Piezoelectric Micromachined Ultrasound Transducers (pMUTs)

Muhammet İpekçi505612003

Electrical Electronics Engineering

Piezoelectric effect

The piezoelectric effect describes the ability of materials to develop electric displacement as a result of an applied mechanical stressThe crystal expands and contracts with a returning sound wave causing an electrical voltage to be emittedReturning sound wave are converted into electrical signals

Inverse Piezoelectric Effect

The property of certain crystals to expand or strain when positive or negative electrical current is applied

Voltage applied to opposite sides of the crystal cause it to expand; polarity is reversed (AC current) causing the crystal to strain

Constant change from expansion to strain, strain to expansion, results in mechanical waves (sound) being produced

Thus, the electrical signal is converted into a sound wave

Piezoelectric sound theory Piezoelectric ceramic buzzer simple structure in which piezoceramic element is sticked to vibration

plate When alternating voltage is applied to piezoceramic element, the

element expands or shrinks diametrically This characteristic is utilized to make vibration plated bend to

generate sounds.

Ultrasound Ultrasound is an oscillating sound pressure wave with a

frequency greater than the upper limit of the human hearing range.

Human hearing range 20-20.000 Hertz

Ultrasound devices frequencies from 20 kHz up to several gigahertz

UltrasoundPrinciple of an active sonar

Ultrasound image of a fetus

What is pMUT ?

Micromachined ultrasound transducers have allowed feasibility for mobile applications of ultrasound devices

imaging range-finding or other

through a decrease in volume, weight, and power consumption. Technological developments for integrated circuit fabrication have allowed further miniaturization and fabrication of 2D and 3D arrays.

pMUTs Structure

Among the available ferroelectric materials

PZT lead zirconate titanate,Pb(ZrxTi1−x)O3 is the most popular due to; its superior dielectric constant,

piezoelectric constants,

thermal stability.

pMUTs StructurePiezoceramic thick films based on lead zirconate titanate (PZT) are of great interest for cost-effective fabrication of integrated sensors and actuators for MEMS (Micro Electro Mechanical Systems) and high frequency ultrasonic transducers.

pMUTs Design

A detailed design of pMUT showing various layers on top of the Si membrane.

pMUTs Design

Each element consists of a silicon membrane, an active PZT film

The SiO2 layer, on top of the silicon membrane

Ti/Pt electrode to the wafer surface at the bottom

Ti/Pt layer is added on top of the SiO2 as a bottom electrode

PZT, in optimized multiple layers, is then spin-coated on the bottom electrode

Finally, a top gold electrode having a predetermined pattern, is deposited on the PZT film and the film poled in the thickness direction

Fabrication of pMUTs

Schematic flow chart of silicon membrane fabrication.

pMUT fabrication involves building a silicon membrane with electroded PZT layers on top

Silicon wafers (p-type 1 0 0, 395–405 m) were wet oxidized at 1050 ◦C to grow a 500 nm thick oxide

The oxide layer was removed from one side of the wafer using a buffered oxide etch (BOE).

Borosilicate glass that forms on the surface 1125 ◦C for 1 h.

Standard photolithography techniques were used to create an oxide mask on the backside of the wafer

The wafers were then etched with the anisotropic silicon etchant ethylenediamine pyrocatechol (EDP)

Fabrication of pMUTs

Schematic flow chart for the fabrication of the PZT-driven membrane from a micromachined substrate.

PZT thin films are then deposited via spin coating of the PZT sol.

Top electrodes were deposited by sputtering 10 nm of TiW and 200 nm of Au.

These films were then patterned using standard photolithography techniques to create a top electrode with leads off the membrane

The PZT film was also patterned to expose the bottom electrode using a HCl:HF:H2O etchant.

Fabrication of pMUTs

Cross-sectional secondary electron beam microscopy picture of 2-μm-thick PZT 52/48 thin film

The micromachined bridge of a suspendedmembrane with the etched Pt/PZT/Pt sandwich

Performance of pMUTs

Schematic of pMUT flexure with associated representations of input sine wave signal, ferroelectric hysteresis loop (indicating domain switching), and mechanical displacement as a function of input voltage. Points A and A’ refer to 0 V applied, points B and D refer to the coercive voltage, and points C and E refer to maximum applied voltage.

Performance parameters

The frequency at which the transducer is the most efficient as a transmitter of sound is also the frequency at which it is most sensitive as a receiver of sound. This frequency is called the natural or resonant frequency of the transducer.

The range of frequencies in the emitted ultrasound wave is called the bandwidth and is defined to be the full width of the frequency distribution at half maximum (FWHM).

Performance parameters

The resonance frequency of the device is directly determined by analyzing its time response under free vibration after a pulse excitation has been applied, while the bandwidth is estimated from the frequency response of the normal velocity of a central point on the membrane.

The resonance frequency of the transducer is governed by the thickness of the PZT. The fundamental resonance mode exists when the thickness of the PZT is equal to half the wavelength.

Performance parameters

Membrane width is an important design parameter as it strongly affects the membrane stiffness and, hence, the device resonance frequency, acoustic impedance, bandwidth, and coupling coefficient

Performance parameters

Performance parameters

Performance parameters

thicker crystal – lower frequency

thinner crystal – higher frequency

crystal thickness = ½ for the frequency

higher propagation speed – higher frequency

slower propagation speed – lower frequency

Typical propagation speeds of 4-6 mm/sFrequency (MHz) = crystal’s propagation speed (mm/s)

2 x thickness (mm)

5x5 2D pMUT array in air

Surface displacement mode shapes of a 200μm pMUT element in air atshowing different modes of operation.

5x5 2D pMUT array in Water

Surface displacement mode shapes of a 200μm pMUT element in water atshowing different modes of operation.

Applications of pMUTs

Medical applications For medical imaging purposes,

the ultrasound transducers would be included on a probe tip.

A device would be required to have a high frequency to insure clear images of such subject matter as veins and small tumors.

Applications of pMUTs

Criminal applications A second possible use for the device is for biometric

fingerprint identification. A micromachined ultrasound transducer could supply a small,

portable, and highly accurate fingerprint scanner that can not only image dermal, but also subdermal layers of the finger

F . Akasheh, T. Myers, J. D. Fraser, S. Bose, and A. Bandyopadhyay, “Development of piezoelectric micromachined ultrasonic transducers,” Sens. Actuators A, vol. 111, pp. 275–287, 2004.

P. Muralt, N. Ledermann, J. Baborowski, A. Barzegar, S. Gentil, B.Belgacem, S. Petitgrand, A. Bosseboeuf, and N. Seter, “Piezoelectric micromachined ultrasonic transducers based on PZT thin films”

David E. Dausch, Senior Member, IEEE, John B. Castellucci, Derrick R. Chou, Student Member, IEEE, and Olaf T. von Ramm “Theory and Operation of 2-D Array Piezoelectric Micromachined Ultrasound Transducers“

Piezoelectric Micromachined Ultrasound Transducers for Medical Imaging by Derrick R. Chou

References

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