A PZT ENERGY HARVESTER MEMS FOR LOW AMPLITUDE ACCELERATIONS: FABRICATION AND CHARACTERIZATION

G. A. Ardila Rodríguez, H. Durou, A. Ramond, P. Dubreuil, D. Belharet, C. Rossi, D. EsteveCNRS-LAAS; Université de Toulouse; 7 avenue du colonel Roche, 31077 Toulouse Cedex 04, France

Abstract: In this paper we present the design, successful fabrication and characterization of a silicon based vibration harvesting device for aeronautic applications and especially for Structure Health Monitoring. Our piezoelectric energy harvester has been designed to harvest the typical mechanical ambient vibrations encountered in flight conditions (0.05g to 0.5g at 40Hz-60Hz). The first prototype can deliver 100nW (300mVrms) over an optimal resistive load (1MΩ) under 0.5g and 50Hz. This unsatisfactory result can be increased by better controlling the cantilever etching parameters as discussed in section 5.

Keywords: Mechanical Energy harvesting, MEMS, PZT

INTRODUCTIONRemote wireless networks are a very attractive and

flexible technology for applications requiring embedded sensors in inaccessible locations.One of the possible applications of wireless networks is the Aircraft Structure Health Monitoring (SHM) [1]. This application imposes inherent constraints (vibration amplitude <0.5g and frequency around 50Hz) and some limiting geometric constraints (thickness) to avoid aircraft aerodynamics alterations.The use of batteries having short lifespan is a high limitation for SHM wireless sensing networks. One emerging option is to harvest the ambient energy (vibration [2], heat [3], between others) to make the system completely autonomous over a theoretical infinite lifespan.

Many prototypes of energy harvesters from mechanical vibration have been reported, most of them use thin film of piezoelectric material deposited directly on the substrate and characterize the device under high mechanical input [3] (>1g) which is not compatible with our application.Here we propose a design, fabrication and characterization of a 4 cantilevers device made of thick PZT ceramic/Si designed to produce sufficient voltage at 0.5g; 50Hz.

DESIGNOur structure consists in 4 serial-connected PZT

cantilevers with a shared proof mass (Fig. 1). This design permits to amplify the voltage level supplied by the device compared to a one cantilever design. The total surface of the system is limited to 4.84cm2

and its thickness to 2.8mm to avoid aircraft aerodynamics alteration. The cantilevers are 350µm wide and 300µm thick (100µm of Si and 200µm of

PZT). The proof mass can be set from 160mg to 8g. The device total weight is 0.7g without proof mass.

Fig. 1: Structure of the PZT energy harvester.

FABRICATIONThe fabrication process starts from a 4” SOI

(Silicon On Insulator) substrate. It uses simple MEMS process steps combined with a conventional packaging process and laser etching technologies. It was first proposed and developed by our group in [4] and now is modified as follows (Fig. 2): 1) First, thin films are deposited on a SOI substrate: LPCVD SiNx (0.15µm) for electrically isolate the substrate and sputtered Ti/Au (0.1µm/0.8µm) to create the metallic contacts (Fig. 2a). 2) The second step consists in successive plasma etching: RIE (Reactive Ion Etching) of the deposited SiNx on both wafer sides and DRIE (Deep Reactive Ion Etching) of the top layer Si (100µm) that will further create the cantilevers (Fig. 2b). 3) The next step is the manual placement and bonding of a 200µm thick PZT film

PowerMEMS 2009, Washington DC, USA, December 1-4, 20090-9743611-5-1/PMEMS2009/$20©2009TRF 197

previously metalized with Cr/Au (500A/5000A) (610-HD supplied by TRS Technologies). (Fig. 2c).

Fig. 2: Fabrication of the energy harvester. Cut view and top side view. a) Thin film deposition. b) RIE/DRIE etching. c) PZT film placement. d) PZT laser etching. e) DRIE backside etching. f) Electrical connections. g) Proof mass placement.

This PZT was chosen for its relatively high d33 and d31 coefficients (690pC/N and -340pC/N respectively) [5]. The placement of the PZT film is made using a Tresky 3000 machine currently used in

microelectronics packaging, the bonding is made using a conducting thermal curing epoxy from Epotek (H20E) with silver filler. From this step on, special care was taken not to expose the PZT material to temperatures beyond its curie temperature (150°C) to avoid a modification of its properties. 4) Then the PZT is laser etched around the cantilevers using a femtosecond laser (Fig. 2d). This type of laser was chosen for its low temperature characteristics caused by an instantaneous ablation [6]. 5) Then the silicon and the SOI oxide (SiO2) are plasma etched (DRIE) from the backside to liberate the cantilevers (Fig. 2f). 6) The sixth step is the electrical serial connections between the cantilevers (Fig. 2g). A wedge wire-bonding technique was used with a gold wire maintaining the chuck at low temperature (110°C). 7) The final step is the manual placement and bonding of the proof mass (W) over the cantilevers (Fig. 2e).

Photos of some of the first prototype fabrication steps are presented in Fig. 3 and Fig. 4. Figure 3a. shows the PZT film after laser etching and removing of the unnecessary parts. Fig. 3b. presents a SEM image of a piece after laser cutting, some damages on the substrate are shown as the laser energy is increased to better etch the PZT film. Fig. 4. presents a SEM image of the final liberated structure.

Fig. 3: Photos of realizations. a) PZT film placement over the MEMS structure and laser etching. b) SEM image of the etched film.

a)

b)

PZT Laseretched film

PZT

Si SiO2

198

Fig. 4: SEM image of the finished prototype after the cantilever liberation using DRIE.

CHARACTERIZATIONThe fabricated device is connected to a varying

resistive load and is characterized under different vibrations conditions. The experimental setup consists in a metallic support attached to a voltage controlled shaker V101 from LDS (Fig. 5). The acceleration amplitude and frequency are measured using a 3-axis accelerometer (MMA7260QT from Freescale Semiconductor) integrated into the support. Fig. 6 gives one example of characterization for the silicon based piezoelectric energy harvester and is compared to a commercial piezoelectric harvester supplied by MIDE (v25wg) with a surface of 26cm² and a mass of 4.25g [7]. Both devices where adjusted to resonate at 50Hz, at this driven frequency the measurements for both devices are given in Table 1 and Table 2.

Fig. 5: Image of the MEMS energy harvester mounted on the experimental platform. In the calibration tests, the proof mass is a combination of small masses and is considerable thicker.

Fig. 6: Example of the generated electrical power in function of the resistive load at 0.1g. a) Commercial harvester .b) Fabricated prototype.

Table 1: Measurements made over the MIDE commercial device. Optimal Load = 50kΩ. Proof mass = 3.37g.

Input (g)

Voltage (Vrms)

Power(µW)

Power density(nW/g.cm²)

0.1 0.2 1 5.10.5 1.4 40 200

Table 2: Preliminary measurements made over the Si fabricated device. Optimal Load = 1MΩ. Proof mass = 1.47g.

Input (g)

Voltage (mVrms)

Power(nW)

Power density(nW/g.cm²)

0.1 100 10 10.5 300 100 9.5

DISCUSSIONSeveral drawbacks on the fabrication scheme

presented elsewhere [4] where solved for this article: First, concerning the laser etching process; the laser energy and time exposition were increased, and at the same time, multiple lines over the undesirable PZT parts were patterned to easily remove them. Secondly, considering the cantilever liberation; a plasma etching

MEMS harvester

PZT/Si cantilevers

Resistive load

Support

Shaker

Accelerometer

a)

b)

199

(DRIE) was used instead of wet etching (KOH) because it induced undesirable too high constraints due to KOH temperature bath (80°C) leading to the rupture of some cantilevers.

Unfortunaltely, The DRIE didn’t go perfectly and over-etched the lateral dimensions of the beams as well as the silicon supporting the PZT. The Si thickness has been measured at 34µm instead of 100µm expected. The cantilevers mean width has been measured at 219µm instead of 350µm expected. These prototype dimensions explain the low electrical harvested power. To confirm this assumption, FEM simulations using COMSOL multiphysics software, have been made for 2 prototypes geometries: one with ideal dimensions (see Fig. 1), one with the measured dimensions (Si: 34µm thick, 219µm wide). As shown on Table 3 and Fig. 7, harvested power quickly drops when the thickness and width of the Si beam are reduced. The simulation results confirm the experimental one and also confirm that if the DRIE process is improved we could reach 13.5µW with the same prototype design at 0.1g and 50Hz.

The low mechanical coupling of the used glue (Young modulus = 5.2GPa, Poisson coefficient 0.28, density 2.5g/cm3) is partly responsible for the power loss.

Table 3: FEM simulations output power compared to experimental measurements.

Generator (0.1g@50Hz) OutputFEM 1 (ideal) 13.56µWFEM 2 (with measured dimensions) 112 nWPrototype (experimental results) 100 nW

CONCLUSIONS AND PERSPECTIVESThe design, fabrication and preliminary

characterizations over a first PZT energy harvester prototype are presented for aircraft SHM applications, though it could be used in multiple wireless networks applications where mechanical vibrations are available. The chosen 4 serial-connected cantilever structure increases the power and voltage compared to a single cantilever structure. The DRIE liberation of the cantilevers needs to be improved to harvest 13.56µW instead of 100nW ( see Table 3).

10-6 10-5 10-4 10-310-8

10-7

10-6

10-5

Si layer thickness [µm]

RMS

outp

ut p

ower

[W]

0.1g @ 51Hz -- 1 beam -- PZT is 200µm thick

Fig. 7: COMSOL FEM simulations: influence of the Si substrate thickness on the output power.

ACKNOWLEDGEMENTSThe authors would like to thank the Aerospace

Foundation (FRAE) for supporting this research.

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[3] Hudak N and Amatucci G 2008 Small-scale energy harvesting through thermoelectric, vibration, and radiofrequency power conversion J. Appl. Phys. 103 101301

[4] Ardila Rodríguez G A et al Fabrication and simulation of a PZT energy scavenging MEMS Technical Digest PowerMEMS 2008 (Sendai, Japon, 9-12 Novembre 2008) 193-196

[5] http://www.trstechnologies.com/Materials/Piezoceramics/properties.php, Webpage accessed on September 22, 2009.

[6] Kim H et al 2008 Laser-machined piezoelectric cantilevers for mechanical energy harvestingIEEE Trans. Ultrason. Ferroelectr. Freq. Control 55 1900-1905

[7] http://www.mide.com/products/volture/vulture_catalog.php, Webpage accessed on September 23, 2009.

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