A 6-DOF PIEZOELECTRIC MICRO VIBRATORY STAGE BASED ON MULTI-AXIS DISTRIBUTED-ELECTRODE EXCITATION OF PZT/SI UNIMORPH T-BEAMS

E.E. Aktakka, R.L. Peterson, and K. Najafi

Center for Wireless Integrated MicroSensing and Systems (WIMS2) University of Michigan, Ann Arbor, MI, USA

ABSTRACT

This paper reports the design, optimization, fabrication and preliminary testing of a six degree of freedom (6-DOF) multi-axis micro vibratory stage leveraging a new type of piezoelectric actuation based on distributed excitation electrodes and PZT/Si unimorph T-beams. This new design concept also allows both angular and linear bending actuation of a single piezoelectric unimorph with very large in-plane and out-of-plane displacements. The PZT/Si beam thickness ratio and width ratio are optimized, and the 6-DOF actuation modes are characterized via FEA. The vibratory stage is fabricated via a process that involves bonding/thinning of bulk-PZT over silicon features and includes PZT micro-patterning via an improved DRIE process. Preliminary tests show static displacement of ±1.0° for X/Y-tilting mode and ±22 μm for Z-translational mode, while the minimum resonance frequency is >0.9 kHz, and the maximum power consumption is <250 μW. The presented device compares favorably with respect to previously reported multi-axis stages in terms of motion capability, displacement, size, input voltage and power consumption. KEYWORDS

Actuation, multi-axis, piezoelectric, bonding, lapping. INTRODUCTION

Multi-axis actuation devices are required in a wide-variety of applications, including manipulation of micro-grippers [1], vibration damping for image stabilization [2], disk-drive protection [3], and probe-based microscopy/data-storage [4]. A new emerging application is to provide on-chip physical stimuli to MEMS inertial sensors for their on-chip calibration and improving their long-term stability. Here, the aim is to periodically measure and actively calibrate the change in bias and scale factor of a gyroscope or an accelerometer in the field, by applying known rotations or accelerations to the sensor. With this aim, chip-scale rotary motors have been reported recently which could be used for in situ calibration of rotational rate and rate-integrating single-axis gyroscopes [5], and dual-axis accelerometers [6]. As an alternative approach to rotary motors, micro vibratory stages can provide sufficient dithering actuation ranges in multiple DOFs in a single device and a very compact size, with additional advantages of low power consumption and easy transfer of electrical signals from the sensor to the outside world (Figure 1). The capability of multi-axis actuation can enable on-chip physical calibration of not only a single-axis sensor, but also multiple-DOF inertial sensors. Furthermore, in addition to the variation of scale factor and bias values, the cross-axis sensitivity of the inertial sensors can also be measured and calibrated.

Figure 1: Simplified system overview to use a multi-axis vibratory stage for calibration of an inertial sensor.

Recently, we demonstrated a device that can provide 3-DOF out-of-plane motions based on piezoelectric crab-leg suspensions with rectangular beam cross-sections [7]. In this paper, we introduce two new concepts to boost the motion capability of this stage into 6-DOF. First, we implement distributed excitation electrodes to enable both angular and linear in-plane and out-of-plane actuation of the stage. Second, we use a T-shaped cross-section in the PZT/Si unimorph suspensions to increase the in-plane displacement range by ~3× compared to a unimorph with a rectangular-shaped cross-section.

Figure 2: Top and bottom views of the micro-fabricated 6-DOF multi-axis actuation stage. MULTI-AXIS STAGE DESIGN

The moving platform is held and actuated by four crab-leg suspensions, where each leg consists of two piezoelectric unimorph actuator units (Figure 2). The beams are actuated in the transverse (31) mode by application of a vertical electric field across the piezoelectric layer. Here, to obtain extended motion capability in multiple axes, we introduce the partitioning of the top electrode of each piezoelectric unimorph into four sections. With different excitation polarities on these electrodes, it is possible to actuate a single unimorph beam in both angular and linear axes, and in both in-plane and out-of-plane axes (Figure 3). The gap between partitioned electrodes is optimized as 30 μm by FEA simulations (Figure 4). With this gap value, the actuation displacement is nearly at its maximum possible value, while the generated mechanical strain on the PZT layer is limited to <1% during excitation. The strain is intentionally limited as a precaution to prevent cracking due to high actuation stress and to minimize other mechanical failures related to fatigue and creep.

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Compared to a unimorph beam cross-section, a bimorph piezoelectric blarger displacements in both in-plane axes. However, it is highly challenging a bimorph beam with equal top and bolayer thicknesses, and to deposit patterboth top and bottom surfaces in a wafRecently, a meso-scale cantilever beasingle bulk piezoelectric block with asection is used to provide in-plane motion with angular but not translatio[8]. In this paper, we introduce a unimorph cross-section, which enablesroute for micro-fabrication in addition tof the actuation signals. This design allothe in-plane actuation without compromplane displacement at the same electric f

In a unimorph beam, to obtain maxiactuation range and to minimize static bto residual stress at the bond layer, it is the z-axis centroid of the beam exactly (i.e. between Si and PZT). This design roptimum PZT/Si thickness ratio (tPZT/tSi)

Figure 3: Top view of the micro motiomulti-axis distributed-electrode excitatio

Figure 4: Optimization of the gap betwetop electrodes on a piezoelectric unimorp

with rectangular beam can provide and out-of-plane

to micro-fabricate ttom piezoelectric

rned electrodes on fer-based process. am made from a a T-shaped cross-

and out-of-plane onal displacement T-shaped PZT/Si a more practical to simpler control ows us to increase mising the out-of-field (Figure 5-6). imum out-of-plane beam-bending due necessary to keep at the bond layer

rule determines an ) (Figure 5).

on stage, and the on of a beam.

een the partitioned rph beam.

In a T-beam with optimudetermined tPZT, the in-plane fdecreased by utilizing a smalle(WSi) compared to the width of

12

where E is the elastic modecreases, the out-of-plane flexto larger tSi requirement. Thusfor WSi, determined as 20 μm in

3

By utilizing multiple T-becrab-leg suspensions of a platdesigned to provide 6-DOFrequired voltage polarities onobtain different actuation modedisplacements are summarized

Figure 5: Simulated in-plane &a PZT/Si cantilever beam with

Figure 6: Simulated in-plane &a PZT/Si cantilever beam with

Figure 7: Simulated 6-DOF mustage by distributed-electrode e

um tPZT/tSi ratio and a pre-flexural rigidity (EI) can be er width in the passive layer f the active layer (WPZT).

1 1

odulus. However, as WSi xural rigidity increases due s, an optimum value exists n this design (Figure 6).

1 2

eam actuators as symmetric tform, a multi-axis stage is F-motion capability. The n individual electrodes to es and their simulated FEA in Figure 7.

& out-of-plane actuation of

rectangular cross-section.

& out-of-plane actuation of

T-shaped cross-section.

ulti-axis actuation of the excitation.

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FABRICATION In order to define the piezoelec

fabrication method that is best suited fostate-of-the-art piezoelectric materials their actuation capability in the selec(Figure 8). The thin-film deposited piezrelatively low d31 strain coefficiencommercially available bulk substratesmaximum deposited thin-film thickness not possible to obtain the optimum piezoratio in a thick suspension beam that isthe payload of the inertial sensor. Therefthe designed 3×3-mm2-sized stages process of solder-bonding and thinnisubstrates on SOI [9] to form a flat piezpre-patterned Si features. A 3-μm thicserves as electrical isolation between telectrodes and their interconnects to pads (Figure 9a, 10). Previously, we havetching technique for patterning of thifeature sizes on the order of ~100 μm [1more precise patterns in the 25-μm thicetching process with C4F8/Ar/H2 plas(Figure 9b). The patterned PZT beamdefined, although the side-walls have 6inefficient removal of byproducts during

Figure 8: Comparison of different piezovia simulated actuation of a unimorph ca

Figure 9: a) Metal interconnects, anthrough parylene insulation, b) Dry etch

tric material and or this application, are compared for

cted beam design zoelectrics have a nt compared to s. In addition, the

is limited, so it is o/silicon thickness s required to carry fore, we fabricated by adapting our ing of bulk-PZT zoelectric layer on ck parylene layer the top excitation the wire-bonding ve reported a wet-ick PZT films for 0]. Here, to obtain k PZT-film, a dry

sma is developed m edges are well-60° angles due to g etching.

oelectric materials antilever.

nd electrical vias hing of bulk-PZT.

Figure 10: Graphical crosfabricated multi-axis motion sta

TEST RESULTS For preliminary testing,

actuated statically, and 3-DOF are measured via an optical inTable 1), while the in-plane mbe performed. With ±25 V extested to provide > 2° X/Ytranslational motion range. Tsimulated off-axis coupling bmodes. However, by adjustingon individual excitation electrosimulations indicate that it is undesired motions to ~1 nm am

The first resonance frequany additional payload is mtranslational mode at ~0.9 kHzplatform is tested to carry up trade-off of decreased operaparticular inertial sensor applscaled to thicker and wider susthe payload capability of thmaximum dynamic power co250 μW with ±25 V excitation

Figure 11: Measured static tilti

Table 1: Measured* and simulaaxis coupling during non-reson

Non-resonant Actuation (±25 V) Displacem

Linear Motion

Z ± 22 μmX or Y ± 7.5 μm

Tilting Motion

Z ± 0.5° *X or Y ± 1.0°

Figure 12: Measured and simfrequency of the vibratory stage

ss-section of the micro-age.

the fabricated stage is out-of-plane displacements

nterferometer (Figure 11 & motion measurements are to xcitation input, the stage is -tilting and > 40 μm Z-There is a 0.4% to 7%

between different actuation g the applied voltage levels odes with 10 mV precision, possible to minimize these

mplitude or less. uency of the stage without measured to occur in Z-z (Figure 12). The moving to 10 mg payload, with a

ational bandwidth. For a ication, the design can be spension beams to increase he moving platform. The nsumption is measured as at 0.9 kHz.

ing actuation range.

ated** displacements & off-nant actuation of the stage.

ment Off-axis Coupling

m * 0.02° Z-tilting ** m ** 0.5 μm Y/X-linear **** 1.7 μm Z-linear ** * 0.04° Y/X-tilting **

mulated first mode natural e.

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CONCLUSION In this paper, a 6-DOF multi-axis micro actuation

stage design is introduced, which is based on distributed-electrode excitation of PZT/Si unimorph T-beams. A prototype with 3×3-mm2 active device area is fabricated via bonding, wafer-level thinning, and dry-etch patterning of a bulk PZT substrate on a 4-inch silicon wafer. The preliminary test results indicate that the fabricated stage can provide ±1.0° X/Y-tilting and ±22 μm Z-translational motion. When compared with previously-reported micro-fabricated multi-axis actuators (Table 2), the introduced device can provide higher DOF capability, larger displacements, and a similar bandwidth in a compact device volume, while requiring smaller actuation voltages and lower power consumption. ACKNOWLEDGEMENTS

This research is supported by DARPA PASCAL award #W31P4Q-12-10002. The fabrication was performed at the University of Michigan’s Lurie Nanofabrication Facility (LNF), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported in part by the National Science Foundation.

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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: Comparison of the introduced micro actuation stage with other multi-axis motion stages. (*tested, **simulated)

Reference Actuation Method

Size of Stage + Suspensions

Actuated DOF

Non-resonant Linear Motion

Non-resonant Tilting Motion

First Natural Frequency

Excitation Input

This Work Piezoelectric 3 × 3 mm2 3* / 6** > ±7.5 μm > ±0.5° 0.9 kHz 25 V [11] Electromagnetic 16 × 16 mm2 6 ±5 μm ±0.25° 0.9 kHz 0.3 A [12] Piezoelectric 15 × 15 mm2 6 1.5 μm ±0.02° 1.4 kHz 80 V [13] Electrostatic 8 × 8 mm2 3 18 μm ±1.7° 0.5 kHz 85 V [2] Electrostatic 8 × 8 mm2 2 25 μm N/A 1.0 kHz 84 V

[14] Piezoelectric 2 × 2 mm2 1 80 μm N/A 0.2 kHz 20 V

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