A SILICON MEMS MICRO POWER GENERATOR FOR WEARABLE MICRO DEVICES

Transcription

1 Journal of the Chinese Institute of Engineers, Vol. 30, No. 1, pp (2007) 133 A SILICON MEMS MICRO POWER GENERATOR FOR WEARABLE MICRO DEVICES Wen-Sheh Huang*, Kung-Ei Tzeng, Ming-Cheng Cheng, and Ruey-Shing Huang ABSTRACT In this paper, a prototype vibration-induced micro power generator is fabricated and tested to investigate the feasibility of a wearable silicon micro power source. This prototype device consists of an energy-collecting electroplated copper coil of 0.1 mm thickness, a nickel-iron alloy (NiFe) suspension spring and a commercially available neodymium iron boron (NdFeB) magnet of dimension 2 mm 2 mm 1 mm. The overall size of the micro power generator is 6 mm 6 mm 1mm. The average generated power is 0.32 uw when the device is tapped gently with fingers. This tapping mimics the vibration when the device is worn on a person, and demonstrates power generation feasibility by general human activity. Average power of 1.44uW was measured when the device was placed on a vibration bed excited with a sinusoidal signal of 50 µm amplitude. Key Words: micro power, micro generator, vibration, MEMS I. INTRODUCTION Miniaturized micro power source devices are of great interest for a variety of wearable MEMS devices and distributed sensor system applications. A mechanical micro power source that converts natural environmental or parasitic vibration kinetic energy to electrical energy is superior to the chemical based power sources (ex: lithium batteries, micro fuel cell, micro gas turbine) due to its unlimited lifetime and no need for replenishment of energy sourcing materials. Among others, a micro-generator utilizing electromagnetism to transform vibration mechanical energy into useful electrical energy is very attractive and easy to devise. A vibration electromagnetic micro-generator essentially is composed of a suspension spring, an energy pick-up coil, and a magnet mounted on the suspension spring. When the generator is excited by vibration the suspended magnet will vibrate and have a relative motion with respect to the energy pick-up coil, which surrounds the magnet. Hence the vibration will induce a change of magnetic flux through *Corresponding author. ( d869001@oz.nthu.edu.tw) The authors are with the Institute of Electronics Engineering, National Tsing Hua University, Hsinchu 300, Taiwan the coil and induce a voltage output. Several works have been carried out on the development of vibration-induced electromagnetic micro power generators. Shearwood and Yates (1997) first used polyimide membrane as the spring element and evaporated gold metal film to form the coil. Due to the membrane stretching induced spring-stiffening effect,the maximum vibration amplitude was much less than expected. Besides, increased generator internal resistance from the planar coil also degrades the power output efficiency. The device tested on a linear vibration bed has a maximum power output of 0.3uW at a resonant frequency of 4k Hz. For a wearable MEMS device application, a reduced resonant frequency is necessary for efficient energy scavenging from human motion induced vibration. Instead of using polyimide membrane and thin film planar coil, Ching et al. (2002) used laser micro machined copper spring and a printed circuit board (PCB) based coil. This spiral copper spring open structure reduced the aerodynamic damping and increased the spring elongation range with extended long spring length. Besides, a PCB based coil s radius is much larger than a thin film coil s thickness and reduces generator internal resistance. The average output power reported has significant improvement, but the overall device volume, up to 1 cm 3, is huge for miniaturized micro

2 134 Journal of the Chinese Institute of Engineers, Vol. 30, No. 1 (2007) Magnet Copper coil k D m (a) Spring z(t) y(t) x(t) Fig. 2 A typical vibration kinetic model of micro power generator Fig. 1 (b) Schematic diagrams of the proposed micro-power generator structure (a) cross-section view of the micro-generator, (b) 3D schematic view of the micro-generator system process integration. Similar structures are also found in (Glynne-Jones et al., 2004) which used etched stainless steel cantilever as the spring element and a hand wire-wound coil to pick up vibration energy. This miniaturized generator s volume is still too large for integrated micro power source applications. In this paper, we have focused on a miniaturized micro power generator using thick electroplated copper as the planar coil material to reduce generator s internal resistance. Also, instead of using copper as spring material, we have adopted nickel iron as the spring material. Since nickel iron has a larger yield strength, the spring element will have a larger linear vibration range in the limited device area. Commercial magnetic field simulation tools ANSOFT EM3D are used to investigate the spatial distribution of the magnetic field, which helps the design optimization of the spatial configuration of coil and magnet. Our design is a two-wafer bonded structure, as shown in Fig. 1; the magnet suspension spring is fabricated on one wafer, whereas the planar coil is on the other wafer. They are then bonded together with epoxy adhesive bonding. A prototype device was fabricated and tested to investigate the feasibility of a wearable micro power source. II. DESIGN OF MICRO POWER GENERATOR 1. Vibration kinetic to electric energy transduction Figure 2 shows a typical vibration kinetic model of a micro power generator. Function y(t), x(t) describes motion of vibrating housing and seismic mass, respectively. The relative motion between seismic mass m and vibrating housing is described by z(t), which is also expressed as x(t) y(t). According to Newton s law of force, the vibration equation for the seismic mass is m x(t).. = kz(t) f d (t), (1) where f d (t) represents the damping force and is modeled as Dż(t), here D is the damping coefficient. If we substitute (z(t) + y(t)) for x(t) we get m z(t).. = kz(t) Dż(t) m.ẏ(t). (2) Apply Laplace transform on both sides and rearrange to get Z(s) Y(s) = ms 2 ms 2 + Ds + k. (3) Equation (3) can be simplified by substituting the resonance frequency ω n = k/m and damping factor ξ = (D/2mω n ), then Z(s) Y(s) = s 2 s 2 +2ξω n s + ω 2. (4) n The vibration amplitude can be obtained by finding the absolute value of Z(s)/Y(s). Replacing Laplace variable s with complex frequency jω, we can find the absolute value of Z(s)/Y(s) from Eq. (4) Z 0 Y 0 = ω 2 (ω n 2 ω 2 ) 2 +(2ξω n ω) 2. (5) Here Z 0, Y 0, represent the relative vibration amplitude and excitation amplitude respectively. ω is

3 W. S. Huang et al.: A Silicon Mems Micro Power Generator for Wearable Micro Devices 135 the excitation frequency. Substitute ω and ω n with the dimensionless term ω c = ω/ω n, then Eq. (5) becomes Z 0 Y 0 = ω c 2 (1 ω c 2 ) 2 +(2ξω c ) 2. (6) The dissipated power can be calculated by integrating the work done per cycle and multiplied by frequency, Z 0 Z 0 P =2f f d dz =2f Dzdz. (7) Z 0 Z 0 The integrated result has been reported in several peapers (William et al., 2001; Li et al., 2000; Mitcheson et al., 2004) as P = ξω c 3 Y 0 2 ω 3 m (1 ω c 2 ) 2 +(2ξω c ) 2 (8) However, Eq. (8) describes the power dissipated by the damping force, not exactly the electric power generated. This damping power includes the air damping, mechanical hysteresis loss, and magnetic damping. The useful electric output power, in terms of emf, only derives from the damping induced by magnetic force, and resides in the coil. Faraday s law predicts that the induced emf is proportional to the time derivative of magnetic flux. This time derivative can be further expressed as the product of space derivative and relative velocity between magnet and coil, as shown in the following equation, where V is the voltage induced by magnetic flux change V = dφ = dt dx dφ dz dt = Φ z. (9) The magnetic force exerted on seismic mass can be found by dividing electric power generated V 2 /(R + sl) by the relative velocity ż between magnet and coil. Substituting V into Eq. (9), this magnetic force is expressed as a typical damping force equation as in Eq. (10). Where V, R, L represent generator output voltage, resistance and inductance, F = = In Eq. (10), V 2 (R + sl) 1 z Φ 2 (R + sl) z = D e. ż. (10) D e = Φ 2 /(R + sl). (11) D e represents the damping from magnetic force, which is induced from coil current. If the spatial flux change rate Φ is constant along the direction of vibration, the vibration governing equation, Eq. (2) can be solved by replacing ξ with ξ t, and ξ t = ξ e + ξ p, where ξ e = D e /(2mω n ), and ξ p = D p /(2mω n ) are the damping factors of magnetic damping and parasitic damping, respectively. The modified amplitude of vibration from Eq. (6) is then: Z 0 Y 0 = ω c 2 (1 ω c 2 ) 2 +(2ξ t ω c ) 2. (12) And the voltage amplitude from Eq. (9) is: V 0 = Φ ωω c 2 Y 0 (1 ω c 2 ) 2 +(2ξ t ω c ) 2. (13) Instead of using Eq. (7), the electric power generated can be estimated by P = V 02 /2(R + sl) P = Φ 2 ω 2 ω c 4 Y 0 2 (1 ω c 2 ) 2 +(2ξ t ω c ) 2 1 2(R + sl) = 1 2mω n Φ 2 R + sl mω 2 ω n ω c ω 3 2 c Y 0 (1 ω 2 c ) 2 +(2ξ t ω c ) 2 = D e mω 3 ω 3 2 c Y 0 2mω n (1 ω 2 c ) 2 +(2ξ t ω c ) 2. (14) Substitute ξ e = D e /(2mω n ), we obtain the final electric power generated P = ξ e ω c 3 Y 0 2 ω 3 m (1 ω c 2 ) 2 +(2ξ t ω c ) 2. (15) Similar results are also reported in the literature (William et al., 2001; Li et al., 2000; Mitcheson et al., 2004). Thus the electric power generated is only part of the dissipated power. Eq. (15) compared with Eq. (8) shows that it is a modification of damping factor with ξ t = ξ e + ξ p, and accounts for both the electric power generated and parasitic power dissipated. ξ p represents the parasitic energy dissipation including air damping and mechanical hysteresis loss, and ξ e is the damping from magnetic force induced by the coil current. When the device is operating in resonance, ω c = 1, Eq. (15) is then simplified as: P res = ξ ey 0 2 ω 3 m 4ξ t 2. (16) To find the maximum power generation, take differentiation on Eq. (16)

4 136 Journal of the Chinese Institute of Engineers, Vol. 30, No. 1 (2007) dp res = Y 0 2 ω 3 m(ξ p ξ e ) dξ 3. (17) e 4ξ t The maximum power generation will occur at ξ e = ξ p. The estimated damping ratios in the work (William et al., 2001) give typical values of ξ air , ξ elec , and ξ mem These data showed that ξ e << ξ p for their micro power generator with planar coil, and there is plenty of room for improvement to increase the electric damping factor ξ e and thus the kinetic to electric transduction efficiency. From Eq. (11), the coupling efficiency may be improved by increasing spatial flux change rate Φ and reduction of generator internal resistance R. 2. Design Consideration of Coil The design parameters of a planar spiral coil include number of turns, metal line width, line thickness, and the gaps between metal lines. These physical geometries determine the internal resistance of the coil and the spatial flux change rate enclosed by the coil. Based on Ohm s law, the total coil resistance of micro power generator is expressed as: R = ρ l n A = ρ 4n(D 0 +(g + w)(n 1)) wt, (18) where ρ is the resistivity of coil material, l n is the total length of coil, A is the coil metal line cross section area, D 0 is the length of the most inner coil turn, g is the gap between coil metal lines, w is the line width, t is the line thickness, and n is the number of coil turns. For a micro generator with a fixed chip area, reduction of metal line width and gap may increase the number of coil turns and thus the output voltage. However, with this design the metal line cross-section area is reduced, therefore the internal resistance will increase at the same time. If coil number is increased by a factor of λ, output voltage output will increase by a factor of λ, the coil length will increase by a factor of λ; to fit into the same chip area the cross section area of metal line is reduced to 1/λ due to the shrinkage of metal line width. Thus, the internal resistance will increase by a factor of λ 2, and for a fixed chip area, according to the average power equation P = V 2 /2R, the overall power output will not increase with the number of coil turns, because the increased factor on output voltage is canceled by the factor on the increased internal resistance. To effectively increase the coil number yet maintain its internal resistance, the metal line width needs to be increased and the gap between lines reduced. The required line gap may reach the lithographic resolution limit. Thus, for a fixed chip area constraint, the Output voltage (mv) Output voltage Output power Number of coil turns Fig. 3 Output voltage and power versus number of coil turns coil number is limited by process minimum resolution. Increasing the metal line thickness is another way to reduce the coil resistance. In conventional PVD metallization processes, film thickness is around 0.3 ~ 3 um. Electroplating with thick PR mold is a promising technology for micro power generator requirement, since the plated metal line thickness can be increased from less than 10 um to 100 um. To investigate coupling efficiency improvement by increasing spatial flux change rate Φ, the total flux change rate is summed across the flux change rate of each coil turns, as described in Eq. (19) dφ dφ = i dx Σ. (19) i dx Spiral planar coil turns located differently with respect to the magnet in the center lead to different spatial flux change rates. The outer coil s flux change rate is less than the inner ones, since magnetic field strength decays with increasing distance away from the magnet. Besides, the outer coil turns have larger resistance due to their longer metal line length and thus are less efficient in contributing power generation. These less efficient outer coil turns may degrade the overall power output performance as the number of coil turns exceeds an optimum value. In our coil design optimization, the spatial distribution of magnetic flux density was first computed with the commercial magnetic field analysis software ANSOFT. Then the spatial magnetic flux change rate of each single coil turn was further computed taking the integration along the whole coil path. The simulated output power and voltage of a typical micro power generator is shown in Fig. 3. Here the magnet is a block of dimension 2 mm 2mm 1 mm, the assumed relative velocity between coil and magnet is 0.25 m/s. The length of each side of the first coil turn is 3 mm, the line thickness is 100 µm, the line width is 30 µm, and the gap between metal lines is 20 µm. In Fig. 3, the output voltage increases monotonously with the number of coil turns, whereas the Output power (uw)

5 W. S. Huang et al.: A Silicon Mems Micro Power Generator for Wearable Micro Devices 137 Wafer I Wafer II Si 3 N 4 Si Si 3 N 4 Si (a) (e) V groove (b) (f) V groove NiFe Copper (c) (g) Si membrane etch Si membrane etch (d) (h) Si SiN NiFe SiO 2 Cu Fig. 4 Process flow for wafer I (a) ~ (d) and wafer II (e) ~ (h), (a) backside nitride pattern for through wafer hole (b) KOH etch for Si membrane (c) Ni-Fe plating for suspension spring (d) final Si membrane etch by RIE for spring release (e) backside nitride pattern to form through wafer hole (f) KOH etch for Si membrane (g) double metal process with 100 µm copper plating as metal II (h) final Si membrane etch by RIE output power shows a maximum peak value. The output power starts to drop when the number of turns exceeds an optimum value. This is because the increased outer coil turns located much further away from the magnet induce less emf but increase the significant internal resistance. Shown in Fig. 3, in this design case, the output power starts to decrease, as the number of coil turns is more than Design Consideration of Suspension Spring As the output power is directly proportional to the relative velocity between the magnet and the coil under vibration, it is desirable to design a spring structure to maximize the possible relative velocity between magnet and coil within the linear range and limited area. For a wearable MEMS power source, vibration amplitude of hundreds of micrometers is necessary. The commonly used silicon beam and polysilicon beam are not suitable for this application because of stiff material properties. To minimize the maximum stress in the spring during the operation, the spring length should be maximized within the available area. In this study, we have devised a two-wafer bonded structure to form our prototype micro generator, thus enabling us to have a long spring yet within a limited area. In the bottom wafer, the suspension spring overlaps the top wafer area where the coil is laid down. This design extends the spring length in the limited area. As for the spring material, we chose NiFe alloy for its more steel like mechanical strength and higher yield strength compared with other readily available materials such as silicon, polysilicon or copper. Although the yield strength of NiFe is less than steel (2.1 GPa), plated NiFe has a yield strength around 300 MPa, which is much higher than plated copper with less than 100 M Pa in yield strength. The spring length, thickness, and width are all possible design parameters for resonance frequency tuning. Based on Eq. (15), a match between device s resonant frequency and the excitation frequency of environmental vibration is important for maximum energy transduction efficiency. III. DEVICE FABRICATION The key process steps for two wafers are illustrated in Fig. 4 and described below. In wafer I for magnet suspension spring, a 1 µm thick low stress nitride was deposited on the wafer by low pressure chemical vapor deposition (LPCVD). This nitride film is used as the masking layer during KOH etching. After patterning the silicon-etching window on

6 138 Journal of the Chinese Institute of Engineers, Vol. 30, No. 1 (2007) (a) (b) Fig. 5 Electroplated 100 µm copper coil the backside nitride as shown in Fig. 4(a), the wafer is immersed in KOH etchant for silicon etching. Concerning mechanical strength, 30 µm Si membrane was left for subsequent wafer process handling as shown in Fig. 4(b). A 2000/1000 Å thick seed layer of Cu/Ti was then deposited on the front side. The suspension spring was formed by Ni-Fe electroplating employing 20 µm thick patterned photoresist as the plating mould. After plating the photoresist and seed layer were then stripped as in Fig. 4(c). Finally, the previous 30 µm supporting Si membrane and nitride on the front side were etched through by RIE to release the suspension nickel-iron spring, as shown in Fig. 4(d). Similarly, in wafer II for energy collecting coils, the supporting silicon membrane was formed first by KOH etching, as in Figs. 4(e) and 4(f). A layer of 4000 Å aluminum was then deposited by DC sputter and patterned. This aluminum line is used to connect the center of spiral coil to the outer bonding pad. A 2000 Å thick PECVD oxide of inter metal insulation layer was deposited. Via holes were then patterned and followed by a deposition of 2000/1000 Å Cu/Ti electroplating seed layer. To reduce the micro power generator s internal resistance, a 100um thick layer of copper for energy-collecting coil was electrodeposited using thick patterned photoresist (AZ9260) as the plating mold. The photoresist and seed layer were then stripped after electroplating, as in Fig. 4(g). Finally, the 30 µm supporting Si membrane and nitride were etched by RIE, thus completing the energy collecting coil wafer with through wafer hole at the center of coil as shown in Fig.4 (h). As for the RIE process during final membrane release, the 30um thick silicon may be time-consuming for conventional RIE tools. However with an optimized etch recipe, etching rate as high as 1.2 um/min is possible with our RIE tools. This may significantly reduce process time as compared with 0.1 ~ 0.3 um/ Fig. 6 (c) Photographs of prototype device (a) Coil wafer (b) suspension spring wafer (c) suspension spring with magnet mounted (d) assembly device micro power generator min etch rate with conventional RIE tools and make the single-wafer RIE process more efficient. A commercially available neodymium iron boron (NdFeB) magnet of dimension 2 mm 2 mm 1 mm (High Mag Tech. Corp, N35) was mounted on the center plate of the spring wafer. The second energy collecting coil wafer was then aligned and adhesive bonded to form the complete micro generator. This device fabrication process has improvements from the previous reported results in two aspects. One is the thick electroplated copper coils that significantly reduce the internal resistance; the plated coil thickness is about 100 µm and internal resistance is only 2 ohm as shown in Fig. 5. The other improvement is the spring fabricated with electroplated nickel-iron alloy. Nickel-iron alloy has higher yield strength compared with that of copper, thus larger vibration amplitude is possible. IV. MEASUREMENTS AND DISCUSSION Figure 6 shows photographs of our prototype device. This device is composed of a coil chip, a spring chip, and a permanent magnet. Fig. 6(a) shows the coil chip. The inner etched through square hole for the permanent magnet is 3 3 mm. The internal resistance of the 20 turns coil is 2 ohm. Fig. 6(b) shows the spring chip with suspended nickel-iron spring. The plated nickel-iron spring is released by the wafer etched-through squared hole, and seen from the backside of the chip. In Fig. 6(c), a commercial NdFeB magnet is mounted on the suspension spring. (d)

7 W. S. Huang et al.: A Silicon Mems Micro Power Generator for Wearable Micro Devices RMS Output Power (uw) Frequency (Hz) Fig. 7 Feasibility of power generation test by direct finger tapping caused vibration Fig. 8 RMS output power when vibration frequency is swept from 50 Hz to 300 Hz Fig. 6(d) shows the complete assembled device. The overall dimension of the micro generator is 6 mm 6 mm 1 mm. The most direct method to test the feasibility of wearable MEMS micro vibration power generation is to gently tap the device with fingers, mimicking the vibration generated while the device is worn on a person. As shown in Fig. 7, with a load resistance of 4 ohm connected on the output leads, the sinusoidal output voltage with 1.6 mv peak voltage is generated. This corresponds to an average output power of 0.32 uw. When the prototype micro-generator is placed on a sinusoidal vibration test bed with vibration amplitude of about 50 µm, the RMS output power of our prototype device is about 1.44 uw. The relative velocity between the coil and the magnet at resonant from LDV (Laser Doppler Vibrometer) measurement is 0.52 m/s. Based on the equations developed, this corresponds to a predicted output power of 1.58 uw, which is in close agreement with the measurement. Fig. 8 shows the RMS output power when vibration frequency is swept from 50 Hz to 300 Hz. The peak output power appears at the frequency around 100 Hz, which can be shifted to a desired frequency by calculated design changes. Based on the vibration kinetic model, the estimated damping factors for our prototype device are ξ e , ξ t Compared with other reported values (William et al., 2001), though ξ e is still much less than ξ t, however, the ratio ξ e /ξ t gets much closer to the optimum condition ξ e = ξ t, as described in Eq. (17). The power density of our prototype device is about 39 uw/cm 3, which is still small in comparison with chemical based power sources (ex: lithium batteries, micro fuel cell, micro gas turbine). This output level may be of interest for low power devices such as RF ID, which require power levels on the order of a few uw. A 2 2 array with four may supply enough power for such applications, with a size around 1cm square and a weight of 0.3g. Though the size/weight is larger than other portable power sources such as batteries, the vibration micro generator still fills the requirements of portable power sources. Besides, it should be emphasized that a vibration based micro power generator operating at low frequency can scavenge energy from the external environment. This direct energy harvesting needs no external connection for charging or supply of liquid or gas fuel material. Compared with other vibration micro power generators utilizing planar coils as the energy pick-up element (William et al., 2001), our prototype device has a larger power output. Although mini generators with wire-wound coils have shown much higher power output (Ching et al., 2002; Glynne-Jones et al.,2004), they are much larger in volume as well, because they have hundreds of stacked coil turns, compared with only 20 planar coil turns in our device. However, the process of planar coils is more compatible with conventional power IC devices (ex. transistor, capacitor, diode...) on silicon substrate; therefore it is possible to integrate voltage boost up converter circuits if desired. V. CONCLUSIONS The improvements of coil design and use of new spring material in this study have increased energy coupling and transformation efficiency from vibration to electricity significantly. Larger power generation

8 140 Journal of the Chinese Institute of Engineers, Vol. 30, No. 1 (2007) was achieved by the increased energy transform efficiency from kinetic to electric due to reduced internal resistance and more efficient coupling. This makes the electric power generation directly from human body movement feasible and promising, although the available power in our current design is still not sufficient for practical applications. ACKNOWLEDGEMENTS The authors would like to thank Industrial Technology Research Institute for its support in providing nickel iron plating facilities and partial funding of this work. We also thank the staff of the National Science Council Central Regional MEMS Research Center for technology support. REFERENCES Ching, N. N. H., Wong, H. Y., Li, W. J., Leong, P. H. W., and Wen, Z., 2002, A Laser-Miromachined Multi- Modal Resonating Power Transducer for Wireless Sensing Systems, Sensors and Actuators, A Vol , pp Glynne-Jones, P., Tudor, M. J., Beeby, S. P., and White, N. M., 2004, An Electromagnetic, Vibration-Powered Generator for Intelligent Sensor Systems, Sensor and Actuator A, Vol. 110, pp Li, W. J., Wen, Z., Wong, P. K, Chan, G. M. H., and Leong, P. H. W., 2000, A Micromachined Vibration-Induced Power Generator for Low Power Sensors of Robotic Systems, World Automation Congress: 8th International Symposium on Robotics with Applications, Hawaii, USA. Mitcheson, P. D., Green, T. C., Yeatman, E. M., and Holmes, A. S. 2004, Architectures for Vibration- Driven Micropower Generators, Journal of Microelectromechanical Systems, Vol. 13, Issue- 3, pp Shearwood, C., and Yates, R. B., 1997, Development of an Electromagnetic Micro-Generator, Electronics Letters, Vol. 33, No. 22, pp Williams, C. B., Shearwood, C., Harradine, M. A., Mellor, P. H., Birch, T. S., and Yates, R. B., 2001, Development of an Electromagnetic Micro- Generator, IEE Proceedings of the Circuits Devices & Systems, Vol. 148, pp Manuscript Received: Oct. 13, 2005 Revision Received: Mar. 20, 2006 and Accepted: Apr. 10, 2006