Piezoelectric MEMS with Giant Piezo Actuation

MEMS with piezoelectric materials - Integrated Actuation ‘’ Sensing ‘’ Transduction

MEMS on Si – 1. Capacitive with metallic electrodes 2. Piezoelectric with active piezo-materials (like PZT)

Applications - Ultrasound Medical Imaging Microfludic Control Mechanical Sensing Energy Harvesting

Piezoelectricity “Piezo” – Pressure; Piezo-electricity - pressure electricity

Direct and Converse piezoelectric effect

Important Piezoelectric ParametersPiezoelectric Figure of Merit

Piezoelectric Strain Constant (d) – Magnitude of the induced strain (x) by an external electric field. x= d.E

Piezoelectric Voltage constant (g) – field per unit strain. g =d/(εε0) ; ε= permittivity

Electromechanical Coupling Factor (k) – Conversion rate between elec. & mech. energy. = (stored elec. Energy)/(Input mech. Energy) = d2/ (εε0).s

Energy Transmission Coefficient (λ) – Maximum k in actual device = ʃ E.dp = (εε0E + d.x)E

Efficiency (Ƞ) - (output mech. Energy)/(Consumed elec. Energy)

Displacement and Stress/Strain relation (at low fields)

Clamping to the Substrate changes it all !!

31 and 33 modes of piezoresponse

Piezoelectric Properties of representative materials

MEMS Based on PbZr(1-x)Tix03

Interesting Properties at MPB –1.High Dielectric Susceptibility2.High Remnant Polarization3.High Piezoelectric Coefficient

Pertinent issues encountered while integrating on Si –1. Suitable buffer layer owing to higher lattice mismatch.2. Suitable bottom electrode maintaining epitaxial nature .3. Suitable growth condition leading to defect-minimal interface.

MEMS Based on PZT

Earlier Works Appl. Phys. Lett., Vol. 74, No. 23, 7

Appl. Phys. Lett. 68 (10)

Appl. Phys. Lett. 63 (26),

(001) Si

SiO2

YSZLSCO

PZT

LSCO

Appl. Phys. Lett. 63 , 189

Appl. Phys. Lett., Vol. 76, No. 11, 13

Electron Diffraction Patterns

TiN/SiSTO/MgOYBCO/MgOPZT/YBCO

J. Micromech. Microeng. 20 (2010) 055008

Hys

tere

sis

Loop

Process Flow for MEMS Micro-fabricationProcess Flow for MEMS Micro-fabrication

PZT

Canti

leve

rPZ

T M

embr

ane

Problem with Giant Piezo MEMS – Relaxor materials are difficult to integrate in Si matrix for device fabrication

MEMS with Giant Piezo Coefficients

PMN:25%PTPMN:33%PTPYN:46%PT

PZT

Giant Piezoelectricity for Hyperactive MEMS

Material: PMN:33%PTOrientation : (001) {according to S. E. Park, T. R. Shrout, J. Appl. Phys. 82, 1804 (1997)}

Problem : Growth of Pyrochlore Phase.

Approaches : 1) The use of SrTiO3 buffer layer. 2) a high miscut in Si substrate. (To incormorate volatile components in the film like PbO suppressing formation of pyrochlore)

Zero Miscut 4o miscut

Atomically sharp interfaces.

Existence of a built-in-bias – advantages and drawbacks.

HRTEM at the Interface

Dielectric and Ferroelectric Measurement

Piezoresponse

Possible reason of higher piezo-activity- 1. Substantial self-polarization 2. Built in bias

Highest e31,f measured after poling = -27 +/- 3 c/m2

How far the properties hold w.r.t. microfabrication ?

Fabrication

Cantilever deflection with external bias

Conclusions Epitaxial growth of PMN-PT on (001)Si using STO buffer. Improved growth through introduction of a high miscut in Si. Manifestation of giant piezoelectric properties. Higher figure of merit suitable for device integration. Preserved properties after microfabrocation.

Coda and Future Challenges High piezo-actuation through use of relaxors may enhance device sensitivity Denser device integration in IC – actuator arrays through easing downscaling. Low power consumption owing to reduction in actuation charge density. Smaller electromechanical devices with better performances.

Exploitation of higher 33 mode response of PMN-PT rather 31 mode. Tuning the elastic properties of passive layers (SiO2, electrode, STO)to

enhance in figure of merit further. Using SOI for complex device structures with desired passive layer thickness. Beyond EMS devices – tune and modulate multifunctional properties with

giant electrostriction and dynamic strain control.