Integrated Electromagnetic Generator for Energy Scavenging

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Jiwu Lu, Alexey Y. Kovalgin, Jurriaan Schmitz. Abstract ... Netherlands, E-mail: J.Schmitz@utwente.nl the reduction of ..... [1] Joseph A. Paradiso, Thad Starner,.
Modeling of an Integrated Electromagnetic Generator for Energy Scavenging Jiwu Lu, Alexey Y. Kovalgin, Jurriaan Schmitz Abstract The ubiquitous deploying of wireless electronic devices due to pervasive computing results in the idea of Energy Scavenging, i.e., harvesting ambient energy from surroundings of the electronic devices. As an approach to possible practical realization of such an energy scavenger, we aim at the fabrication of an electromagnetic (EM) generator. The generator will be fully on-chip integrated, CMOS compatible, suitable for silicon post-processing, and will have a natural resonant frequency below 100 Hz. To the best of our knowledge, such an EM generator will be the first fully on-chip integrated energy harvester. We have designed an integrable EM generator and have developed a process flow entirely based on low temperature post-processing technology. Our EM generator is composed of a permanent micro-magnet, a spring, and copper coils. The resonant frequency of the generator will be tuned by the spring constant k and the mass of the magnet. The AC/DC module of COMSOL software was used to perform numerical simulations in order to determine optimal relative dimensions of the generator and the best possible position of the permanent micro-magnet with respect to the copper coils. Keywords: CMOS post-processing, electromagnetic generator, energy scavenging, deposition, micromachining, permanent magnet

1. Introduction In the realization of pervasive computing[1], powering a huge number of the small electronic devices by wires or replacing the low capacity batteries routinely is an impossible mission. In the past decade, the advances in VLSI fabrication, such as Jiwu Lu is with University of Twente, MESA+ Institute for Nanotechnology, Hogekamp 3214, P.O. Box 217, 7500 AE Enschede, the Netherlands, Tel.: +31.53.489.2729; Fax: +31.53.489.1034; E-mail: [email protected] Alexey Y. Kovalgin is with University of Twente, MESA+ Institute for Nanotechnology, Hogekamp 3218, P.O. Box 217, 7500 AE Enschede, the Netherlands, E-mail: [email protected] Jurriaan Schmitz is withUniversity of Twente, MESA+ Institute for Nanotechnology, Hogekamp 3248, P.O. Box 217, 7500 AE Enschede, the Netherlands, E-mail: [email protected]

the reduction of the transistor power consumption and the low duty cycles of the wireless electronic devices, reduced the power requirement to the sub-miliwatt range [2]. The low-power dissipation provides the opportunity for energy harvesting from the ambient environment, which surrounds the electronic devices. There are various energy sources for energy harvesting, and each source has certain advantages and disadvantages[3]. Utilizing mechanical vibrations is the most versatile approach due to the ubiquity of vibrations in nature. Based on the energy conversion mechanism (i.e., from kinetic energy of the vibrations to electric energy), generators can be classified into 3 categories: electromagnetic generators [2], electrostatic generators [4], and piezoelectric generators [3]. Piezoelectric generators are the most

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promising and successful macro devices. In the past several years, while trying to miniaturize the piezoelectric generators, one significant problem occurred. Namely, the resonant frequency of such micro-scale energy generators can hardly match with the frequency of the mechanical vibration sources in the environment. The latter has a typical value below 100 Hz, and the vibration frequency of a micro-scale energy generator is well above 1 kHz [5]. In this work, a new EM generator design is proposed, operating at a natural resonant frequency below 100 Hz. The generator is targeted to be fully on-chip integrated, using CMOS compatible post processing technology.

The spring is made of the spiral or bispiral coils, which are fabricated by electroplating, and SU8-2000 photoresist will be used as a sacrificial material. Both thickness of the copper spring and weight of the micro permanent magnet will determine a vibration frequency of the EM generator. Compared to the conventional cantilever beam used as the spring of a EM generator, our EM generator can easier provide a low vibration frequency.

2. Design of the new EM generator Fig.1 depicts the design of our EM generator. It is composed of coils, springs, and a permanent magnet.

Fig. 2: Copper spring [6] The permanent micro magnet is Co80%Pt (20-x)%Px%, and will be electro-deposited during the device realization instead of a manually assembly. Such a process can provide a thickness of the permanent magnet about 20 μms [7] or up to 50 μms [8]. The magnetic field can exceed 0.5 T. The electro-deposition is done at a temperature well below 420 oC, which satisfies the thermal budget requirement of silicon postprocessing [9].

Fig.1: Schematic diagram of the micro scale EM generator The coils are multi-layer copper coils realized by electroplating, sputtering and ion etching. SU8-2000 is used as the insulator between the successive layers.

Fig. 3: Schematic diagram of the permanent magnet 3. Simulations In order to find the optimum relative positions of the magnet and the coils, numerical simulations have been carried out

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structure consisted of 39000 elements; the number of degrees of freedom to be solved was 640000. While performing the simulations in a vertical direction, the equilibrium position is set to 10 μm above the top surface of the magnet. The magnetic flux through the coil and EMF of the coil versus time are shown in Figs. 5 and 6.

using a COMSOL FEMLAB code [10]. The code calculated the magnetic field from the Maxwell equations under the quasi static assumptions [11], which are applicable due to the small size of the EM generator and the low vibration frequency: μ x − x' B = ∇ × A = 0 ∫∫∫ J M (x' ) × dv ' 3 ' 4π volume x−x (1) μ0 x − x' + j m (x' ) × ds ' ' 3 4π ∫∫ s x−x Where A is the magnetic vector potential; J M ≡ ∇ × M is the magnetic volume current density; jm (x ) ≡ M × nˆ is magnetic surface current density; M is the remanent magnetization.

Magnetic flux through one Single Coil [nWb]

Magnetic flux v.s Time of Vertical Vibration in One Period Time 6

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Using equation (1), the stray field of a permanent magnet has been created by a ViziMag code, see Fig.4.

Fig.5: Magnetic flux (vertical vibrations) through a single-turn coil. EMF VS time for one single turn at 100 Hz 8 6

Electomotive Force [uV]

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Fig.4: Stray field of a permanent magnet 3.1 Simulations by COMSOL An AC/DC module of COMSOL 3.3a has been used to perform the numeric simulations. In the model, the coil vibrated inside the stray field of a cylindrical magnet. For the simulations, the height of the permanent magnet was set to 50 μm, and its radius was also 50 μm. The vibration amplitude of the coil was 5 um at a frequency of 100 Hz. There are two possible vibration directions of the coil with respect to the magnet: a vertical vibration and a horizontal vibration. Both the directions have been simulated. The mesh of the simulated

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Fig.6: Electromotive force (vertical vibrations) of a single-turn coil at 100 Hz. When performing the simulations in a horizontal direction, the coil is placed at 5 μm above top of the permanent magnet. The magnetic flux through the coil and EMF of the coil versus time are shown in the Figs. 7 and 8, respectively.

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Magnetic density through one single coil of Horizontal Vibrationvs tim e

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Fig.7: Magnetic flux (horizontal vibrations) through a single-turn coil. EMF vs tim e for one single turn coil of horizontal vibration at 100 Hz

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From Fig.6, the maximum EMF of the vertical vibrations is about 5 μV, so one can obtain the maximum power output from V2 Pmax _ vertical = = 62.5 pW . R The power can be extracted is half of the power disspipated: V2 Pextracted _ vertical = = 31.2 pW R From Fig.8, the maximum EMF of the horizontal vibrations is about 0.5 μV. Therefore, the power output in this case is much lower: V2 Pmax _ horizontal = = 0.625 × 10 −7 pW R The power can be extracted is half of the power disspipated: V2 Pextracted _ horizontal = = 0.311 pW 2R

-1 -1,5 -2 Tim e in units of T/80

Fig.8: Electromotive force (horizontal vibrations) of a single-turn coil at 100 Hz. Comparing the Figures, one can see that the horizontal vibrations are not as clear as that ofhe vertical vibrations. This is due to the limited meshed size in the simulation.A refinement of the mesh has been done, the new mesh consisted of 120000 elements with the number of degrees of freedom increased to 970000. However, this resulted in low computer memory, even for a sever with 8Gb memory. 3.2 Power estimation Assuming a 4 × 4 μm cross section of the coil, the coil resistance R is given by 60 × 2π × 10 −6 L = ≈ 0.4 Ω . R= σS 5.998 × 10 7 × 16 × 10 −12

3.3 Discussion From the results, the EM generator with a 50 × 50 × 50 μm 3 size permanent magnet and one single turn of the copper coil has a power output in the range of pW. If the vibration amplitude is about 5 μm, the vertical vibrations have a power output approximately 100 times larger than that of the horizontal vibrations. In order to increase the power output to the μW range, a bigger permanent magnet is necessary. Because the magnet height is limited to 50 μm due to the electrodeposition process, an array of small magnets can be realized. This array can be integrated inside an square of 1 × 1 mm 2 . Such a "big magnet" will have the surface area ~400 times larger:

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V 2 1 ⎛ dφ ⎞ 1 ⎛ d (B ⋅ S ) ⎞ 2 P= = ⎜ ⎟ = ⎜ ⎟ ∝ (area ) R R ⎝ dt ⎠ R ⎝ dt ⎠ The calculated power output will then increase up to 10 μW: 2

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Pmax = 31.25 pW × 400 * 400 = 5 μW 4. Conclusions In this paper, a newly-designed EM Generator for energy harvesting has been proposed. The fabrication technique has been described. From the carried out COMSOL simulations, size of the generator is to be chosen in the millimeter range, in order to have a μW power output. 5. Acknowledgements The authors would like to thank their colleagues from the Semiconductor Components group of the University of Twente, for their helpful discussions and suggestions. Our thanks to Marion NijhuisGroen of the MESA+ clean room staff and to Jacob Bart of the Micro&NanoFluidic Process Technology group, both at the University of Twente. We express out gratitude to Dr. A. Klaver for the discussions regarding the permanent magnet deposition. This project (TET.6630) is financially supported by the Dutch Technology Foundation (STW).

[4] P. D. Mitcheson, P. Miao, et al., MEMS electrostatic micropower generator for low frequency operation, 2004, Sensors and Actuators A 115, 523-529 [5] Steven R Anton, Henry A Sodano, A review of power harvesting using piezoelectric material(2003-2006), 2007, Smart Materials and Structures 16, R1-R21 [6] Johnny M.H. Lee, Steve C.L. Yuen, et al. Vibration-to-Electrical Power Conversion Using High-Aspect-Ratio MEMS Resonators, 2003, Proceedings of Power MEMS 2003 [7] P.L. Cavallotti, et al., Electrodeposition of magnetic multilayers, (1998) Surface and Coatings Technology, 105 (3), pp. 232-239. [8] F.M.F.Rhen, E.Backen, et al., Thick-film permanent magnets by membrane electrodeposition, 2005, Journal of Applied Physics 97, 113908 [9] Jurriaan Schmitz, Adding functionality to microchips by wafer post-processing, 2007, Nuclear Instruments and Methods in Physics Research A 576, 142-149 [10] http://www.comsol.nl/ [11] http://web.mit.edu/6.013_book/www/ ,

References [1] Joseph A. Paradiso, Thad Starner, Energy Scavenging for Mobile and Wireless (2005) IEEE Pervasive Electronics, Computing, 4 (1), pp. 18-27. [2] R. Amirtharajah, A. P.Chandrakasan, Self-Powered Signal Processing Using Vibration-Based Power Generation, 1998, IEEE Journal of Solid-State CircuitsPage(s):687 - 695. [3] Roundy, S., Energy Scavenging for Wireless Sensor Networks, 2003, Kluwer Academic Publishers, Boston MA.

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