WAFER LEVEL FABRICATION OF HIGH PERFORMANCE MEMS USING BONDED AND THINNED BULK PIEZOELECTRIC SUBSTRATES

E.E. Aktakka*, H. Kim, and K. Najafi

Center for Wireless Integrated Microsystems (WIMS) University of Michigan, Ann Arbor, Michigan, USA

ABSTRACT

We report a batch-mode fabrication technology for integration of bulk piezoelectric materials into MEMS devices, and test results of high-performance out-of-plane piezoelectric actuators fabricated with this technology. Low-temperature (200C), reliable AuIn and Parylene bonding of PZT wafers/dies on Si wafers is achieved, and lapping is used to obtain <10 m PZT films. Conservation of the piezoelectric properties is confirmed with a hysteresis measurement. Additionally, square and circular shaped PZT diaphragms with 4mm×4mm, 2mm×2mm, and 1mm×1mm sizes operating in the d31-mode are fabricated with a 2-mask fabrication process. Greater than 12 m peak-to-peak deflection is obtained by actuation of a 1mm2 diaphragm at resonance (110.9kHz) with a power consumption of <7mW.

KEYWORDS

Piezoelectric, Solder bonding, Parylene bonding, Lapping, Actuators, Sensors, PZT. INTRODUCTION

Bulk piezoelectric ceramics, unlike deposited piezoelectric thin films (<few µm), provide greater electromechanical force, structural strength, and charge capacity, which are highly desirable in many MEMS applications including high-force actuators, harsh-environmental sensors, and micro-power scavengers. Previous studies for integration of bulk ceramics in MEMS focused on bonding by surface activation [1], hydrophilic conditioning [2], epoxy-resin [3], Cytop [4], AuSn [5], or InSn [6]. However, these manufacturing techniques have faced significant challenges such as a non-patternable bond layer, low bond strength due to high stress and voids in the bond layer, or out-diffusion of lead and re-polarization issues due to high temperature processing. In this study, we have developed low-temperature, fluxless, patternable, and reliable AuIn (conductive) and Parylene (non-conductive) bonding of bulk PZT-5A on Si wafers, both in die and wafer level (70mm×70mm), and have demonstrated that bonded PZT wafer/pieces can be thinned down at wafer-level to less than 10 m.

PZT-SI WAFER-LEVEL BONDING

AuIn solder and Parylene bonding processes (Fig.1) are performed in a vacuum environment (700mTorr) with a bonding pressure of 0.75MPa at 200C for 1 hour using

both 4�” PZT substrates and pieces (Fig.2). Typically, bonding of bulk PZT layers is challenging because commercial PZT layers have rough surface (~4.5 m) and experience loss of polarization at high temperatures. Both intermediate bonding layers of AuIn solder and Parylene can re-flow and planarize surface roughness (Fig.3). Additionally, the weight percentage of indium in the final AuIn layer is <20%, so the bond has a re-melting temperature >500C [7], which allows further high temperature processing.

Fig.1: Deposition of bond layers on Si and PZT wafers.

Fig.2: Die-level and wafer-level bonding of PZT on 4-inch Si.

Fig.3: Cross-sections of 130 m thick PZT bonded on Si.

Bonded PZT-5A samples do not require re-polarization, since the bonding temperature is around half of the Curie temperature (350C). The bond shear strength is measured and summarized in Table 1 for both AuIn and Parylene bonded samples. During shear tests, more than 50% of the AuIn samples failed due to cracking that occurred in the PZT or the Si, which indicates that the bond layer has greater strength than the bonded material itself. After bonding, ferroelectric properties are confirmed by measuring the remnant polarization (37.7 C/cm2) and the coercive field (16.8kV/cm) (Fig.4a).

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Table 1: Shear strength test results of PZT pieces bonded on Si.

AuIn Bond Parylene Bond # of Test Samples 112 43

Bond Area 2mm × 2mm 3mm × 3mm Avg. Shear Strength 4.44 MPa 1.47 MPa Standard Deviation 2.01 MPa 0.81 MPa Max Shear Strength 10.05 MPa 3.40 MPa

Dominant Failure Mode During Tests

Cracking in PZT/Si (>50% of samples)

Bonding Layer Peeling off

DIE-LEVEL BONDING AND THINNING Previous studies to pattern thick/thin PZT films for

fabrication of piezoelectric devices include ultrasonic machining [8], laser ablation [9], FIB milling [10], dry-etching [11], and wet-etching [12]. However, each of these methods has its own drawback such as micro-cracks due to mechanical machining, unintentional v-shaped cutting, impractically long processing time, or intolerable under-cutting. In order to enable piezoelectric device fabrication without the necessity of patterning PZT, bonding of 200 m×200 m and 500 m×500 m (both 130 m thick) PZT samples on a Si wafer has been demonstrated. Additionally, bonded pieces and wafers are lapped down (Fig.4b) to thickness values from tens of microns down to <10 m. During this process, diamond slurry with a particle size of 3 m, is used on a copper lapping wheel, and a lapping rate of 4 m/min is achieved. Surface roughness of final PZT thinned-film is 74.3nm (average) with 182.9nm maximum hill/valley height. Uniform film thickness is obtained at wafer-level with +/�–5 m variance towards the edges. The thickness uniformity can be improved by checking the flatness of the lapping wheel regularly, applying uniform pressure on the process wafer, and keeping the rotation speed of the process wafer constant.

Fig.4: (a) Hysteresis curve after bonding; (b) 500 m × 500 m PZT pieces are bonded on Si, and lapped down to <10 m

THIN FILM PZT ACTUATORS Different size (4x4mm2/2x2mm2/1x1mm2) square and

circular shaped d31-mode piezoelectric out-of-plane actuators were designed (Fig.5), and fabricated at wafer-level. Dimensions of inner and outer actuation electrodes are optimized via multi-physics FEA simulations (Fig.6). The 2-mask fabrication process includes deposition of bonding layer metals on PZT and SiO2/Si wafers, bonding, lapping (PZT~15.5 m), deposition of actuation electrodes (Cr/Au), release of PZT-AuIn membranes by back-side DRIE of Si, and oxide removal in the cavities by dry/wet etching. The fabricated devices are shown in Fig.7.

Fig.5: Design of d31-mode piezoelectric diaphragm actuators

Fig.6: Optimization of inner and outer electrode dimensions on the diaphragm by multi-physics FEA simulations.

Fig.7: (a) PZT thickness on device wafer; (b) Cross-section of 1mm diameter diaphragm; (c) Array of 2mm×2mm diaphragms.

Actuated displacements of fabricated diaphragms are measured using a laser vibrometer. In order to confirm that the piezoelectric properties of the bulk material are fully conserved, measured out-of-resonance displacement of a diaphragm is compared with its FEA simulation at static actuation (Fig.8). The simulation results are represented with the shaded area, in order to account for +/�– 0.5 m variation in PZT and AuIn thickness values, due to the rough interface between these layers.

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Fig.8: Comparison of measured and simulated displacements. Resonance frequencies of fabricated actuators are

determined by measuring diaphragm deflections while operating at 2VPP (Fig.9). Then each diaphragm is operated at its own resonance frequency in air with increasing actuation voltage (Fig.10). It has been observed that inner electrode actuation results in comparable or greater deflection than outer electrode actuation with the same input voltage. However, outer electrode actuation results in up to 2× greater power consumption due to its larger capacitance area (Fig.11).

Fig.9: Frequency response of diaphragms with different shapes.

Fig.10: Deflection vs. Voltage for a 16mm2 square diaphragm

Measured temperature response of the piezoelectric actuators is shown in Fig.12. Temperature sensitivity of resonance frequencies is observed to be fairly linear, and measured as 6160ppm/C, 2940ppm/C and 1729ppm/C for 4mm, 2mm and 1mm diameter diaphragms, respectively. This high temperature dependence is believed to be caused by the variation of residual stress with temperature, which is due to different thermal expansion

coefficients of PZT-5A (4.0 /K), Au (14.2 /K), In (32.1 /K), and Si (2.6 /K). As temperature increases, the

tensile stress in the bond layer is reduced, and the diaphragm stiffness decreases. This decrease in stiffness can also be observed from the increasing out-of-resonance deflection of the diaphragm (Fig.13). PZT-5A has fairly stable characteristics over temperature, and <3% change in the piezoelectric strain coefficient d31 is expected between 20C-100C according to the manufacturer. However, ~20% increase in dielectric constant with temperature, causes a slight increase in power consumption (Fig.13).

Fig.11: Power consumption vs. Frequency of diaphragms.

Fig.12: Sensitivity of resonance frequency to temperature.

Fig.13: Change in low frequency (out-of-resonance) response of a diaphragm with temperature.

In an integrated system, the drive circuitry should be in feedback with the change in transducer�’s frequency. In order to sense the change in mechanical resonance frequency, either the outer electrode reading or the input current through the inner electrode can be used (Fig.14).

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Fig.14: Using the outer electrode as sensing read-out. CONCLUSION

We have successfully demonstrated die-level and wafer-level bonding and thinning of bulk piezoelectric layers on silicon, and confirmed that the piezoelectric properties are fully conserved after this process. Thin film piezoelectric diaphragms with different sizes and shapes are fabricated, and the device performance (Table.2) clearly proves that this technology can be used for fabrication of high-deflection high-stroke actuators and sensors. This technology can be applied to general bulk piezoelectric materials, such as PZN-PT or PMN-PT for higher electromechanical coupling coefficient.

ACKNOWLEDGEMENTS

This project is supported by the Hybrid Insect MEMS (Grant# N66001-07-1-2006) and Micro Cryogenic Cooler (Grant# W31P4Q-06-1-001) programs of DARPA.

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CONTACT *E.E. Aktakka, tel: +1-734-272-3170; aktakka@umich.edu

K. Najafi, tel: +1-734-763-6650; najafi@umich.edu

Table.2: Performance summary Actuation by inner electrode Actuation by outer electrode Actuation by both electrodesDiaphragm Resonance

frequency Quality Factor

Peak-to-Peak Deflection

Power Consumption

Peak-to-Peak Deflection

Power Consumption

Peak-to-Peak Deflection

Power ConsumptionSize Shape

16mm2 Square 15.5 kHz 16.5 11.56 m 19.92 mW 7.38 m 36.30 mW 15.54 m 56.22 mW Circular 16.3 kHz 21.8 17.68 m 16.98 mW 14.75 m 31.45 mW 25.11 m 48.43 mW

4mm2 Square 35.1 kHz 38.5 18.15 m 10.27 mW 15.56 m 20.87 mW 25.22 m 31.14 mW Circular 38.4 kHz 49.9 24.47 m 11.36 mW 25.92 m 21.13 mW 34.21 m 32.49 mW

2mm2 Square 89.2 kHz 77.7 10.20 m 6.10 mW 9.64 m 12.42 mW 13.34 m 18.52 mW Circular 110.9 kHz 76.3 12.23 m 6.74 mW 11.95 m 16.86 mW 15.49 m 23.60 mW

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