IEEE SENSORS JOURNAL, VOL. 11, NO. 1, JANUARY 2011 155

A 2-DOF MEMS Ultrasonic Energy HarvesterYong Zhu, Member, IEEE, S. O. Reza Moheimani, Senior Member, IEEE, and

Mehmet Rasit Yuce, Senior Member, IEEE

Abstract—This paper reports a novel ultrasonic-based wirelesspower transmission technique that has the potential to drive im-plantable biosensors. Compared with commonly used radio-fre-quency (RF) radiation methods, the ultrasonic power transmis-sion is relatively safe for the human body and does not cause elec-tronic interference with other electronic circuits. To extract am-bient kinetic energy with arbitrary in-plane motion directions, anovel 2-D MEMS power harvester has been designed with reso-nance frequencies of 38 520 and 38 725 Hz. Frequency-responsecharacterization results verify that the device can extract energyfrom the directions of X, Y, and diagonals. Working in the diag-onal direction, the device has a bandwidth of 302 Hz, which is twicewider than a comparable 1-D resonator device. A 1- F storage ca-pacitor is charged up from 0.51 to 0.95 V in 15 s, when the har-vester is driven by an ultrasonic transducer at a distance of 0.5 cmin the X-direction, and is biased by 60 Vdc, indicating the energyharvesting capability of 21.4 nW in the X-direction. When excitedalong the Y-axis, the harvester has an energy-harvesting capacityof 22.7 nW. The harvester was modeled and simulated using anequivalent electrical circuit model in Saber, and the simulation re-sults showed good agreement with the experimental results. The ul-trasonic energy harvesting was also investigated using a 1-D piezo-electric micro-cantilever.

Index Terms—Energy harvester, implantable biosensor, micro-electromechanical system (MEMS), ultrasonic transmission.

I. INTRODUCTION

I MPLANTABLE devices and low-power biosensors, usedfor identification, monitoring and treatment of patients, are

critical to future healthcare industries. Supplying energy contin-uously to a biosensor microsystem is a key requirement for itscontinuous and reliable operation. Radio-frequency (RF) wire-less powering and telemetry from outside the human body toan implanted medical device has been shown to enable its unin-terrupted operation without the need for a battery replacement[1]. However, the inductive coupling method can cause electro-magnetic coupling (EMC), which may interfere with other elec-tronic devices in medical environment [2]. Also, exposing the

Manuscript received January 21, 2010; revised April 29, 2010; acceptedJune 14, 2010. Date of publication September 20, 2010; date of current versionNovember 03, 2010. This work was supported by the Australian ResearchCouncil (ARC) through Discovery Grant DP0774287. This is an expandedpaper from the IEEE SENSORS 2009 Conference. The associate editorcoordinating the review of this manuscript and approving it for publication wasDr. Patrick Ruther.

The authors are with the School of Electrical Engineering and Com-puter Science, University of Newcastle, Newcastle, NSW 2308, Australia(e-mail: yong.zhu@newcastle.edu.au, reza.moheimani@newcastle.edu.au;mehmet.yuce@newcastle.edu.au).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2010.2053922

patient to RF radiation may be unsafe for living tissue [3]. Al-ternatively, the ultrasound is free from the EMC and is believedto be relatively safe for human’s health [4], making ultrasonicwireless power transmission a promising candidate for futureimplantable devices.

In a typical power harvester, kinetic energy is often convertedinto electrical energy using electromagnetic, piezoelectric, orelectrostatic mechanisms [5]–[7]. However, the efficiency of vi-bration-to-electricity conversion is quite low in existing micro-generators, since: 1) most of the reported kinetic generator tech-niques suffer from scavenging in a narrow bandwidth from am-bient sources due to their single resonance frequency and 2)most devices are designed to harvest energy in a single direc-tion, which wastes the vibration energy existing in other direc-tions [8]. An exception is the multifrequency piezoelectric har-vester reported in [9] that was shown to broaden the bandwidthby using three piezoelectric bimorph cantilevers with identicaldimensions but different masses. Also, an 800-Hz widebandelectromagnetic micro generator has been realized by imple-menting 40 serially connected cantilevers in different lengths[10]. Recently, Bartsch reported a 2-D electrostatic resonantmicro energy generator to extract energy from ambient vibra-tions with arbitrary motion directions. However, the level of har-vested power was rather low (100 pW) [11].

In this paper, an ultrasonic energy transmission method isintroduced. A novel 2-degree-of-freedom (2-DOF) microelec-tromechanical system (MEMS) device has been designed, fab-ricated, and tested as a proof-of-concept. The fabricated devicevibrates due to incoming ultrasonic waves and can scavenge en-ergy in arbitrary directions in a plane as well as broaden thebandwidth with two resonance frequency peaks. A 1- F storagecapacitor is charged up from 0.51 to 0.95 V within 15 s in asample experimental test conducted, indicating the energy har-vesting capability of 21.4 nW. Also, the energy-harvesting per-formance has been evaluated with an equivalent electrical circuitmodel in Saber, showing good agreement between simulationand experimental results.

II. DESIGN

To harvest vibration energy in arbitrary directions in a plane, a2-DOF motion mechanism has been incorporated in our design,as illustrated in Fig. 1(a). The seismic mass moves in the X-and Y-axes to absorb vibration energy in any in-plane direction.The X and Y mechanisms and the seismic mass are all discretemechanical elements, attached to each other via elastic flex-ures. These flexures significantly decouple the X and Y move-ments of the mass. Electrostatic comb variable capacitorsand collect and vibration contributions separately andflow the currents into the load, as illustrated in Fig. 1(b). The

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156 IEEE SENSORS JOURNAL, VOL. 11, NO. 1, JANUARY 2011

Fig. 1. 2-DOF motion mechanism to harvest any direction in-plane vibrationenergy. (a) Schematic. (b) Equivalent circuit model.

Fig. 2. (a) System schematic in Architect. (b) 3-D view.

capacitors are polarized by a dc voltage source. In a practicalapplication, the polarization can be achieved using permanentlycharged electrets, which remove the need for an external biasvoltage, as illustrated in [12], [13]. Also, an electrets-free powerharvester has been proposed by using an inductive fly-back cir-cuit returning part of harvested charge back to the bias capacitor[14]. A similar method may be incorporated into the design pro-posed here.

The electromechanical structure of the harvester wasdesigned and optimized using Architect Saber in a Coventor-Ware environment. The Architect parametric libraries allowsystem-level designers to simulate and rapidly evaluate multipledesign configurations using a top-down, system-level approachand to deliver precise behavioral models to the device designer.1

The detailed structure design was started from SchematicCreation: building an accurate system-level model using Para-metric Electro-Mechanical Library components (e.g., beams,rigid plate mass, sense combs), as illustrated in Fig. 2(a). Then,the schematic was processed, and a 3-D view of the devicewas created as shown in Fig. 2(b). This structure topology isinspired from the decoupled gyroscope structure describedin [15]. The central seismic mass is suspended by 16 beamsattached to four corner anchors. The specific topology of thebeams splits a certain direction of seismic mass movement intothe comb capacitance changing in the X and Y frames. Themovable comb fingers are restricted by these beams and only

1Online. Available: http://www.coventor.com/

Fig. 3. Mechanical structure design showing two movement modes (a) and (b)along the �- and � -axes.

TABLE IDESIGN PARAMETERS FOR 2-DOF ENERGY HARVESTER

move in parallel with the fixed comb fingers, without the riskof pulling in together. This way, symmetry of the structure ispreserved, while the X and Y sense capacitors are kept indepen-dent from each other but simultaneously contribute the chargingof the load. As the structures are totally symmetrical, and the dcbias was applied to the central seismic mass, the associated elec-trostatic forces cancel each other in each axis. Thus, there is noeffect on the mechanical behavior caused by the dc bias.

Table I shows the key design parameters for the energyharvester prototype. As a part of the design process, a modalanalysis via CoventorWare was carried out on the mechanicalstructure which determined the primary resonance frequencyat 39238 Hz (X mode) and the secondary resonance frequencyat 39266 Hz (Y mode). The X and Y modes obtained from aCoventorWare simulation are shown in Fig. 3. The simulationresults show that the mechanical coupling in the combs fromone mode to another is less than 2%. For example, in the Xmode, the combs in Frame Y move less than 2% of the move-ment of the combs in the X mode due to mechanical couplingand vice versa. The -axis is less stiff than the - and -axes,and the resonance frequency in the -direction is 22.48 kHz,which is far away from the and resonance frequencies.Thus, vibrations in the -direction can be ignored when theharvester is excited by 38.5-kHz acoustic waves.

In Fig. 3, there are two primary resonance frequencies, onefor each mode. Each resonance frequency is determined by thestiffness of the corresponding suspend beams and the mass in themovement direction. For instance, in the X mode, the resonancefrequency depends on the stiffness of the beams in parallel withthe -axis, and the mass including the central mass and twomovable X comb fingers. In our design, all 16 beams were de-signed to have identical dimensions. However, the masses of X

ZHU et al.: 2-DOF MEMS ULTRASONIC ENERGY HARVESTER 157

Fig. 4. SOI fabrication process through MEMSCAP.

and Y movable comb fingers were separately adjusted to obtainthe requisite resonance frequencies. To broaden the bandwidth,the masses were designed to have different dimensions alongthe - and -axes (the width of the X frame is m widerthan that of the Y frame, as shown in Table I).

III. FABRICATION

The device was fabricated in a commercial silicon-on-in-sulator (SOI) MEMS foundry (MEMSCAP) process2 with a25- m-thick silicon device layer and minimum gap of 2 m.Fig. 4 provides an illustrated summary of this process, whichis given here.

1) Surface metal pads are patterned on a highly doped n-type25- m silicon device layer to allow for ohmic contact.

2) Deep reactive ion etch (DRIE) is performed from the frontside of the wafer to define both the anchored and movablefeatures of the structure.

3) A protective polyimide layer is applied to the front side.4) A deep trench underneath the movable structures is created

by etching through the substrate using DRIE.5) The exposed buried oxide is removed using a wet HF etch.6) The polyimide coat on the front side is removed by oxygen

plasma, thereby allowing the movable structure to be fullyreleased. Then, a large contact metal pad (i.e., electricalgrounded contact) is patterned on the substrate to reduceparasitics.

The image of the packaged device and a section of it takenunder a scanning electron microscope (SEM) are provided inFig. 5(a). A large number of holes have been incorporated intothe seismic mass to enhance the mechanical coupling betweenultrasonic and seismic mass. A photograph of the packaged de-vice is provided in Fig. 5(b). To reduce the effect of parasiticcapacitances on measurement, the chip was glued mechanicallyon a package using a conductive silver tape, and connected elec-trically to the pins of package by gold bonding wires.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

Here, we describe the experimental setup and characteriza-tion of the fabricated MEMS energy harvester. Frequency re-

2Online. Available: http://www.memscap.com/en_mumps.html

sponse measurement was first conducted using a probe stationto obtain initial test results. To validate the design in terms ofenergy harvesting, energy management was conducted using apower rectification circuit. The energy-harvesting performancewas evaluated and then compared with a simulation results by anequivalent electrical circuit model in Saber. The ultrasonic en-ergy harvesting was also investigated using a 1-D piezoelectricmicro-cantilever, and a possible implantable ultrasonic powertransmission scheme is proposed.

A. Frequency Response Characteristics

The measurement set up is illustrated in the inset of Fig. 6.A SUSS PM5 Analytical Probe System was adopted for the ini-tial test. The chip was fixed on the surface of an optical table,and then probes were placed on the device pads for electricalconnection to provide access to the device for measurements.An ultrasonic transducer (SQ-40 T) was utilized to generate ul-trasonic waves. This transducer has a resonance frequency ofaround 40 kHz, which is conveniently close to the resonance fre-quencies of the designed 2-D resonator. Thus, high energy cou-pling is expected to be achieved between the ultrasonic trans-ducer and the power harvesting device. The ultrasonic trans-ducer was driven by a 4-V peak-to-peak ac signal generated bythe HP35670A Dynamic Signal Analyzer to produce ultrasonicwaves. The seismic mass absorbs this ultrasonic energy and vi-brates in the – plane. While the mass is moving, the combscapacitance is also changing. The comb capacitors are polarizedby a 100 V DC voltage source, and the moving combs force thecurrents into the input node of the signal analyzer (1-M inputimpedance).

Fig. 6 shows the obtained frequency response of the har-vester, with the ultrasonic transducer aiming at varied direc-tions, as illustrated in the inset. With the transducer having anangle of 0 with the -axis, one resonance frequency peak isobserved, located at around 38.52 kHz. The small peak existsat 38.75 kHz, which is caused by the mechanical cross axiscoupling. The results of 90 case are similar to those of 0case. When the direction comes to 45 , two resonance frequencypeaks are observed. The 10-dB bandwidth is 302 Hz, whichis twice wider than a comparable 1-D resonator. The results of

and directions were the same, having two peaksat 38.52 and 38.75 kHz. However, the magnitude between thesetwo peaks is lower than that in the 45 case, because of the an-tiphase currents contributed by the two connected combs.

Note that the measured resonance frequencies are differentto those predicted in the Table I. This is mainly due to thefabrication process, which is not free of errors. For instance,over-etching in DRIE tends to make the beams narrower thanwhat they are designed to be. Consequently, a lower stiffness inbeams results in lower resonance frequencies.

B. Energy Management

Ultrasonic energy is transformed into electrical energy in theform of ac current in Fig. 6. However, in a real application,the rectification of ac to dc voltage must be done for energystorage prior to powering a biosensor. Fig. 7 shows the ac–dcvoltage-rectifier circuit diagram and the circuit board for the en-ergy management. The rectifying bridge circuit consists of two

158 IEEE SENSORS JOURNAL, VOL. 11, NO. 1, JANUARY 2011

Fig. 5. (a) SEM image of the fabricated energy harvester. (b) Photograph of the packaged device.

Fig. 6. Magnitude curves of the output voltage versus frequency.

Fig. 7. (a) Energy-harvesting circuit diagram. (b) Circuit board with ultrasonictransducer and packaged MEMS chip.

Fig. 8. Voltage profiles of 1-�F storage capacitor with different gaps (0.5, 1.0,and 1.5 cm) between ultrasonic transducer and the MEMS energy harvester indifferent directions.

STMicroelectronics 1N5711 small-signal Schottky diodes and a1- F storage capacitor. These Schottky diodes have low forwardvoltage drop of approximately 0.2 V, thereby allowing largest dcvoltage on the storage capacitor .

Fig. 8 illustrates the voltage profiles of the 1- F storagecapacitor with different air gaps (0.5, 1.0, or 1.5 cm) betweenthe ultrasonic transducer and the MEMS energy harvester.The bias voltage was 60 V and the ultrasonic transducer wasactuated by , 38.53-kHz ac voltage, which matched theX-mode resonance frequency of the harvester. The transduceraimed at the -axis of the harvester to maximize the energycoupling. A Stanford low-noise preamplifier with 100- inputimpedance and unit gain is connected between and anoscilloscope.

Due to the ultrasonic attenuation through air, the convertedvoltage on the storage capacitor decreases with the increasinggap, as shown in Fig. 8. After the ultrasonic wave was appliedto the harvester, the dc voltage on storage capacitor was chargedup from 0.51 to 0.95 V in 15 s with an 0.5-cm air gap betweenthe transducer and harvester. Because of the stray capacitancebetween the actuation ac signal and the MEMS harvester, the ac

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Fig. 9. Electrostatic harvester equivalent circuit model in SaberSketch.

signal was fed through to the rectification end, leaving an initial0.51-V charge on the storage capacitor.

The power stored in the capacitor is calculated as

(1)

where is the charging time. Thus, the energy stored on thestorage capacitor changed from 0.13 to 0.451 J in 15 s, indi-cating that the power absorption capability of this energy har-vester is 21.4 nW.

Fig. 8 also shows the harvesting results along the -axis. Thebias voltage was 60 V and the ultrasonic transducer was actuatedby , 38.77-kHz ac voltage, which matched the Y-moderesonance frequency of the harvester. The dc voltage on storagecapacitor was charged up from 0.51 to 0.97 V in 15 s with a0.5-cm air gap between the transducer and harvester, indicatingthat the harvester has an energy harvesting capacity of 22.7 nW.

In a practical device, a power supply of 0.95 V is enoughfor powering low-power IC circuits. With the current state-of-the-art IC technology, devices can work with dc voltages lowerthan 1 V [16].

C. Equivalent Electrical Circuit Model

Modeling a MEMS variable capacitor is the key to simulatingelectrostatic MEMS devices. Spice [17] and Verilog HDL [18]models have been successfully developed to model the vari-able capacitors. However, both methods require the designer towrite complex codes. Saber in CoventorWare Architect providesa variable capacitor component model, which is a building blockfor multidomain system-level simulation in a user-friendly elec-trical simulation environment [19]. A schematic of the equiv-alent circuit model for our electrostatic harvester is shown inFig. 9. The variable capacitor component models an electricalcapacitor, whose capacitance value is the value on the input pincap, and the pin is connected to an ideal variable sourcegenerating a sinusoidal signal. The output of is a no-unitvariable, allowing us to define the variable capacitor’s function.For instance, the displacement of moveable combs was mea-sured to be 5 m at 38.53-kHz resonance frequency, with anoriginal overlap of 40 m. According to the combs dimensions,the corresponding calculated capacitance amplitude is 0.053 pFwith an offset of 0.43 pF. Thus, the is set to be 0.053-pFamplitude, 0.43-pF offset, and 38.53-kHz frequency. is theparasitic capacitor and is set to be 4 pF. Fig. 10 illustrates thesimulation result for the electrostatic harvester operating undera 1-cm air gap. This shows a good agreement between simula-tion and experimental results.

Fig. 10. Comparison between experimental results and simulation results forthe electrostatic harvester.

Fig. 11. Piezoelectric cantilever energy harvesting circuit diagram.

D. 1-DOF Piezoelectric Cantilever Harvester

To validate the ultrasonic energy harvesting in another way,a harvester based on a commercially available Veeco DMASPmicro-actuated silicon probe3 was investigated. The device con-sists of a micromachined silicon cantilever designed for atomicforce microscopy. A piezoelectric zinc oxide (ZnO) layer sand-wiched between two Ti/Au metal electrodes is deposited on thesurface of this micro-cantilever. Although it was originally de-signed for atomic force microscopy, this device has been usedin other applications including a wide range pressure sensor,whose resonance frequency is a function of the pressure andthe temperature [20]. Piezoelectric transducers have been usedwidely in vibration control applications due to their ability totransform electrical energy to mechanical energy and vice-versa[21]. Recently, there has been significant interest to use them aspower harvesting devices [22], [23].

The experimental setup and circuit connections are illustratedin Fig. 11. The micro-cantilever has a resonance frequencyof 50.5 kHz. Thus, an ultrasonic transducer (PROWAVE500 MB120) with a resonance frequency of 50 kHz was se-lected to maximize the energy coupling to the micro-cantilever.The piezoelectric cantilever was actuated by this ultrasonictransducer with a 1-cm air gap and aimed at the surface of thecantilever. The beam absorbs the ultrasonic energy and vibratesas shown in Fig. 11. The ZnO layer on the cantilever undergoesa periodic stress due to the vibration, and thus AC current flowsinto and charges the storage capacitor through the rectificationdiodes. Note that, in this device, there is no dc bias connectedto the cantilever, which is an appealing feature compared with

3Online. Available: https://www.veecoprobes.com/

160 IEEE SENSORS JOURNAL, VOL. 11, NO. 1, JANUARY 2011

Fig. 12. Measured results comparison between electrostatic harvester andpiezoelectric harvester.

the electrostatic method. However, only one direction energycan be harvested.

The measured charging voltage profiles of the storage capac-itor for electrostatic and piezoelectric harvesters are depicted inFig. 12. For comparison, both devices operate under the same1-cm air gap, and the storage capacitor in both experiments was1 F. Herein we only plot the increasing voltage after ultrasonicwave takes effect on the harvester, eliminating the feed throughoffset voltage as shown at the beginning in Fig. 8. As illustratedin Fig. 12, the piezoelectric harvester charges the storage capac-itor faster than the electrostatic harvester.

V. CONCLUSION

The modeling, fabrication, and characterization of an ul-trasonic-based transducer used for energy harvesting wereintroduced and discussed. The 2-D resonator can extract ultra-sonic energy from all directions in the device plane as well asbroaden the bandwidth, increasing the efficiency of the energyscavenging. Using this device, a power level of 21.4 nW wasconverted on a 1- F storage capacitor by ultrasonic energytransmission in an air gap of 0.5 cm. The harvester was modeledand simulated using an equivalent electrical circuit model inSaber, and the simulation results showed good agreement withthe experimental results. A possible application for this deviceis ultrasonic energy transmission through living tissue to powerimplantable biosensors, which will be investigated in futureresearch.

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Yong Zhu (M’10) received the B.Sc. degree inphysics and M.Eng. degree in microelectronics fromthe Dalian University of Technology, Dalian, China,in 1999 and 2002, respectively, and the Ph.D. degreein microelectronics from Peking University, Beijing,China, in 2005.

He was a Research Associate with the Depart-ment of Engineering, University of Cambridge,Cambridge, U.K., in 2006 and 2007, where hewas involved with the project of microfabricatedelectrometer and mass sensor. He is currently a

Research Academic with the School of Electrical Engineering and ComputerScience, University of Newcastle, Newcastle, NSW, Australia. His researchinterests include micromachined resonators, power harvesters, nanopositioners,capacitive sensors, RF MEMS, as well as interface circuits design for MEMS.

ZHU et al.: 2-DOF MEMS ULTRASONIC ENERGY HARVESTER 161

S. O. Reza Moheimani (SM’00) received the Ph.D.degree in electrical engineering from the Universityof New South Wales, Australian Defence ForceAcademy, Canberra, A.C.T., Australia, in 1996.

He is currently a Professor with the School ofElectrical Engineering and Computer Science, Uni-versity of Newcastle, Newcastle, NSW, Australia,where he holds an Australian Research CouncilFuture Felowship. He is Associate Director ofthe Australian Research Council (ARC) Centrefor Complex Dynamic Systems and Control, an

Australian Government Centre of Excellence. He has held several visitingappointments at IBM Zurich Research Laboratory, Switzerland. He has au-thored or coauthored two books, several edited volumes, and more than 200refereed articles in archival journals and conference proceedings. His currentresearch interests include applications of control and estimation in nanoscalepositioning systems for scanning probe microscopy, control of electrostaticmicroactuators in microelectromechanical systems (MEMS) and control issuesrelated to ultrahigh-density probe-based data storage systems.

Prof. Moheimani is a Fellow of the Institute of Physics, U.K. He is a recipientof the 2007 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY Out-standing Paper Award and the 2009 IEEE CSS Control Systems TechnologyAward, jointly with a group of researchers from IBM Zurich Research Labs. Hehas served on the Editorial Board of a number of journals including the IEEETRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY and has chaired severalinternational conferences and workshops.

Mehmet Rasit Yuce (S’01–M’05–SM’09) receivedthe M.S. degree from the University of Florida,Gainesville, in 2001, and the Ph.D. degree fromNorth Carolina State University (NCSU), Raleigh,in 2004, both in electrical and computer engineering.

He is currently a Senior Lecturer with the Schoolof Electrical Engineering and Computer Science,University of Newcastle, Newcastle, NSW, Aus-tralia. Between August 2001–October 2004, heserved as a Research Assistant with the Departmentof Electrical and Computer Engineering, NCSU. He

was a Postdoctoral Researcher with the Electrical Engineering Department,University of California, Santa Cruz, in 2005. He has author or coauthoredapproximately 50 technical papers. His research interests include wirelessimplantable telemetry, wireless body area network (WBAN), analog/digitalmixed-signal VLSI for wireless, biomedical, and RF applications.

Dr. Yuce was the recipient of received a NASA Group Achievement Awardin 2007 for developing an SOI transceiver.