VIBRATION ENERGY HARVESTING WITH PZT THIN FILM MICRO DEVICE

P. Muralt1*, M. Marzencki2, B. Belgacem1, F. Calame1, and S. Basrour2

1Ceramics Laboratory, Swiss Federal Institute of Technology EPFL, Lausanne, Switzerland 2TIMA laboratory, Grenoble, France

Abstract: A micro power generator for harvesting vibration energy by means of a piezoelectric laminated cantilever with inertial mass was designed, fabricated, and characterized. The device was micro machined from a silicon-on-insulator structure coated with a 2µm thick piezoelectric PZT thin film. Interdigitated electrodes (IDE) were applied to achieve higher voltages. A piezoelectric coupling constant k2 of 5% was derived from resonance/antiresonance curves. A voltage of 1.6 V, and an output power of 1.4 µW were measured at 2g acceleration and 870 Hz with a cantilever having a 0.8x0.4 mm large active area using the device close to antiresonance. Keywords: piezoelectrics, thin films, energy harvesting INTRODUCTION

During recent years, energy harvesting from vibration and motion sources has attracted much interest, particularly as micropower sources [1]. The main applications are wireless communication and sensors. Supply powers of <100 µW are sufficient to operate wireless nodes in the silent mode. The duty cycle of such nodes can be quite small, so that mW supply levels enable some autonomy. Motion and vibration are the most versatile and ubiquitous ambient energy source available, if light harvesting is excluded by the application [2]. The mechanical to electrical energy transformation is most efficiently done by piezoelectric materials. In a microscale generator, an elastic structure containing a piezoelectric film is strained by coupling to the external vibration by means of induced inertial motion at resonance (Fig. 1). Such sources must match their resonance frequency to the external vibration spectrum. Vibrations from machinery usually have a frequency of ≈ 100 Hz or less which is a rather low for micro systems, given that resonant frequencies tend to increase with shrinking dimensions and masses.

The energy transformed from a mechanical to an electrical form is proportional to the piezoelectric coupling k2, which in the case of MEMS structures is not the material constant of the piezoelectric layer but includes the rigidities of the other elastic materials involved in the deformation and the relative dimensions, including the volume fraction of the piezoelectric material in the total elastic body (see [3]). The energy of the resonating mass-spring system is oscillating between its kinetic form and its potential form, the latter being the sum of elastic and electrical

energy. The capture of electrical energy by an attached circuit is damping the oscillator.

Fig. 1: Principle of micro power generator based on inertial vibration harvesting using interdigitated electrode on top of piezoelectric thin films.

There are always some parasitic damping

mechanisms like damping by air if the device is not in vacuum to which energy harvesting is in competition. It is in general found that the damping coefficient by energy harvesting (i.e. somewhat smaller than k2) must be equal or larger than the parasitic damping coefficient [4]. Microcantilevers operated in air have quality factors of around 50 to 100 meaning that the parasitic damping coefficients (inverse of Q) are equal to a few percent in this case. The usage of cantilevers including materials such as PZT which offer superior k2 of 5% and more are thus helpful in two ways. Firstly they provide more energy in the electrical tank to be harvested, and secondly they allow for better matching parasitic damping when operating the device in air. A further important issue is the matching of the effective resistance of the harvesting circuit to the internal resistance of the piezoelectric. High coupling piezoelectric devices exhibit a large

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frequency gap between the resonance frequency where the internal resistance is lowest and currents are highest, and the antiresonance frequency where the internal resistance and the voltages are highest. The optimal working point is not necessarily the resonance as often assumed. An important point in thin-film piezoelectric harvesters is the choice of the electrodes. PZT parallel-plate capacitors exhibit a large capacitance, resulting in low voltage outputs. One has to keep in mind that vibration harvesting devices have an ac current output that needs to be rectified for energy storage in a battery. All such rectifying semiconductor devices need at least 500 mV for efficient rectification [5], even when using charge transfer switches for voltage multiplication. As a consequence, it is much better to use interdigitated electrodes for harvesting with high permittivity piezoelectrics. Such devices were first demonstrated by Kim and coworkers [6]. This paper reports on a micro device according to Fig. 1 based on PZT thin films and SOI substrates, with the goal to achieve a large piezoelectric coupling factor, and an exploitable range of voltage and power output.

Fig. 2: Scanning Electron Microsope (SEM) image of the interdigitated electrode. FABRICATION

The chosen design is based on a Silicon On Insulator (SOI) wafer with a 5µm thick device layer. The device layer supplies the elastic layer for the piezoelectric bending transducer and spring. First, a 1µm wet oxide layer was grown on the 100 mm SOI wafer. This layer is at the same time barrier layer against Si-PZT interdiffusion and stress compensation layer [7]. To avoid Pb diffuse into SiO2, a 10-20 nm thick TiO2 layer was deposited prior to grow the 2µm thick PbZr0.53Ti0.47O3 (PZT) film by sol gel techniques [8, 9]. A platinum top electrode was sputter deposited and dry etched (Fig. 2). PZT was etched in a HCl:HF acid (details see [10]). The barrier SiO2 layer and the Si device layer were patterned by dry etching (Fig. 3).

Finally the cantilevers were released by deep silicon etching (Bosch process) leaving a piece of the handle wafer with full thickness at the end of the cantilever. This piece serves as inertial mass, following and idea demonstrated for accelerometers [11].

Fig. 3: SEM image of the front side after PZT deposition, etching of the barrier oxide layer and the device silicon layer. ELECTRICAL CHARACTERIZATION

The fabricated devices were tested on a controlled vibration source – a DataPhysics V20 shaker controlled by a custom LabVIEW application. The same application provided a closed loop acceleration amplitude control and data acquisition through a National Instruments card PCI-6024E. For each measure a variable load was connected between the electrodes of the generator and the output voltage was observed through INA116 (BurrBrown) high impedance instrumentation amplifier. The output power was calculated from the RMS value of the voltage measured and the resistive load value. The output power was measured as a function of frequency and load resistance. The fundamental resonance was observed at around 870 Hz (Fig. 4). At small load the power output peaks at the resonance frequency, at large load the power peaks close to the antiresonance frequency. The electromechanical coupling coefficient can be derived from the frequency separation of the two peaks:

�

k 2 ≈2( fa − f r )

f r (1)

The curves of fig. 4 yield a value of k2 = 5 %. This is a quite reasonable value and equals about the values obtained in piezoelectric micromachined transducers, using, however, PZT on Pt bottom electrodes [12].

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Fig. 4: Output power as a function of frequency and load resistor.

A optimal power output was observed at antiresonance, on an optimal load of 2MW in order to obtain high output voltages. The output power, presented in Fig. 5 is equal to 1.4 µW for 2g acceleration, the voltage reaching 1.6 V.

Fig. 5: Voltage and output power as a function of acceleration. DISCUSSION

The obtained powers are in the expected range. A high enough voltage was achieved thanks to interdigitated electrodes and harvesting close to the antiresonance frequency. The minimal acceleration to arrive at the required 0.5 V amounts to about 0. 4g. The active transducer area amounts to 0.4x0.8 mm, thus delivering a power density of 160 µW/cm2/g. In the present design the inertial mass is too large, covering about 2/3 of the area. Future developments must include masses of higher density. Although our micro harvester operates at much lower frequencies

than earlier versions [6], a further decrease is desirable in order to exploit more frequent vibrations below 100 Hz. A heavier mass would also contribute to achieve such a goal. In addition, the silicon layer could be thinned down. The large coupling constant k2 of 5 % means that we can well use such a transducer for operation in air where damping coefficients of a few percent may occur. CONLCUSION

A well functioning piezoelectric energy harvesting device was obtained based on PZT thin films deposited by sol-gel techniques. A high piezoelectric coupling could be demonstrated. Voltage and power level are in a good range for potential exploitation.

Acknowledgements

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