Seung-Hyub Baek

Center for Electronic Materials

Korea Institute of Science and Technology (KIST)

15th Korea-U.S. Forum on Nanotechnology

Epitaxial piezoelectric heterostructures

for ultrasound micro-transducers

• Personal information • Banking

Security issue for mobile electronics

Voice

Face

Iris

Vein

Fingerprint

• Highly secured

• Convenient

Fingervein

Biometrics-based authentication systems

Fingerprint

• Easy to hack

Similar to medical imaging

Not affected by sweat or dirt

Convenient

Small, low-power consumption

3 dimensional recognition

Fingerprint/vein/blood flow

Ultrasonic transducer array

Ultrasound-based fingerprint/vein recognition

Ultrasound Probe

Human

Body

Ultrasonic

Beam

Reflected

Signal

Principle of ultrasound imaging

2D Transducer Array !

Beam-forming for 3D imaging

Wiring issue

Not scalable

Issues on 2D array with bulk piezoelectric materials

Electrodes Membrane

Insulation

Cavity

Si substrate Deflection

Vac+Vdc

Electrodes Piezoelectric layer

SiO2

Cavity

Si substrate Deflection

Vac

cMUT pMUT

Electrostatically-driven Piezoelectrically-driven

• Hard to fabricate

• High DC-field (electric shock!)

• Electrical charging

• Non-linear behavior

• Relatively easy to fabricate

• No DC-field necessary

• Stable

• Linear behavior

Micro-Machined Transducer (MUT)

Vein recognition without gel !

vs.

pMUT-based fingerprint/vein imaging system

Ultrasonic transducer array

Air Air

Dermis

vs.

vein

• Giant piezoelectric material.

PMN-PT Single crystal is 10 times

higher than PZT ceramic.

(d33= ~2000 pC/N, k33= ~0.92)

Giant piezoelectric relaxor-ferroelectrics

Pb(Mg1/3Nb2/3)O3 - PbTiO3 Single Crystals

S-E. Park, T.R. Shrout , J. Appl. Phys. 82, 1804 (1997).

S-E. Park, T.R. Shrout , J. Appl. Phys. 82, 1804 (1997).

> 1.7% strain Field-Induced Phase Transition in (001) PZN-8% PT Single Crystal

Giant piezoelectric relaxor-ferroelectrics

• Anisotropic piezo-property: d33 [001] > d33 [111]

Polycrystalline (001) Epitaxial

Giant piezoelectric relaxor-ferroelectrics

Epitaxial PMN-PT thin films on Si

Si substrate Diamond Cubic structure Lattice parameter: 5.4302 Å

Conducting oxide Perovskite structure ~3.930 Å

Piezoelectric material Perovskite structure ~4.02 Å

Insulating oxide Perovskite structure 3.905 Å

MBE

Sputter

Off-Axis Sputtering for high-quality films

Heater Block

Sputtering gun Energetic particles

Diffused particles

Challenge for PMN-PT film growth

Voltage (V)

Pola

riza

tion (

µC/c

m2)

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

-60

-40

-20

0

20

40

60

Leaky!

Volatile PbO

Miscut substrate to fix volatile PbO

Step as a preferential nucleation site

X-ray diffraction with 2D area detector for

PMN-PT /SrRuO3/Si

Exact (001) Si 4o miscut (001) Si

SrRuO3

PMN-PT

2 nm Si

SrRuO3

PMN-PT

100 nm

SrTiO3

Rocking Curve

FWHM

0.23o

0.58o

Cross Sectional TEM images of PMN-PT on silicon

TEM by X.Q. Pan, Michigan 0.32o (bulk PMN-PT single crystal)

-600 -400 -200 0 200 400 600 -60

-40

-20

0

20

40

60

Po

lariza

tio

n (

µC

/cm

2)

Electrical field (kV/cm)

3000

0

1000

2000

0

0.1

0.2

0.3

-100 0 100

Electrical field (kV/cm)

Die

lectr

ic c

on

sta

nt

Lo

ss ta

ng

en

t

Strong Imprint- Unipolar Operation

• Highly insulating (>600 kV/cm)

• Strong imprint: unipolar operation

• Reduced dielectric constant for better detection

Piezoelectric and energy harvesting FOM

MEMS performance

• Simulations match experimental data using

bulk PMN-PT parameters.

• PMN-PT piezoelectric properties unaffected

by processing.

100 101 102

0.0

0.2

0.4

0.6

0.8

Tip

dis

pla

cem

ent

(µm

)

Voltage (V)

Modeled electrostatic cantilever

Modeled PMN-PT cantilever

Actual PMN-PT cantilever

0 0.02 0.04 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Hei

ght

(µm

)

Length (mm)

0V

2.8V

Cantilever clamping point

Length:35µm

0.375 µm / V deflection

0.0 1.5 3.0 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

∆Z

(μm

)

S.H. Baek et al. Science 334, 958 (2011)

Development of relaxor-ferroelectrics

1. Mn: Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3

• Giant piezoelectricity &

• high temperature field stability

2. All sputtering process

PMN-PZT heterostructure on SOI

-40 -20 0 20 40

-40

-20

0

20

40

Pola

rization (C

/cm

2)

Drive Voltage (V)

20 30 40 50 60 7010

0

101

102

103

104

105

106

107

Inte

nsity (

cps)

2

YSZ (

002)

YSZ (

004)

Si (004)

CeO

2 (

002)

CeO

2 (

004)

LSM

O (

001)

LSM

O (

002)

PM

N-P

ZT (

001)

PM

N-P

ZT (

002)

XRD P – E Curve

1 μm PMN-PZT/LSMO/CeO2/YSZ/Si

Summary • We have fabricated epitaxial PMN-PT piezoelectric heterostructures on silicon with unparalleled piezoelectric properties. (d33: 1200 pm/V and e31: -29 C/m

2) (2-3 times better than best piezoelectric films ever reported and

on Si !)

• High strain piezoelectric heterostructures on silicon can be used

for integrated MEMS devices for actuation, sensing and imaging • High frequency ultrasound transducer 2-D arrays • Hyper-Active NEMS • High frequency filters • Energy Harvesting

Buffer layer for epitaxial Oxide on Si

Si 5.43 Å

CeO2 5.41 Å

YSZ 5.15 Å

SRO, LSMO, PZT 3.86~3.93 Å

Si substrate

Perovskite oxides

Buffer Layers

Doppler effect for blood flow imaging

Principle of ultrasound imaging

(1960)

(2003) Ceramic PZT transducer array At 1 MHz

20x20 first 2-D array (currently 256x256 = 65,536 subdiced elements)

S.W. Smith, Duke Univ.

GE Medical