Piezoelectric Sensors and Actuators

Piezoelectric Sensors and Actuators

Outline Piezoelectricity Origin Polarization and depolarization Mathematical expression of piezoelectricity Piezoelectric coefficient matrix Cantilever piezoelectric actuator analysis Typical piezoelectric materials Quartz, PZT, PVDF, ZnO Examples of mechanical analysis

Piezoelectricity Piezoelectricity Direct effect: Stress causes the generation of charge or voltage. Inverse effect: Applied voltage (electrical field) causes stress (stain). Strong field (1000V/m) v.s. very small strain (10-7 ) Crystallities Piezoelectric materials consist of crystallities with unbalanced positive and negative charge centers. Poling (polarization) Using strong electrical field and elevated temperature to align crystallities to the direction of the electrical field. Piezoelectric material would exhibit enhanced piezoelectric effect in the poling direction. E

Depolarization E Depolarization mechanism Electrical field A strong field opposite to the polarization direction Mechanical Vibration and shock Thermal A temperature higher than Curie point Aging The piezoelectric property of piezoelectric materials changes (degrades) with time and repeated use. The crystallities gradually become randomly aligned.

Origin of Piezoelectricity + E E Crystal materials Centrosymmetric: without piezoelectric effect NonCentrosymmetric: with piezoelectric effect Origin of Piezoelectricity Direct effect The stress/strain causes unbalanced charge distribution at the surface. Tensile and compressive stress generate opposite electrical fields. Inverse effect Strong electrical field causes the stretch or compression of the crystallities. Opposite electrical field directions generate either elongation or compression - - +

Direct effect D = [d]t + [ε]e Mathematical Expression of Piezoelectricity z (3) Direct effect Polarization due to electric field D j electrical displacement (polarization) (C/m 2 ) d ij piezoelectric constant (C/N) T j normal/shear stress component (N/m 2 ) ε ij permitivity (Farad/m) E j electric field (V/m)

Inverse effect S = [s]t + [d]e Mathematical Expression of Piezoelectricity Hook s law Inverse effect S j Strain (no unit) d ij piezoelectric constant (C/N) d T j normal/shear stress component (F/m 2 ij E j : (C/N)(V/m) = ) (CV)/(Nm)=1 E j electric field (V/m)

Typical Piezoelectric Material Piezoelectric material for MEMS Thin film Easy to deposit and patterning Reasonable piezoelectric constant, especially d 31 ZnO PZT

Quartz Material properties Natural piezoelectric material No need for poling Excellent thermal stability Used in bulk form for resonators 10k~200MHz Not suitable for MEMS Low piezoelectric constant Hard to form thin-film

PZT Material properties A family of piezoelectric materials Different composition and different piezoelectric property Thin film material can be deposited and patterned using a number of methods. High dielectric constant High piezoelectric coefficient (d 33 )after poling, suitable for making acoustic sensors Pb(Zr 0.40,Ti 0.60 )O 3 3 Pb(Zr 0.52,Ti 0.48 )O 3 S 3 = d 33 E 3 (Transmitting) D 3 = d 33 T 3 (Receiving)

ZnO Material properties Thin film material can be deposited using sputtering Film might have high internal stress C-axis(z) almost perpendicular to substrate surface No poling required Can be easily attacked by many etchants, increasing the difficulty of patterning 3

Cantilever Piezoelectric Actuator Model (1 st order estimation) Metal E 3 S 1 = d 31 E 3 Actuation force: F = kδ(x)

Examples of Piezoelectric Sensor/Actuator Based on ZnO Sensor Input: external force F Output: voltage across the two electrodes φ r Actuator Input: voltage across the two electrodes Output: Deflection of the beam l A l B 31 φ δ

Examples of Piezoelectric Sensor/Actuator Based on ZnO Sensor T 5 = F/(wt) (shear stress) Actuator No Strain occurs in 1 and 3. No motion.

Part II Piezoelectric accelerometers Piezoelectric acoustic sensors Microphone and hydrophone Surface acoustic wave (SAW) devices Piezoelectric tactile sensor

Piezoelectric Accelerometer Using ZnO Piezoelectric layer Structural layer Sacrificial etching of SiO 2 Sensitivity: 0.95fC/g Resonant frequency: 3.3kHz Cantilever must bend up to function g = 9.8m/sec 2 Bulk sacrificial etching of Si Sensitivity: 13.3fC/g or 46.7fC/g Resonant frequency: 2.23kHz 0r 1.02kHz Reference 6-7: Devoe, et al, JMEMS, 1997

Piezoelectric Accelerometer Using PZT Structural layer Reference 6-6: Wang, et al, JMEMS, 2003 Circular ring spring structure design Reduce lateral movement and cross sensitivity Increased stiffness Use PZT (higher piezoelectric coefficient) Sensitivity: 0.77~7.76pC/g, f R : 35.3~37kHz Bulk etching from the backside Use observation window to control the thickness of proof mass

Piezoelectric Acoustic Sensor and Generator Acoustic wave Electrical voltage Acoustic wave Electrical voltage Acoustic sensors and generators are the major applications for piezoelectric devices both at macro and micro scale. Efficient and direct energy conversion Transmitting and receiving can be done in one device Compact design and configuration Suitable frequency range Single-chip integration Examples Microphones: 0~20kHz Hydrophones: ~MHz (used in water. Higher frequency provides higher resolution) Surface Acoustic Wave (SAW) or flexural plate wave (Lamb wave) Devices Reference 6-3: Bernstein, et al, IEEE-THFST, 1997

Piezoelectric Acoustic Sensor Electrical voltage Packaged 6 6 array for underwater imaging Acoustic wave Reference 6-3: Bernstein, et al, IEEE-THFST, 1997 Membrane size: 0.2~2mm Piezoelectric material PZT d 33 (140~240pC/N) ε r :1400.

Piezoelectric Acoustic Sensor Ti/Pt P++ doping Ti/Pt PZT Reference 6-3: Bernstein, et al, IEEE-THFST, 1997

Piezoelectric Microphone/Speaker Cantilever type diaphragm Lower force constant Higher sensitivity Lower resonant frequency (890Hz) Material system ZnO as the piezoelectric material Polysilicon as bottom electrode Aluminum as top electrode SiN as cantilever membrane material Reference 6-9: Lee, et al, JMEMS, 1996

Piezoelectric Tactile Sensor Reference 6-31: Kolesar, et al, JMEMS, 1995 Use an entire piece of PVDF film as the piezoelectric material. PVDF is difficult to etch and pattern. Both top and bottom electrodes are patterned into array of pixels. The electrodes are directly connected to amplifier circuit. Adjacent taxels should be separated from each other by 300µm to reduce coupling.

Piezoelectric Deformable Mirror Utilize large d 33 of PZT Electrodes are used for both poling and actuation Reference 6-10: Mescher, et al, IEEE MEMS Conference, 2002

Surface Acoustic Wave (SAW) Device Transmitter Receiver Sensing region Transmitter Receiver Sensing region SAW wave and Lamb wave can be launched and received by two sets of interdigitated finger electrodes on piezoelectric material. The propagation characteristics (amplitude and frequency) are affected by the material properties along the propagation path.

Surface Acoustic Wave (SAW) Device Transmitting Receiving