piezoelectric mems: materials, devices, and applications

David Horsley, UC Davis September 2014 Berkeley Sensor & Actuator Center

Piezoelectric MEMS: Materials, Devices, and Applications

Professor David HorsleyCo-director, Berkeley Sensor & Actuator Center (BSAC)

Mechanical and Aerospace EngineeringUniversity of California, Davis

e-mail: [email protected]://mae.engr.ucdavis.edu/~memslab


Outline• Piezo-MEMS devices today

– companies & foundries in the market today– products on the market today– Comparison of piezo materials: PZT & AlN

• Future piezo-MEMS devices– Piezoelectric micromachined ultrasonic transducers (PMUTs)– Air-coupled devices for ranging & gesture sensing– Fluid-coupled devices for biometrics

2


Horsley Group Research

Ultrasonics

New materials & fabrication

Optical MEMS

Magnetic MEMS

Gyro & magnetometer

Diamond MEMS

Microactuators

Si fluidics

3


Piezoelectric MEMS Commercialization Status

4


Film Bulk Acoustic Resonators (FBAR)

• Avago produces 1 Billion AlN FBAR’s per year.

LTE Band Duplexer (Avago)


6Images: Harmeet Bhugra, IDT

IDT: Timing Oscillators


Other Commercial Piezo-MEMS Devices

7

Panasonic Gyroscope

Microgen Energy Harvester

poLight Varifocal Lens


2013 2014 2015 2016 2017 2018

Timeline

Dro

nes

Thin Film PZT – Sensors & ActuatorsPlayers roadmap – expected year for market entry

Ultr

asou

ndM

EM

S

Iner

tial

ME

MS

IR

dete

ctor

sM

EM

S

foun

drie

s

…

Expected year of foundry service readiness for thin film PZT technology

Expected year of foundry service readiness for thin film PZT technology

TFP Technology - Gyros

(Yole Developpement, Nov. 2013)


Piezoelectric MaterialsMechanical strain ↔ electrical polarization

10

e = cd

d: [pm/V]

e: [C/m2]

stiffness tensor

D = eS + εTEstrain

Piezoelectric coefficients


Comparing Piezoelectric Materials: Materials for Acoustic Transducers

Metric Property Units AlN PZT ZnOTransmitter Sensitivity e31,f C m-2 -1.05 -14.9 -1.0

ε33 - 10.5 1020 10.9Receiver

Sensitivitye31,f / ε33ε0

GV/m -11.3 -1.64 -10.3

[S Trolier-McKinstry & P. Muralt, J. Electroceram. 12(7-17), 2004.] 11


AlN Deposition: Reactive Sputtering

• Deposition rate > 50 nm/min• Low substrate temperature: 400°C• Bottom electrode materials: Mo, Pt, Al• AlN crystalline structure characterized by XRD Rocking Curve

– Typically ~1.5° FWHM for highly c-axis oriented film 12

V. Felmetsger, OEM Group

XRD of AlN on Mo

Magnetron S-gun

Plasma

Wafer

Sputtering Chamber


Piezoelectric MicromachinedUltrasonic Transducers (PMUTs)

13


Ultrasonic MEMS: Current Research Areas• Air-coupled PMUTs

– Diameter ~0.5 mm– Operating frequencies: 40 kHz – 800 kHz– Applications: gesture recognition, ranging, autofocus, gas metering

• Fluid-coupled PMUTs– Diameter ~50 microns– Operating frequencies: > 10 MHz– Applications: medical imaging, biometrics (fingerprint sensing)

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• Medical imaging• Ranging/obstacle avoidance• Robotics• Non destructive testing• Gesture recognition

Ultrasound Applications

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• Advantages:– Large output pressure– Directional, if desired

• Disadvantages:– Inefficient coupling to air– Matching layers required– Too big for consumer electronics– Dumb sensor. Lots of external electronics required.

Existing Ultrasound Transducers

10 mm

senscomp.com 5.2 mm

muratamericas.com

maxbotix.com

20 mm

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• Suspended plate structure• Increased coupling due to low acoustic impedance• Array fabrication possible• Micro-scale features

Micromachined Ultrasound Transducers

17


Air-Coupled Acoustic TransducersWant:• Large output pressure despite air’s low acoustic impedance

→ Large transducer displacement→ Piezoelectric Actuation

[Wygant et al., IEEE TUFFC 2009]

[Shelton et al., 2009 IEEE Ultrasonics Symp.]

[Przybyla et al., IEEE Sensors J. 2011]

Capacitive (CMUT) Piezoelectric (PMUT)

MoAlNAlN

Al

vs.

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fo ≈ 200 kHz

BW ≈ 10 kHz

–

Aluminum Nitride (AlN) PMUT Cross Section

Si

Al

450μm250μm

+

AlN

AlNMo

AlNAl

1μm

2μm

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Chip-Scale Arrays Enable 3D Ranging

• 2D array of transducers:– Output power on-axis: N2

– Number of elements sets beam width

• Beam width ~180○/N for linear array

– Individual electrodes enable electrical beam steering

– Spacing ~λ/2 = 0.9mm for 180○beam steering

37-Element Array

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Single element 2D array

S. Shelton, A. Guedes, R. Przybyla, R. Krigel, B. Boser, D.A. Horsley, 2012 Solid-State Sensors Actuators & Microsystems Workshop, Hilton Head SC, June 2012.

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Narrow beam improves SNR & spatial resolution

Phased Arrays are Directional


2D arrayOutput SPL Scales with N

0 5 10 15 200

5

10

15

20

Number of Transducers

Rel

ativ

e O

utpu

t Pre

ssur

e (a

.u.)

Measured Data Linear Fit

S. Shelton, A. Guedes, R. Przybyla, R. Krigel, B. Boser, D.A. Horsley, 2012 Solid-State Sensors Actuators & Microsystems Workshop, Hilton Head SC, June 2012.

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Phased Arrays are DirectionalNarrow beam improves SNR & spatial resolution


Range Measurement – Received Signal

Time

Time of Flight

≅ mm


Range Resolution

Voltage

Time


x-angle [deg]

Ran

ge [m

]

-40 -30 -20 -10 0 10 20 30 400

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

y-an

gle

[deg

]

-15

-10

-5

0

5

10

15

20

25

www.chirpmicro.com

Single Pulse Image

25

For 1 m max range design, @50 cm:

Z-axis: 0.4 mm rms

X-axis: 0.2o rms

Y-axis: 0.8o rms


Ultrasonic Fingerprint Sensor

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• Requires PMUTs similar to medical transducers– Center frequency > 20 MHz– 50 m resolution

• Significant performance advantages over existing fingerprint sensors– Sub-surface dermal imaging.– Wet/dry fingers can be imaged.


State of the art: Commercial Ultrasonic Fingerprint Sensor

Reference: Ultra-scan®, U.S. patent 5224174

lens

transducer

Mechanical motion

Pros:Dermal detection

Cons: Large system mechanical scanning

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Bulk Piezo Fingerprint Sensors

Sonavation Inc.

Drawbacks:

• Interconnect is challenging

• Readout based on resonator Q (no advantage over capacitance)

• High manufacturing cost.

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72 x 9 PMUT Array

Cross-section of 40 µm PMUT

Y. Lu et al, 2014 Solid State Sensor, Actuator, and Microsystem Workshop. 29


Coupling the Array to Skin

PDMSPDMS

Polyethylene

Fluid

Hydrophone

Test Setup Concept

250 m

30


Mapping the Pressure Field

31

40 m Needle Hydrophone

x/z

x/y

PMUT Array


Experimental Beamforming: AlN PMUTs

12V input

15 elements

8 channels

140 um pitchLateral distance (m)

Axi

al d

ista

nce

(mm

)

-200 -100 0 100 2001.3

1.35

1.4

1.45

1.5

1.55

1.6

1.65

10

20

30

40

50

60

70kPa15 elements/ 8 channels, 140 m Pitch

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100 µm


System diagram: pulse-echo imaging

PMUT Array

Collaboration w/ Bernhard Boser, UC Berkeley33


1 D ultrasonic imaging

1 cm

Lateral distance (mm)

Axi

al d

ista

nce

(mm

)

-5 -4 -3 -2 -1 0 1 2 3 4 5

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

0.1

0.2

0.3

0.4

0.5

0.6

100 µm scanning step

4 6 8 10 12 14 16-600

-400

-200

0

200

400

600

Pul

se e

cho

(mV

)

Time (s)

Pulse echoEnvelope

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2 D Scanning

1 cm

Lateral distance (mm)

Long

itudi

nal d

ista

nce

(mm

)

-4 -2 0 2 4

-2

-1

0

1

2

0

0.2

0.4

0.6

0.8

1

X scanning step 150 µmY scanning step 200 µm

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Summary & Conclusions• Current piezo MEMS devices:

– Several piezo-MEMS devices are now in high volume production.– Growing maturity of manufacturing base will enable new devices.

• Future piezo MEMS devices: PMUTs– Today’s ultrasound sensors lack capabilities for consumer

electronics applications.– In air: Tiny 0.5 mm PMUTs have up to 1 m range.– In tissue: pulse-echo imaging demonstrated w/ 1.8V supply. – PMUTs are a disruptive technology for gesture sensing and

biometrics.

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Acknowledgements• Prof. Bernhard Boser’s group (UC Berkeley)

– Richard Przybyla, Hao-Yen Tang, Igor Izyumen• Horsley group (UC Davis)

– Stefon Shelton, Yipeng Lu, Ofer Rozen, Andre Guedes, Stephanie Fung

• Sponsors– NSF & DARPA– Texas Instruments– Invensense– Capella Microsystems– Qualcomm

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piezoelectric mems: materials, devices, and applications

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