Piezoelectric MEMS: Materials, Devices, and Applications Professor David Horsley Co-director, Berkeley Sensor & Actuator Center (BSAC) Mechanical and Aerospace Engineering University of California, Davis e-mail: [email protected] http://mae.engr.ucdavis.edu/~memslab

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Horsley Group Research Optical MEMS

Gyro & magnetometer

Microactuators

Si fluidics

Ultrasonics

New materials & fabrication Magnetic MEMS

3

Diamond MEMS David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Piezoelectric MEMS Commercialization Status 4 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Film Bulk Acoustic Resonators (FBAR) LTE Band Duplexer (Avago)

• Avago produces 1 Billion AlN FBAR’s per year. David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

IDT: Timing Oscillators

Images: Harmeet Bhugra, IDT David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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Other Commercial Piezo-MEMS Devices Panasonic Gyroscope Microgen Energy Harvester

poLight Varifocal Lens 7 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

(Yole Developpement, Nov. 2013)

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

Inertial MEMS

Ultrasound MEMS

IR detectors

Drones

MEMS foundries

Expected year of foundry service readiness for thin film PZT technology

TFP Technology - Gyros

… David Horsley, UC Davis

2013

Timeline

2014

2015 September 2014

2016

2017 Berkeley Sensor & Actuator Center

2018

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Piezoelectric Materials Mechanical strain ↔ electrical polarization Piezoelectric coefficients

D = eS + εTE strain

stiffness tensor

e = cd d: [pm/V] e: [C/m2] David Horsley, UC Davis

September 2014

10 Berkeley Sensor & Actuator Center

Comparing Piezoelectric Materials: Materials for Acoustic Transducers Metric Transmitter Sensitivity Receiver Sensitivity

Property

Units

AlN

PZT

ZnO

e31,f

C m-2

-1.05

-14.9

-1.0

ε33 e31,f / ε33ε0

-

10.5

1020

10.9

GV/m

-11.3

-1.64

-10.3

[S Trolier-McKinstry & P. Muralt, J. Electroceram. 12(7-17), 2004.] David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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AlN Deposition: Reactive Sputtering XRD of AlN on Mo

Sputtering Chamber Wafer Plasma

Magnetron S-gun V. Felmetsger, OEM Group

• • • •

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

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) 13 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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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September 2014

Berkeley Sensor & Actuator Center

Ultrasound Applications • • • • •

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

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September 2014

Berkeley Sensor & Actuator Center

Existing Ultrasound Transducers

20 mm

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

10 mm maxbotix.com

• Disadvantages: – – – –

Inefficient coupling to air senscomp.com Matching layers required Too big for consumer electronics Dumb sensor. Lots of external electronics required.

5.2 mm

muratamericas.com 16 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Micromachined Ultrasound Transducers

• • • •

Suspended plate structure Increased coupling due to low acoustic impedance Array fabrication possible Micro-scale features 17

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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

→ Large transducer displacement → Piezoelectric Actuation

Piezoelectric (PMUT)

Capacitive (CMUT)

vs.

Al AlN AlN

Mo

[Shelton et al., 2009 IEEE Ultrasonics Symp.] [Wygant et al., IEEE TUFFC 2009] David Horsley, UC Davis

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

September 2014

Berkeley Sensor & Actuator Center

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Aluminum Nitride (AlN) PMUT Cross Section Al

+

2μm

– 1μm 250μm

AlN Mo AlN

Si

450μm

fo ≈ 200 kHz BW ≈Al10 kHz AlN David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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Chip-Scale Arrays Enable 3D Ranging 37-Element Array

• 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

20 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Phased Arrays are Directional Narrow beam improves SNR & spatial resolution

2D array

Single element

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. David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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Phased Arrays are Directional Narrow beam improves SNR & spatial resolution

Relative Output Pressure (a.u.)

20 Measured Data Linear Fit 15

10

5

0 0

5

10

15

20

Number of Transducers

Output SPL Scales with N

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. David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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Range Measurement – Received Signal

≅ mm

Time

Time of Flight David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Range Resolution

Voltage

Time

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

0.8

25

0.7

20

0.6

15

0.5

10

0.4

5

0.3

0

For 1 m max range design, @50 cm:

0.2

-5

Z-axis: 0.4 mm rms X-axis: 0.2o rms

0.1

y-angle [deg]

Range [m]

Single Pulse Image

-10

Y-axis: 0.8o rms 0

David Horsley, UC Davis

-40

-30

-20

-10 0 10 x-angle [deg] September 2014

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30

40

-15

www.chirpmicro.com Berkeley Sensor & Actuator Center

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Ultrasonic Fingerprint Sensor • 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.

26 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

State of the art: Commercial Ultrasonic Fingerprint Sensor

Pros: Dermal detection Cons: Large system mechanical scanning

Mechanical motion

lens

transducer

Reference: Ultra-scan®, U.S. patent 5224174 27 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Bulk Piezo Fingerprint Sensors Drawbacks: • Interconnect is challenging • Readout based on resonator Q (no advantage over capacitance) • High manufacturing cost.

Sonavation Inc. 28 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

72 x 9 PMUT Array

Cross-section of 40 µm PMUT

Y. Lu et al, 2014 Solid State Sensor, Actuator, and Microsystem Workshop. David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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Coupling the Array to Skin Hydrophone Fluid PDMS

Polyethylene

250 m

PDMS

Concept

Test Setup

30 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Mapping the Pressure Field

x/z

40 m Needle Hydrophone

PMUT Array x/y 31 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

Experimental Beamforming: AlN PMUTs 15 elements/ 8 channels, 140 m Pitch

1.65

kPa 70 60

Axial distance (mm)

1.6 50 1.55

100 µm

1.5 1.45 1.4

40

12V input

30

15 elements

20

8 channels

1.35 10 1.3

-200

-100 0 100 Lateral distance (m)

200

140 um pitch 32

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

System diagram: pulse-echo imaging

PMUT Array

Collaboration w/ Bernhard Boser, UC Berkeley David Horsley, UC Davis

September 2014

33 Berkeley Sensor & Actuator Center

1 D ultrasonic imaging 600 Pulse echo Envelope

Pulse echo (mV)

400 200 0 -200

1 cm

-400 -600

4

6

8

10 Time (s)

12

14

16

0.6

3

Axial distance (mm)

2.8

0.5

2.6 0.4

2.4 2.2

0.3

2 1.8

0.2

1.6 1.4

0.1

100 µm scanning step

1.2 -5 David Horsley, UC Davis

-4

-3

-2

-1 0 1 Lateral distance (mm)

2

September 2014

3

4

34

5 Berkeley Sensor & Actuator Center

2 D Scanning 1

Longitudinal distance (mm)

2 0.8 1 0.6 0 0.4 -1 0.2 -2 -4

-2 0 2 Lateral distance (mm)

4

1 cm

0

X scanning step 150 µm Y scanning step 200 µm 35 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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.

36 David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center

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 37

David Horsley, UC Davis

September 2014

Berkeley Sensor & Actuator Center