A CMOS-Compatible Piezoelectric Vibration Energy Scavenger Based on the Integration of Bulk PZT Films on Silicon

Ethem Erkan Aktakka, Rebecca L. Peterson, Khalil Najafi

Center for Wireless Integrated Microsystems (WIMS)

University of Michigan, EECS Dept., 1301 Beal Ave., Ann Arbor, MI 48109-2122, USA Tel:+1-734-763-6650, Fax:+1-734-763-9324, E-mail: aktakka@umich.edu, najafi@umich.edu

Abstract

This paper presents the micro-fabrication and testing of a CMOS-compatible high-performance piezoelectric inertial power generator. This is believed to be the first wafer-level micro-scale generator integrating a bulk piezoelectric ceramic, PZT. The technology offers advantages in fabrication flexibility and device performance over existing piezoelectric thin film deposition methods. The process involves aligned solder-bonding and thinning of bulk PZT pieces on Si. By conserving the bulk piezoelectric properties of the PZT material, high generator output voltage and high output impedance are obtained, simplifying rectification and regulation. Additional benefits of this process include the capability to obtain thick (5 m to 100 m) piezoelectric films without chemical patterning. Here, we describe a harvester with a Si proof mass, which generates 0.15µW from an input acceleration of 0.1g at 263Hz, and 10.2µW from an input acceleration of 2g at 252Hz. The unpackaged active volume of the generator (beam + mass) is 12.1mm3. The fabricated device has the highest Normalized Power Density for a PZT-based MEMS harvester reported to date, to the authors�’ knowledge.

Introduction

Bulk piezoelectric ceramics provide greater electromechanical force and charge capacity than deposited piezoelectric thin films (e.g. sputtered AlN or sol-gel PZT) (Table 1), and thus are highly desirable in many MEMS applications including micro-power scavengers, since the power output is directly proportional to the square of the electromechanical coupling coefficient (k31) (1).

22 2

RESONANCE 31 EFFAccelerationPower k . m . . Q

Res. Frequency∝ (1)

Previously, we introduced a batch-mode fabrication technology for wafer-level integration of bulk piezoelectric substrates on silicon in order to fabricate high performance diaphragm actuators (2). However, for fabrication of a cantilever beam or any other released structure, one needs to pattern the PZT, which is a challenging chemical process. Patterning technologies for thick/thin PZT films include ultrasonic machining (3), laser ablation (4), FIB milling (5),

DRIE (6), and wet-etching (7). Each of these methods has drawbacks such as micro-cracks due to mechanical machining, unintentional v-shaped cutting, impractically long processing time, or intolerable under-cutting. We have developed an aligned piece-wise bonding and lapping process for bulk PZT pieces on a silicon wafer to eliminate the necessity of patterning the PZT after bonding. This process can be applied to pieces as small as 200 mx200 m. Furthermore, the bulk piezoelectric properties are conserved in the final bonded and thinned PZT thin film with no re-polarization required. Table 1: Comparison of piezoelectric micro-fabrication processes

Fabrication Technology

The 6-mask fabrication process (Fig. 1) starts with etching

recesses on the Si wafer via DRIE, at places where PZT pieces will be bonded later in the process. The depth of the recess determines the total thickness of the beam, which is the sum of bond layer and PZT thicknesses. After oxidation of the Si wafer for electrical insulation, the bonding layer metals are deposited on the PZT and SiO2/Si wafers. Solder bonding details can be found in (2). The 130 m thick PZT substrate is then diced into pieces of the desired size and shape. These pieces are placed and aligned on the process wafer with the aid of a guide-wafer, and AuIn transient liquid phase (TLP)

PiezoelectricCoupling k31

2Material

Thickness Process Temp.

Process Challenges/ Application Limits

Bulk Ceramics

(This Work)

0.15 �– 0.35 (Wide range of selection)

5-100 m 200C (Bond. Temp.)

Min. feature size, Uniform thinning

Screen Printing < 0.090 10-100 m 600 �–

900C Pattern resolution, Slurry composition

Sol-gel PZT 0.060 �– 0.090 < 4 m 400 �–

700C Shrinkage/Cracking,

Substrate Effect Sputtered

PZT 0.070 < 2-3 m 450 �– 650C

Crystal orientation,Film uniformity

Sputtered AlN 0.017 - 0.030 < 2 m 20 �–

400C

Non-ferroelectric, Properties depend

on thickness Sputtered

ZnO 0.049 < 2 m 20 �– 275C

Fast diffusion of ZnLow DC resistivity

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solder bonding is performed at 200°C for 45min. Since PZT- 5A material has its Curie temperature at 350°C, the piezoelectric polarization is conserved even after the bonding process without any re-polarization step. With lapping/polishing, the bulk PZT thickness is decreased down to ~12.8 m. A ~2.2 m thick conformal Parylene layer is deposited for insulation between top and bottom electrodes, and patterned with O2 plasma. The top electrode is patterned by sputtering and wet-etching of Cr/Au (Fig. 2). Finally, the beams are released by back-side DRIE (Fig. 3). With the release of the structure, some residual stress may bend the beam (Fig. 4). This bending is due to the high mismatch between thermal expansion coefficients (TCE) of the PZT (4 m/m/K), and the bond layers, Au (14.2 m/m/K) and In (32.1 m/m/K). The residual stress can be prevented in a future design, where the passive layer of the unimorph structure is formed of a material with a better TCE match to PZT, such as Si (2.6 m/m/K).

Figure 1: Cross-sectional process-flow.

During the PZT thinning process, the surface of the silicon wafer serves as a stop layer. The thinning speed decreases considerably when PZT layer levels with the silicon surface, because of the sudden drop in the lapping pressure due to the larger area exposed to the lapping wheel. Another reason is the fact that single crystal silicon is a harder material to lap compared to a ceramic like PZT. The use of the silicon wafer as a stop layer enable us to achieve higher accuracy over the final PZT thickness, and also higher uniformity of PZT thickness across the wafer, down to +/-2.5 m. For CMOS compatibility, the DRIE step to form Si recesses can be avoided by following an alternate route, electroplating Ni to act as a thinning slow-down layer. Other factors affecting the wafer-level uniformity are initial wafer bow, the slurry composition, the uniformity of applied pressure over the wafer during lapping, and the condition of the lapping wheel.

Figure 2: SEM image of bonded & thinned PZT layer in the Si recess. A Parylene layer provides insulation between the bottom electrode (bond layer) and the top electrode.

Figure 3: a) Fabricated 4�” PZT-Si wafer before DRIE; b) Energy harvester with 475 m die thickness and 12.11mm3 active volume (beam+mass).

Figure 4: Residual stress due to TCE mismatch causes beam bending

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The top electrode dimensions are optimized for maximum power output via FEA simulations before fabrication (Fig. 5). The electrode coverage not only extends through the whole region where the beam bends during vibration, but also includes an additional distance of 50 m on both the anchor side and proof mass side, where there is still high strain.

Test Results

Fig. 6-10 summarize the characterization of the generator, including effects of load, vibration frequency and amplitude, and proof mass to the power output. Compared to other thin- film piezoelectric materials, thinned-PZT has a higher dielectric constant and enables higher voltage output from the harvester, 622mVPP at 0.1g and 5.18VPP at 2.0g (Fig.6). This high voltage output could be useful for efficient rectification, when the MEMS generator is to be integrated with its power management circuitry.

Figure 5: Optimization of top electrode by parametric FEA simulations.

Figure 6: Output resistance is optimized at 300k for maximum power. The harvester�’s output on optimum load is 622mVPP (210mVRMS) at 0.1 g, and 5.18 VPP (1.75VRMS) at 2.0g vibration input.

The decrease in normalized power density with increasing acceleration is due to the nonlinearly increasing damping and a correspondingly lower quality factor (Figs. 7, 9). The total damping in the system is determined by the sum of mechanical damping due to internal friction in the material, dielectric loss through PZT, electrical damping due to piezoelectric effect, and air damping. In the current design, the material damping is believed to be the dominant factor, since PZT-5A has a maximum mechanical quality factor of 80, a value provided by the vendor.

The measured power density is compared to the state-of- the-art in micro-fabricated power generators (Table 2 and Fig. 11). This work produces the highest reported normalized power density (N.P.D.) of 1.22mW/cm3/g2, among PZT-based micro harvesters, and provides the largest bandwidth.

3 2

2

Power / VolumeNormalized Power Density = (mW/cm /g )Acceleration

(2)

Replacing the silicon proof mass with a higher mass

density material, such as tungsten, via low-temperature backside bonding, is estimated to increase N.P.D. up to 6.31mW/cm3/g2 (Fig.10). This can also lower the resonance frequency of the device down to the 60Hz-120Hz range while preserving the same form factor, which is beneficial for matching frequencies of ambient vibrations available in consumer and industrial applications.

Figure 7: Frequency response at different input vibration levels.

Res

onan

ce F

requ

ency

(Hz)

Pow

er O

utpu

t (W

)

Figure 8: Power output at different input vibration levels.

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Nor

mal

ized

Pow

er D

ensi

ty

(mW

/cm

3 /g2 )

Qua

lity

Fact

or (u

nitle

ss)

Figure 9: Normalized power density decreases with increasing acceleration due to the increase in mechanical damping.

Res

onan

ce F

requ

ency

(Hz)

Pow

er O

utpu

t (W

)

Figure 10: Effect of mass to the power output is experimented by attaching additional weights (each 21.5milligrams) to Si proof mass.

In order to evaluate the practical potential of this technology, a new figure of merit is introduced. This enables a fair comparison between generators operating at different frequencies and with different bandwidths,

3 2

Tech. Perf. = N.P.D. Res. Freq. / Quality Factor N.P.D. Bandwidth (mW-Hz/cm /g )

×≈ ×

(3)

Conclusion

A new process for bulk-PZT MEMS integration and PZT thinning has been successfully demonstrated using a micro-fabricated piezoelectric harvester. The device performance clearly shows that this technology has a great potential for micro-fabrication of high-efficiency vibration generators. For further improvement, the quality factor of the device can be substantially increased by incorporation of a �“hard�” PZT (with lower dielectric loss and mechanical damping), and by vacuum packaging.

10 102 103 104

Vibration Frequency (Hz)

102

101

110-1

10-2

10-3

10-4

10-5

10-6

10-7

Nor

mal

ized

Pow

er D

ensi

ty (m

W/c

m3 /g

2 ) Hand Assembly (Not micro-fabricated)

Electrostatic ElectromagneticPiezoelectric

W Proof Mass(estimated via Fig.10)

Iso-Ambient-Energy lines used to compare energy harvesters with different resonance frequencies

This Work

Si Proof Mass

Figure 11: Comparison of state-of-the-art vibration energy harvesters. Table 2: Comparison of micro-fabricated piezoelectric harvesters. Figure of merit values are derived from the data provided in the references. * Based on test measurements shown in Fig.10.

Acknowledgements

This work is supported by the DARPA Hybrid Insect MEMS program under Grant # N66001-07-1-2006.

References (1) S. Roundy et al., Comput. Commun., 26, pp. 1131-1144, 2003. (2) E.E. Aktakka et al., Transducers �’09, pp. 849-852, 2009. (3) T. Li et al., MEMS �’05, pp. 387-390, 2005. (4) J.-P. Desbiens et al., Sens. Actuators A, 136, pp.554�–563, 2007. (5) J. Marshall et al., Integr. Ferroelectr. 61, pp. 223-230, 2004. (6) J.-K. Jung et al., Jpn. J. Appl. Phys. 40, pp. 1408-1419, 2001. (7) L.-P. Wang et al., Mat. Res. Soc. Symp. Proc. 657, pp.

EE5.39.1- EE5.39.6, 2001. (8) R. Elfrink et al., PowerMEMS �’09, pp. 67-70, 2009. (9) D. Shen et al., J. Micromech. Microeng, 18, 055017, 2008. (10) M. Renaud et al., Sens. Actuators A, 145�–146, pp. 380�–386,

2008. (11) B.S. Lee et al., J. Micromech. Microeng., 19, 065014, 2009.

Ref. Material N.P.D. (mW/cm3/g2)

Res. Freq.

Band-width

Tech. Perf. (mW-Hz/cm3/g2)

This Work

Thinned PZT

6.309 (*)(w/ W mass)

80.1 Hz Q = 32 2.5 Hz 15.79

This Work

Thinned PZT

1.215 (w/ Si mass)

263 Hz Q = 63 4.2 Hz 5.07

(8) IMEC

Sputtered AlN 0.967 325 Hz

Q=109 3.0 Hz 2.88

(9) Auburn

Sol-gel PZT 0.824 461.2 Hz

Q=385 1.2 Hz 0.99

(10) IMEC

Sol-gel PZT 0.387 1.8 kHz

Q=1800 1 Hz 0.387

(11) NTU

Aerosol PZT 0.500 255.9 Hz

Q = -- -- --

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