MEMS for vibration energy harvesting

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Ambient energy harvesting of MEMS(Micro Electro Mechanical ... nature, since MEMS variable capacitors are fabricated with relatively mature silicon ...
MEMS for vibration energy harvesting Lin Li 1, Yangjian Zhang 1, Haisheng San 1, Yinbiao.Guo1, Xuyuan Chen 1,2 1

Pen-Tung Sah Micro-Electro-Mechanical Systems Research Center and School of Physics and Mechanical & Electrical Engineering, Xiamen, Fujian 361005, China 2

Faculty of Science and Engineering, Vestfold University College, P.O. Box 2243, N-3103 Tonsberg, Norway

ABSTRACT In this paper, a capacitive vibration-to-electrical energy harvester was designed. An integrated process flow for fabricating the designed capacitive harvester is presented. For overcoming the disadvantage of depending on external power source in capacitive energy harvester, two parallel electrodes with different work functions are used as the two electrodes of the capacitor to generate a build-in voltage for initially charging the capacitor. The device is a sandwich structure of silicon layer in two glass layers with area of about 1 cm2. The silicon structure is fabricated by using silicon-on-insulator (SOI) wafer. The glass wafers are anodic bonded on to both sides of the SOI wafer to create a vacuum sealed package. Keywords: vibration-to-electric, MEMS, electrostatic, SOI wafer

1. INTRODUCTION Traditionally electrical power has been generated from fossil fuels in large, which produce exhaust gas polluted atmosphere, soil and ocean. Now a variety of different methods exist for harvesting energy from large-scale ambient, such as wind energy, ocean waves, solar power, electrostatic, piezoelectricity, thermoelectricity, and physical motion. There are widely available and uncontaminated but difficult to harvest. Ambient energy harvesting of MEMS(Micro Electro Mechanical System)-based devices which convert mechanical energy into electrical energy have attracted much interest in both the military and commercial sectors. For example, random vibration can be converted into electricity which can be used by automotive sensors, microprocessors, RF transmitters and such as those developed using MEMS technology, which are often very small and require only a little power, but their applications are limited by the depending on battery power. So micro-generators are definitely needed by a completely self-powered wireless and battery-less sensor node. In the past several years, there is an increasing interest in the development of device which can generate electrical energy by using vibrations from the environment. MEMS/MOEMS Technologies and Applications III, edited by Jung-Chih Chiao, Xuyuan Chen, Zhaoying Zhou, Xinxin Li, Proc. of SPIE Vol. 6836, 683610, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.756153 Proc. of SPIE Vol. 6836 683610-1 2008 SPIE Digital Library -- Subscriber Archive Copy

Traditionally, there are three main strategies for the vibration energy harvesting: piezoelectric, electrostatic and magnetic [2]. Piezoelectric energy harvesting converts mechanical energy to electrical by straining a piezoelectric material, but the power collective efficiency lies on the quality of PZT. Electromagnetic energy harvesting has relatively lower voltage and power density level and no long-term stability. For electrostatic energy harvesting, capacitive principle can be quite easy implemented in MEMS (Micro-Electro-Mechanical System) technology. Therefore, capacitive method is strategically used in electrostatic energy converter device. The capacitive energy harvesting relies on the changing capacitance of vibration-dependent variable capacitor. Variable capacitor is initially charged and, as vibrations separate its plates, mechanical energy transforms to electrical energy. The attractive feature of this method is its IC-compatible nature, since MEMS variable capacitors are fabricated with relatively mature silicon micromachining techniques. But it needs complicated and high speed and precision dedicated circuitry. In addition, an external power source is needed to initially charge the capacitors. This disadvantage baffles the capacitive energy harvesting developing progress. Base on the changing capacitance of vibration-dependent variable capacitor, a capacitive energy harvesting devices has been designed. The two electrode plates of the capacitor are two different metals that have different work functions [1]. When they are contact via external connection, the contact voltage or build-in-voltage will be created across the capacitor, so there is no need for the additional power source. In addition, the MEMS technology involved is relatively simple.

2. DESIGN PRINCIPLE FOR CAPACITIVE VIBRATION ENERGY HARVESTER Energy harvesting from vibrations is based on the movement of a "spring-mounted" mass relative to its support housing (Fig 2). The movable electrode is deposited on the mass surface while the immovable electrode is deposited on the housing. A parallel-plate capacitor is formed. Acceleration is produced by external vibration energy that in turn causes the mass component to move and oscillate. With the mass vibrating, the distance between the two electrodes of capacitor changes, followed by the change of the stored electrical energy, which could induce a current in the outer circuit. This energy conversion process causes electrostatic damping forces to the mass-spring-damping systems. As a result, the oscillations are reduced and eventually extinguished. For initially charging the capacitor, the power source is required. A smart design to avoid the external power source has been developed recently. The contact voltage, which is formed by contacting materials of different work functions, is used as the power source to initially charge the capacitor. The work function is the least amount of energy required to remove an electron from the surface of a conducting material, to a point just outside the metal with zero kinetic energy, which is closely related to its Fermi energy level EVacuum yet the two quantities are not exactly the same. One material (metal) with a work function Ф1. Another material (metal) with a different work function Ф2. Both metals are aligned at vacuum level EVacuum when they are not contacted. As soon as the two metals are electrically contacted, such as passing an external conductor, the Fermi levels EFermi equilibrates. Fig. 1 illustrates this behavior schematically. The equilibration of the Fermi levels induces an electrical current. Electrons flow from the material with the lower work function to the material with the higher work function. Therefore, a contact potential difference V establishes between the metal. V is also referred to as contact voltage or built-in voltage. For generating max contact potential, materials of appropriate should be chosen. From the table 1, adequate materials are aluminium (Al) with Ф1 = 4.08 eV and platinum (Pt) with Ф2 = 6.35 eV. Using these two materials will result in a contact voltage of V = 2.27 V. [2]

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Table 1

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Figure 2. Design principle of the vibration energy harvester

3. STRUCTURE DESIGN AND SIMULATION 3.1 Structure design In this work, the capacitive energy harvester is designed as a sandwich structure shown in figure 3. SOI (Silicon-On-Insulator) wafer and Pyrex 7740 glass are used to make the function elements and vacuum sealing package. SOI wafer is in the middle and glass wafers are bonded on both the upper and the lower side of SOI wafer. The SOI wafer consists of beam, mass and the movable electrode. The counter electrode and the capacitor gap are formed in the upper glass wafer. The lower glass wafer defines the beam vibration range. The two glass wafers encapsulates device, and keep the device operating under low pressure conditions. Figure 4 shows the cross-section of vibration energy harvester. In the SOI wafer, the device layer is 7µm in thickness, which will be used to machine the suspending beam. The thickness of the SOI handle wafer is 500µm, which will be used to machine the mass stack. The BOX in SOI wafer has thickness of 1µm, which will be used as electrical isolation layer. Pyrex 7740 glass wafer has the thickness of 300µm.

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There several vital details should be designed, such as how to make movable electrode and counter electrode lead-out, system stiffness, and its natural frequency. In this section, we focus on how to design beam and its dimension, because beam design strongly determines performance of the vibration energy harvester.

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3.2 Beam design Beam design for a given mass determines the performance of the harvester. As described in figure 4, the harvester can be modeled by a mass-spring-damping system. To converting the most available vibrations in the environment, to say 1kHz vibration, the system must be quite soft in the variation direction and very rigid in other directions. We have made different design proposal which will be analyzed by using ANSYS software. The optimized design will be determined. Because of the beam thickness was limited by the SOI thickness, the position of the beam, number of the beam, and the width of the beam will be manipulated in the designs. First of all, structure of four beams on each side of the mass as shown in figure 5.a was proposed. Through ANSYS simulation, this design can not attain the operation requirement because the first order vibration mode is torsion. Hence, this four-beam-structure is eliminated. Another design of four-beam-structure on each corner is tried. The design is illustrated by figure 5.b. ANSYS simulation results show that the luffing motion is obtained in the first-order vibration mode. However, the resonance frequency is much higher than that of the first design with the same device dimension. In order to decrease the resonance frequency, we should decrease the dimension of beam which will cause the instability of the structure as the beams are too small to support the relative huge mass. Therefore, this second design option is excluded too. Learn from above two designs and ANSYS simulations, we have made a design that consists of 8 beams. The distribution of these beams is demonstrated in figure 5.c. We find that the eight-beam structure not only achieves the expected vibration mode but also the decreased natural frequency of desired range. Consequently, the third design will be used for design optimization and prototype fabrication. The optimization has been done as described in detail in the

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following paragraph.

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3.3 Design optimization by using ANSYS simulation Vibration energy harvester is expected to work on the 1kHz input frequency. This eight-beam-structure should be thoroughly simulated using ANSYS software according to the width of the beams and distance between the two beams at the same side of the mass. The ANSYS element library contains more than 150 different element types. Each type of element has a unique number and function. SOLID45 is suitable for the 3-D modeling of solid structures, so we use it as element type. The following material properties of single-crystal silicon will be used as simulation parameters. They are, density ρ = 2.33 kg·m-3, Young’s modulus E = 1.65×1011 N·m-2, and Poisson ratio ε = 0.3. The dimension of the harvester used in simulation is given as, the width and length of the mass is 6 mm, and the thickness of the mass is 508 µm which equals to the thickness of SOI wafer (thickness of the device layer is 7 µm, BOX thickness is 1µm, substrate thickness is 500 µm). The thickness of beam is 7 µm which can not change for it equals the thickness of device layer of SOI wafer. The simulation results show that five vibration modes were visible. The first-order vibration mode is elastic vibration in Z direction, and the other order vibration modes are swinging. Therefore we hope that the resonance frequency of the first-order vibration mode is much smaller than that of other vibration modes. Figure 6 to 10 display the resonance frequency versus beam dimension and the distance between the two beams at the same side of the mass. In figure 6, the resonance frequency dependence on beam length is plotted, where the width of beam is 300 µm, the distance between two beams at the same side of the mass Sp is 4000 µm. Certainly, the resonance frequency decreases with the increase of the beam length. We find that as the length of the beam increases, the difference between the first-order frequency and the second-order frequency decreases. When the length of beam is fixed as 500 µm, the distance between the two beams at the same side of the mass Sp is 4000 µm, the ANSYS simulation result for the resonance frequency dependence on beam length is plotted in figure 6. The resonance frequency naturally increases with the increase of the beam width. The difference between the first-order resonance frequency and the second-order resonance frequency becomes much more distinct when the width of the beam increases.

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Using 450 µm and 300 µm as beam length and width respectively for the input to ANSYS simulation, relationship between the resonance frequency and the distance of the beams is described in figure 8. When the distance between the two beams at the same side of the mass Sp increases, the difference between the first-order and the second-order resonance frequency increase too. But Sp can not arbitrary increase, as the length and width of mass is fixed as 6 mm, so the maximum of Sp is related to the width of beam. Figure 9 displays the multi-parameter effect on the resonance frequency of the harvester, supposing that the beam length is 450 µm. The optimized values for the group of parameters can be determined by seeing the results from figure 6 to 9. The best design is with the dimension of beam length equal to 450 µm, beam width equal to 310 µm, and distance between the beams Sp equal to 5380 µm. Using the dimension which has decided by above analysis, the first order and second order vibration mode is determined and demonstrated in Figure 10. The resonance frequency of first-order vibration mode is 998.2 Hz which is 732.6 Hz less than the second-order resonance frequency. This approves the correct design for the capacitive harvester.

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4. PROCESS FLOW AND THE PROTOTYPE FABRICATION According to schematic design of the capacitive harvester, a process flow for prototype fabrication has been designed. The harvester is processed in Pen-Tung SAH Micro-Electro-Mechanical Systems Research Center at Xiamen University. 4.1 Top glass wafer patterning process The upper glass is wet etched to create indent for upper leading-out electrode (fig 11.b) by using first mask, to generate the gap of the capacitor (fig 11.c) by using second mask, and to form a mechanical stoppers (fig 11.d) by using third mask which also can be utilized to deposit the Pt electrode as the fix plate for capacitors (fig 11.e). Finally, The leading-out Al connection is processed by using fifth mask (fig 11. f).

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0.3µm thick of SiO2 was grown by dry oxidation on the surface of the SOI-wafer after standard (fig 12.b) cleaning processes. As a next step, beam, groove of leading-out electrical connection and mass are firstly formed by wet-etching SiO2 (fig 12.c). Then 0.3-µm-thick Al is sputtered as movable electrode and leading-out electrode (fig 12.d). After that, beam and mass are secondly machined by deep-dry-etching Si (fig 12.e and f). At the end, mass and beam are release through wet-etching oxide layers (fig 12.g).

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4.3 Bottom glass wafer patterning process The lower glass wafer mass is simply wet-etched to enable a free movement of the mass and an encapsulation (fig 13).

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4.4 Sandwich structure formation process Ultimately, three wafers were bonded to a sandwich-structure via anodic bonding under low pressure. The prototype was fabricated and is shown in figure 14.

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Figure 14. Optical microscopy of top views for different parts of capacitive harvester

5. CONCLUSIONS A capacitive vibration-to-electrical energy harvester has been designed and optimized by suing ANSYS simulation tool to meet the 1kHz variation available in environment. The Pt and Al are used as the two electrode plates of the capacitor to generate a build-in-voltage for initial charging capacitor. An eight-beam-structure after the dimension optimization results in the best performance of the capacitive harvester. The process flow has been developed and the prototype harvester was successfully produced in SAH MEMS research center at Xiamen University.

ACKNOWLEDGE This work is supported in part by the Science and Technology Program of Xiamen City under contract No.3502Z20063006.

REFERENCE [1] Ingo Kuehne, Alexander Frey, Gerald Echstein, Helmut Seidel, “Design and Analysis Of A Capacitive Vibration-To-Electrical Energy Converter With Built-In Voltage”, Solid-State Device Research Conference, pp.138-141, (2006) [2] Bernard C. Yen, Jeffrey H. Lang, A Variable-Capacitance

Vibration-to-Electric Energy Harvester, IEEE

Transactions on circuits and System-I:Fundaymental theory and Applications [3] Cynthia Watkins, Bing Shen, and Rama Venkatasubramanian, Low-grade-heat energy harvesting using superlattice thermoelectrics for applications in implantable medical devices and sensors,Thermoelectrics, 2005. ICT 2005. 24th International Conference IEEE,pp.265- 267,(2005) [4] Shad Roundy, Improving Power Output for Vibration-Based Energy Scavengers, Pervasive Computing, VOL. 4, Issue: 1, pp. 28- 36, (2005) [5] Pritpal Singh, Sanjay Kaneria, Vinay Sagar Anugonda, Huiming M. Chen, Xiquan Q. Wang, David E. Reisner, and Rodney M. LaFollette, Sensors Journal, pp. 211- 222 , (2006) [6] Nirmal Kµmar Mandal, Roslan Abd. Rahman, M. Salman Leon, Experimental investigation of vibration power

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flowin thin technical orthotropic plates by the method of vibration intensity, Journal of Sound and Vibration 285, pp.669-695, (2005) [7] Fabio Peano and Tiziana Tambosso, Senior Member, Design and Optimization of a MEMS Electret-Based Capacitive Energy Scavenger, Microelectromechanical Systems, VOL. 14, NO. 3, pp. 429- 435 ,(2005) [8] Ghislain Despesse, Thomas Jager, Jean-Jacques Chaillout, Jean-Michel Léger Skandar Basrour[J]:Design and Fabrication of a New System For Vibration Energy Harvesting, [9] HE Fang, Huang Qing-an, Qin Ming: Fabrication of Uniform Crystal silicon menbrance using two-step silicon direct bonding Technolgy, Chinese Journal of Electron Devices [10] Thomas Velten, Hans Heinrich Ruf, David Barrow, Nikos Aspragathos, Panagiotis Lazarou, Erik Jung, Packaging of Bio-MEMS: Strategies, Technologies and Applications, Advanced Packaging, VOL 28,pp.533-546,(2005)

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