Presentation on theme: "Piezoelectric Micromachined Ultrasound Transducers (pMUTs)"— Presentation transcript:
1Piezoelectric Micromachined Ultrasound Transducers (pMUTs) Muhammet İpekçiElectrical Electronics Engineering
2Piezoelectric effectThe 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
3Inverse Piezoelectric Effect The property of certain crystals to expand or strain when positive or negative electrical current is appliedVoltage applied to opposite sides of the crystal cause it to expand; polarity is reversed (AC current) causing the crystal to strainConstant change from expansion to strain, strain to expansion, results in mechanical waves (sound) being producedThus, the electrical signal is converted into a sound wave
4Piezoelectric sound theory Piezoelectric ceramic buzzersimple structure in which piezoceramic element is sticked to vibration plateWhen alternating voltage is applied to piezoceramic element, the element expands or shrinks diametricallyThis characteristic is utilized to make vibration plated bend to generate sounds.
5UltrasoundUltrasound is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range.Human hearing range HertzUltrasound devices frequencies from 20 kHz up to several gigahertz
6UltrasoundPrinciple of an active sonarUltrasound image of a fetus
7What is pMUT ?Micromachined ultrasound transducers have allowed feasibility for mobile applications of ultrasound devicesimagingrange-finding or otherthrough 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.
8pMUTs 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.
9pMUTs 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.
10pMUTs DesignA detailed design of pMUT showing various layers on top of the Si membrane.
11pMUTs DesignEach element consists of a silicon membrane, an active PZT filmThe SiO2 layer, on top of the silicon membraneTi/Pt electrode to the wafer surface at the bottomTi/Pt layer is added on top of the SiO2 as a bottom electrodePZT, in optimized multiple layers, is then spin-coated on the bottom electrodeFinally, a top gold electrode having a predetermined pattern, is deposited on the PZT film and the film poled in the thickness direction
12Fabrication of pMUTspMUT fabrication involves building a silicon membrane with electroded PZT layers on topSilicon wafers (p-type 1 0 0, 395–405 m) were wet oxidized at 1050 ◦C to grow a 500 nm thick oxideThe 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 waferThe wafers were then etched with the anisotropic silicon etchant ethylenediamine pyrocatechol (EDP)Schematic flow chart of silicon membrane fabrication.
13Fabrication of pMUTsPZT 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 membraneThe PZT film was also patterned to expose the bottom electrode using a HCl:HF:H2O etchant.Schematic flow chart for the fabrication of the PZT-driven membrane from a micromachined substrate.
14Fabrication of pMUTsCross-sectional secondary electron beam microscopy picture of 2-μm-thick PZT 52/48 thin filmThe micromachined bridge of a suspendedmembrane with the etched Pt/PZT/Pt sandwich
15Performance of pMUTsSchematic 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.
16Performance 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).
17Performance 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.
18Performance 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
21Performance parameters thicker crystal – lower frequencythinner crystal – higher frequencycrystal thickness = ½ for the frequencyhigher propagation speed – higher frequencyslower propagation speed – lower frequencyTypical propagation speeds of 4-6 mm/sFrequency (MHz) = crystal’s propagation speed (mm/s)2 x thickness (mm)
225x5 2D pMUT array in airSurface displacement mode shapes of a 200μm pMUT element in air atshowing different modes of operation.
235x5 2D pMUT array in WaterSurface displacement mode shapes of a 200μm pMUT element in water atshowing different modes of operation.
24Applications 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.
25Applications 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
26ReferencesF . 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