This document discusses using lead magnesium niobate-lead titanate (PMN-PT) for piezoelectric MEMS due to its giant piezoelectric properties. PMN-PT thin films were grown epitaxially on silicon substrates using a strontium titanate buffer layer. This resulted in atomically sharp interfaces and preserved the material's giant piezoelectric coefficients after microfabrication into cantilevers. Exploiting the high piezoelectric response of relaxor ferroelectrics like PMN-PT could enable smaller, more sensitive MEMS devices with lower power consumption.

1. Piezoelectric MEMS with Giant
Piezo Actuation

2. MEMS on Si – 1. Capacitive with metallic electrodes
2. Piezoelectric with active piezo-materials (like PZT)
 MEMS with piezoelectric materials - Integrated Actuation
‘’ Sensing
‘’ Transduction
 Applications - Ultrasound Medical Imaging
Microfludic Control
Mechanical Sensing
Energy Harvesting

3. Piezoelectricity
 “Piezo” – Pressure; Piezo-electricity - pressure electricity
 Direct and Converse piezoelectric effect
Important Piezoelectric Parameters
Piezoelectric 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)

4. Displacement and Stress/Strain relation (at low fields)
Clamping to the Substrate changes it all !!
31 and 33 modes of piezoresponse

5. Piezoelectric Properties of representative materials

6. MEMS Based on PbZr(1-x)Tix03
Interesting Properties at MPB –
2.High Dielectric Susceptibility
3.High Remnant Polarization
4.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.

7. MEMS Based on PZT
Earlier Works Appl. Phys. Lett., Vol. 74, No. 23, 7

8. Appl. Phys. Lett. 68 (10)

9. Appl. Phys. Lett. 63 (26),
LSCO
PZT
LSCO
YSZ
SiO2
(001) Si

10. Appl. Phys. Lett. 63 , 189

11. Appl. Phys. Lett., Vol. 76, No. 11, 13
Electron Diffraction Patterns
PZT/YBCO YBCO/MgO STO/MgO TiN/Si

13. J. Micromech. Microeng. 20 (2010) 055008
Process Flow for MEMS Micro-fabrication
Hysteresis Loop

14. PZT Membrane PZT Cantilever

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

17. PMN:25%PT
PMN:33%PT
PYN:46%PT
PZT

18. Giant Piezoelectricity for Hyperactive MEMS
Material: PMN:33%PT
Orientation : (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

19. HRTEM at the Interface
 Atomically sharp interfaces.
Dielectric and Ferroelectric Measurement
 Existence of a built-in-bias – advantages and drawbacks.

20. 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

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

22. Cantilever deflection with external bias

23. 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.