO, MEMS INTEGRATED MICRO-SOLENOIDS AND HELMHOLTZ MICRO-COILS V. Badilita1*, K. Kratt1*, T. Burger1, J.G. Korvink2,3, and U. Wallrabe1 Department of Microsystems Engineering – IMTEK, Laboratory for Microactuators, University of Freiburg, GERMANY 2 Department of Microsystems Engineering – IMTEK, Laboratory for Simulation, University of Freiburg, GERMANY 3 Freiburg Institute for Advanced Studies – FRIAS, University of Freiburg, GERMANY 1

ABSTRACT High-aspect-ratio 3D geometrically perfect solenoidal micro-coils are fabricated for the first time in a fully MEMS-integrated technology. Vertical micro-coils with up to 15 windings and diameters down to 100µm have been wound using an automatic wirebonder around SU8 and PMMA cylindrical posts. Using this method we also fabricate Helmholtz micro-coils capable to generate magnetic fields of 1mT, according to simulations. We demonstrate the potential to use large arrays of solenoidal micro-coils for energy harvesting applications by placing them in a sinusoidal magnetic field. The induced voltage is 1.4mV at 300kHz for a 7 windings micro-coil, in agreement with theoretical calculations.

KEYWORDS 3D micro-coils, wire-bonding, energy harvesting.

INTRODUCTION The development of 3D high aspect ratio microsolenoids and Helmholtz micro-coils proved to be a challenging task for the MEMS community mainly because the traditional microfabrication techniques inherently generate 2D structures. Many groups have focused their research efforts towards obtaining 3D microcoils in the past decade. Peck et al. [1] and Seeber et al. [2] have used a hand-winding technique to produce microcoils by wrapping a wire around a capillary. This procedure is not compatible with batch-fabrication since each coil must be treated individually, with consequences on the yield and reproducibility of the manufactured coils. An interesting but rather complicated technique reported by Rogers et al. [3] involved micro-contact printing combined with a rolling process and subsequent electroplating to define coils around a capillary tube. An innovative approach by Dohi et al. [4] combines surface micromachining and a post-release folding process to create freestanding micro-coils. Ehrmann et al. recently reported [5] a MEMS-compatible technology to create micro-solenoids and Helmholtz micro-coils using three electroplated copper layers and vias through SU8 isolation layers. While this is a batch-fabrication technique, due to the planar nature of the processes involved, the aspect ratio of the coils, therefore their 3D character is rather limited. Our group has recently reported [6] a method to fabricate coils with sub-millimeter dimensions on PCB

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substrates exploiting the unique capabilities of an automatic wirebonder. However this is not compatible with wafer-scale fabrication, therefore unsuitable for MEMS applications. Here, we report for the first time a fully MEMSintegrated process for micro-coil fabrication using the automatic wirebonder in conjunction with traditional microfabrication techniques. CrAu evaporation on Si or Pyrex substrate together with UV photolithography is used to define the metal pads for coil winding. Cylindrical posts to support the micro-coils are defined using thick SU8 photolithography. Alternatively, the Deep X-Ray Lithography technique has been used to demonstrate PMMA posts to support micro-coils with substantially increased aspect ratio. Although the wire-bonder based technique to manufacture the micro-coils is a serial technique, it is compatible with the previously mentioned standard batch-fabrication techniques due to the fact that it is very fast and reproducible.

MICRO-COIL SUBSTRATE PROCESSING The purpose of this paper is to demonstrate on-wafer integration of 3D high aspect ratio micro-solenoids and Helmholtz micro-coils. To this end, we have used Si and Pyrex substrates, CrAu metallization and two different methods for the manufacturing of the posts supporting the micro-coils: SU8 photolithography and Deep X-Ray Lithography in PMMA. CrAu (50/500nm) was evaporated on a Si or Pyrex substrate. AZ 1518 photoresist was UV-patterned and the CrAu metal was subsequently wet-etched to define the metal pads for winding the micro-coils. A thick SU8 process was employed to define high aspect ratio cylinders as supporting structures for the micro-coils. We have adopted the “constant-volume injection” method described by Lin et al. [7]. That means, instead of using a conventional spin coater, the volume of the SU8 photoresist was first calculated for a given thickness on a 4-inch wafer surface. One has to also consider that the solvent content of SU8 is drained from the film after baking so the final photoresist thickness is decreased by about 20% [7]. In this work, a volume of 6.5ml high viscosity SU8-2150 yields a 650µm-thick photoresist layer after development. After casting the SU8 layer as explained in [7], the wafers are softbaked on a hotplate as follows: the temperature was ramped (1deg/min)

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a)

b)

c)

Figure 1: Micro-coil winding technique: a) thee trajectory is limited to 15 points [6]; b) almost contiinuous trajectory with 24000 points; c) capture of the wirebonder head in movvement during the micro-coil winding process

from room temperature to 65°C, maintainned at 65°C for 15min and then ramped up (1deg/min) to 95°C. The wed by cooling wafers were kept at 95°C for 9 hours follow down back to room temperature with the same ramp of 1deg/min. This long softbake ensures a good selfplanarization of the SU8. Additionally theere is no edgebead effect, which commonly occurs whille using a spin coater. The wafers were subsequently exposeed to a dose of 1300mJ/cm2 for the Si substrates and 20000mJ/cm2 for the Pyrex substrates using an i-line filter. Post-exposure baking (PEB) was carried out using the same ramping parameters but the wafers were maintaineed at 95°C for only 5 hours. The structures have been ddeveloped for 1 hour in SU8 developer. Full and hollow posts with outer diam meters between 100 and 1000µm and sidewall thicknesses down to 60µm have been fabricated with an excellent adhesion to both the Si and Pyrex substrates. Taking into accoount the 650µm height of the posts, we demonstrate an aspect ratio of Employing SU8 almost 11:1 for the 100µm diameter posts. E photolithography to define the supportingg posts for the subsequent micro-coil winding opens the perspective to further fill the inner volume of the coil withh soft-magnetic materials. Therefore, increased aspect ratioo means thinner sidewalls of the supporting post and an im mproved filling factor of the micro-coil. In order to further increase the aspectt ratio we have employed the Deep X-Ray Lithographyy technique to fabricate PMMA posts. A 760µm-thick P PMMA layer is glued onto a Si substrate with CrAu paads defined as described above. The PMMA is structured using X-Ray Photolithography. The processed postss have outer diameters down to 100µm and sidewall thicknesses of down to 20µm leading to an aspect ratio of uup 38:1.

MICRO-COIL WINDING PROCESS S Vertical 3D micro-solenoids Our group has previously reported [66] a method to fabricate micro-coils wound around glasss capillaries on PCB substrates. This is not suitable for on-wafer integration since the capillaries have to be inserted in the substrate by a pick-and-place techniquee. The process described in the present work is fully ME EMS-integrated since the supporting structures are fabricateed by traditional

planar technologies as shown in the previous section. Moreover, we report here a significant step-forward in what concerns the trajectory described by the wirebonder ocess. As shown in [6], one head during the coil winding pro can define a 3D trajectory to be followed by the wirebonder head. For solenoid dal coils, the head moves around the supporting post, the wire w deforms plastically to the shape of the post and rem mains as a solenoid. The previously reported technology y was limited to only 15 points per trajectory of the wireebonder head – Figure 1a. Thus, assuming that three pointss define one winding, each micro-coil is limited to a maxim mum of 5 windings. In this paper we demonstrate a refined arbitrary a trajectory with up to 24000 points (Figure 1b) thatt allows for the fabrication of vertical 3D micro-coils with hypothetically up to 8000 windings. Therefore, the actual number n of windings is only limited by the height of the supp porting SU8/PMMA post. Compared to [6] we are now using commercially available insulated wire (X-Wirre™) and we are bonding on Si or Pyrex substrate with CrAu pads. The bonding parameters have been adapted appropriately as shown in u finish used in [6] is softer Table 1. PCB with a thick CuAu than Si with a relatively thin CrAu layer, therefore the bond forces are smaller in the Si case. The fact that it is actually easier to realize a go ood bond on Si or Pyrex substrate appears clear from Table T 1: the significantly smaller forces and shorter times needed for these B. substrates as compared with PCB To get better adhesion at thee second bond (wedge), we introduced a cleaning stage with h a shift in the wedge-ball direction (similar to [8]). This shift s is a movement of the wirebonder capillary when the wire w is in contact with the metallic pad, performed in ord der to break the insulating layer and expose the metal for a good wedge contact. Although the wirebonding process p is a serial process, it is extremely fast and allow ws, in combination with established microfabrication tech hniques, for manufacturing of large, wafer scale arrays. The T manufacturing time is 200ms for a single coil and less than 12s for the full array shown in Figure 2a. Figure 2b shows a micro-coil with a diameter of d a 650µm–high SU8 post. 150µm and 15 windings around The number of windings is on nly limited by the actual height of the supporting structu ure and by the diameter of the wire. For this work we havee used 25µm gold wire. An

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important characteristic of this process iis that we use insulated wire therefore no pitch has to be introduced between consecutive windings in order tto avoid shortcircuits. Table 1: Bonding parameters optimized forr Si and Pyrex substrate versus PCB [6]

F (mN) Time (ms) US (µm)* Shift (µm) T (°C) *

PCB 450 45 0.8 100

ball Si 250 8 1.8 110

Pyrex 210 5 1.8 110

PCB 3500 70 0.4 100

wedge Si 800 14 0.8 13 110

Pyrex 700 14 0.8 13 110

b)

ENERGY SCAVENGING APPLICATIONS A Planar coil arrays have beeen recently reported for applications as energy converters [9]. The technology reported in n this paper goes one step forward towards miniaturization since it naturally lends itself to the manufacturing of larrge planar arrays of vertical 3D micro-solenoids. One of th he possible applications of micro-coil planar arrays is in the field of energy scavenging.

Ultra-sound amplitude at 128kHz, in µm.

a)

a)

Figure 3. a) Helmholtz micro-co oil with 200µm radius; b) Calculated magnetic field along thee axis of the Helmholtz microcoil. z = 0µm denotes the bottom of the micro-coil (substrate level), z = 500µm – the top.

b)

Figure 2. a) Large array of micro-coils on SSi substrate with CrAu pads, wound around SU8 posts with diameters from 1000µm (upper line) to 200µm (bottom line); bb) Side view of a micro-coil with 150µm diameter and 15 windinggs. The SU8 post is 650µm high.

3D Helmholtz micro-coils This versatile technology can be eassily adapted to fabricate Helmholtz micro-coils. In generaal, a Helmholtz pair consists of two coaxial identical circular coils separated by a distance equal to the radius oof the coil. Each coil carries an equal electrical current flowiing in the same direction. Helmholtz coils are used to prodduce regions of uniform magnetic field. Therefore, this techhnology enables on-wafer integration of localized homogeenous magnetic fields by means of Helmholtz micro-coils. micro-coils with Figure 3a shows a Helmholtz pair of m a radius of 200µm and 2 windings per coill. Therefore the spacing between the two coils is also 2000µm. The two micro-coils share the same bonding pads thhus ensuring the same direction of the current flow. The simuulated magnetic field is found to be 0.9mT for a currentt of 100mA as shown in Figure 3b. One has to note that at this scale tthe number of windings is limited by the actual geomettric constitutive constraints of a Helmholtz pair in generaal. Taking into account the wire diameter of 25µm, the tottal width of one micro-coil has to remain small with respectt to the distance between micro-coils – 200µm in this case.

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Figure 4. Linear dependence betw ween the induced voltage and the frequency of the sinusoidal ma agnetic field, as predicted by Faraday’s law.

In order to evaluate this po ossibility, we placed single micro-coils with different diameters and number of windings in a time-dependent magnetic field. For the nt magnetic field we have generation of the time-dependen used a custom-made Helmholtzz pair with a diameter of 2cm. Each coil of the Helmholtzz pair has 120 windings, Cu wire with 50µm diameter using g a coil-winding machine. The single micro-coil was placeed in the uniform magnetic field region inside the Helmholtzz coil. An AC signal with a frequency of 300kHz was used d to generate a sinusoidal magnetic field with the same frequency. According to Faraday’s law, a voltage is in nduced in the micro-coil placed in a time-varying mag gnetic field. The induced voltage increases linearly with th he frequency of the applied sinusoidal magnetic field, as con nfirmed by Figure 4. Coils with different diameteers and different number of windings have been placed in n the sinusoidal magnetic field. The induced voltage for a single micro-coil with a

diameter of 1000µm and 7 windings was m measured to be 1.4mV at 300kHz (Figure 5). This micrro-coil winding method offers the potential to further increaase the induced voltage by increasing the number of windinngs, as stated in the processing section above. Moreover, onne can build 2D arrays of closely packed micro-coils for eneergy scavenging over large areas.

Figure 5. Measured induced voltage vs. coil diameter at 300 kHz. The number of windings N has a strongger influence for larger coils, leading to higher voltages.

CONCLUSIONS We report in this paper a fully ME EMS-integrated technology to fabricate on-wafer 3D micro-solenoids with the axis perpendicular to the substrate. We prove the monstrating onversatility of this technology by also dem wafer Helmholtz micro-coils. The technique presented here consists inn exploiting the capabilities of an automatic wirebonder – for micro-coil fabrication, together with traditional m microfabrication techniques – for micro-coils wafer scale inteegration. This method opens new perspectives inn various fields. We have demonstrated the possibility to use these microcoils for energy harvesting applications. Planar arrays can be fabricated with this technique for largge area energy scavenging devices. Another potential appplication lies in the field of magnetic resonance investigatiion where large 2D arrays of micro-coils would enable parallel, highthroughput sampling and diagnosis at the ceell level. This work closes a long-persisting gapp in the MEMS community by providing a fast, reliable annd reproducible method to fabricate fully wafer scale, high aspect ratio 3D vertical micro-solenoids.

REFERENCES [1] T.L. Peck, R.L. Magin, and d P.C. Lauterbur, “Design and Analysis of Microcoils for NMR Microscopy”, Journal of Magnetic Reson nance, vol. 108, pp. 114124, 1995. [2] D.A. Seeber, R.L. Cooper, L. Ciobanu, and C. H. T of High Sensitivity Penningtona, “Design and Testing Microreceiver Coil Apparaatus for Nuclear Magnetic Resonance and Imaging””, Review of Scientic Instruments, vol. 72, no. 4, pp. p 2171–2179, 2000. [3] J.A. Rogers, R.J. Jackman n, and G.M. Whitesides, “Using Microcontact Printin ng to Fabricate Microcoils on Capillaries for High Reesolution Proton NMR on Nanoliter Volumes”, Appl. Phys. P Lett., vol. 70, no. 18, pp. 2464–2466, 1997. [4] T. Dohi, K. Kuwana, K. Matsumoto, and I. Shimoyama, “A Standing Micro-coil for a High Resolution MRI”, Proc. of Transducers 2007, pp. 1313-1315. [5] K. Ehrmann, N. Saillen, F.. Vincent, M. Stettler, M. Jordan, F.M. Wurm, P.-A.. Besse, and R. Popovic, “Microfabricated Solenoids and Helmholtz Coils for mmalian Cells”, Lab on a NMR Spectroscopy of Mam Chip, vol. 7, pp. 373-380, 20 007. [6] K. Kratt, M. Seidel, M. Em mmenger, U. Wallrabe, and J.G. Korvink, “Solenoidal Micro-coils Manufactured with a Wirebonder”, Proc. of MEMS 2008, pp. 996999. W. Chang, and Guan-Liang [7] C.-H. Lin, G.-B. Lee, B.-W Chang, “A New Fabrication Process for Ultra-Thick Utilizing SU-8 Microuidic Microstructtures Photoresist”, J. Micromech h. Microeng., vol 12, pp. 590–597, 2002. [8] J. Lee, M. Mayer, Y. Zhou, and J. Persic, “Pull Force and Tail Breaking Force Op ptimization of the Crescent Bonding Process with Insu ulated Au Wire”, Proc. of Electronics Packaging Tecchnology Conference, pp. 727 – 730, 2007. [9] B. Mack, K. Kratt, M. Stürmer and U. Wallrabe, “Electromagnetic Micro Geenerator Array Consisting of 3D Micro Coils Oppo osing a Magnetic PDMS Membrane”, in press, Proc. of Transducers 2009.

ACKNOWLEDGMENTS The authors acknowledge the funding by tthe EC through the NEST Project 028533. The authors alsso thank Jürgen Mohr and Martin Börner from Forsschungszentrum Karlsruhe, Institute for Microstructure T Technology for providing LIGA-patterned PMMA substratees.

CONTACT Vlad Badilita: Phone: +49-761-2 203-7435; Fax: +49-761203-7439; email: [email protected] * These authors have contributed d equally to this work.

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