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Development of high frequency focused transducers for single beam acoustic tweezers
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Development of high frequency focused transducers for single beam acoustic tweezers
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DEVELOPMENT OF HIGH FREQUENCY FOCUSED TRANSDUCERS FOR SINGLE BEAM ACOUSTIC TWEEZERS by Hsiu-Sheng Hsu A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (MATERIALS SCIENCE) December 2012 Copyright 2012 Hsiu-Sheng Hsu ii Dedication To My parents, Tu-Wan Hsu and Hsiu-Wan Lu iii Acknowledgements I would like to express my sincere gratitude to my advisor, Dr. K. Kirk Shung and Dr. Qifa Zhou, for giving me a great opportunity to work in the NIH Ultrasonic Transducer Resource Center at USC. I deeply appreciate their supervision, trust and support during the entire time of my research. This work would not have been accomplished without their kindly instruction. I would also like to thank Dr. Edward Goo, Dr. Steven R. Nutt and Dr. Jesse Yen for their agreement to serve as my dissertation committee and taking the time and effort to evaluate my work. I am indebted to all the current and former members in our center, especially Dr. Jonathan Cannata, Dr. Fan Zheng, Dr. Ying Li, Dr. Kwok Ho Lam, Dr. Xiang Li, Dr. Jae Youn Hwang, Dr. Hao-Chung Yang, Dr. Hojong Choi, Dr. Sien-Ting Lau, Dr. Hyung Ham Kim, Ruimin Chen, Jay Williams, Changyang Lee, Chi Tat Chiu, Teng Ma, Xiabing Zhang, Thomas Cummins and Vatcharee Benjauthrit for their help with this work. I also sincerely thank all of my friends at USC for their friendship, encouragement and assistance during my graduate studies. Among them, Dr. Changgeng Liu, Dr. Shih-Jui Chen, Dr. Po-Chiang Chen, Dr. Ting-Wei Yeh, Dr. Anderson Lin, Dr. Po-Ying Li, Haitian Chen, Arash Vafanejad, Yu-Lun Shen deserve special thanks. Finally, I would like to thank my beloved family and family-in-law for their love, patience, support and encouragement throughout these years. Last but not least, I want to express my heartfelt thanks to my wife, Yu-Lin Hsueh for her everlasting love and unstinting support. I also wish to thank my son, Jason. Your presence in my life is a source of joy and inspiration. Thank you for all your patience and support. Love you all. iv Table of Contents Dedication ........................................................................................................................... ii Acknowledgements ............................................................................................................ iii List of Tables .................................................................................................................... vii List of Figures .................................................................................................................. viii Abstract .............................................................................................................................. xi Chapter 1 Introduction ........................................................................................................ 1 1.1 In general .......................................................................................................... 1 1.2 Acoustic tweezers ............................................................................................. 3 1.3 Fundamental physics of single beam acoustic tweezers ................................... 4 1.4 Ultrasonic transducers ....................................................................................... 7 1.5 Outline of this dissertation .............................................................................. 14 Chapter 2 Press-focused High Frequency Needle Transducer ......................................... 15 2.1 Introduction ..................................................................................................... 15 2.2 Materials and methods .................................................................................... 16 2.3 Transducer evaluation ..................................................................................... 21 2.4 In virto ultrasonic biomicroscopy imaging ..................................................... 29 2.5 Microparticles manipulation ........................................................................... 32 2.6 Conclusion ...................................................................................................... 39 Chapter 3 Self-focused Ultrahigh Frequency ZnO Transducer ........................................ 40 3.1 Introduction ..................................................................................................... 40 3.2 Materials and method ...................................................................................... 42 v 3.3 Transducer evaluation ..................................................................................... 47 3.4 Microparticles manipulation ........................................................................... 50 3.5 Conclusion ...................................................................................................... 53 Chapter 4 Silicon Lens Ultrahigh Frequency Transducer ................................................. 54 4.1 Introduction ..................................................................................................... 54 4.2 Materials and methods .................................................................................... 55 4.2.1 Silicon lens ....................................................................................... 55 4.2.2 Silicon lens transducer ..................................................................... 61 4.3 Transducer evaluation ..................................................................................... 65 4.4 Microparticles manipulation ........................................................................... 67 4.5 Conclusion ...................................................................................................... 70 Chapter 5 PMN-PT-PZT Composite Films for High Frequency Transducer Applications ........................................................................................................................................... 71 5.1 Introduction ..................................................................................................... 71 5.2 Experiment ...................................................................................................... 74 5.2.1 Preparation and characterization of xPMN-PT-(1-x)PZT composite films .......................................................................................................... 74 5.2.2 Fabrication and evaluation of transducer ......................................... 76 5.3 Results and discussion .................................................................................... 78 5.4 Conclusion ...................................................................................................... 85 Chapter 6 Summary and Future Work .............................................................................. 86 6.1 Summary ......................................................................................................... 86 6.2 Future work ..................................................................................................... 88 6.2.1 High frequency focused transducers ................................................ 88 vi 6.2.2 Single beam acoustic tweezers ......................................................... 89 Bibliography ..................................................................................................................... 91 vii List of Tables Table 2.1: Material properties of PMN-PT single crystal. ................................................ 17 Table 2.2: Passive material properties used in the transducer designs. ............................ 18 Table 4.1: Effect of etching recipes on etching rate and surface roughness of etched cavity. ................................................................................................................................ 58 Table 4.2: Effect of coaxial cable length on the measurements of the pulse-echo response. ........................................................................................................................................... 67 Table 5.1: Measured properties and performance of composite sol-gel films very high frequency transducers. ...................................................................................................... 85 viii List of Figures Figure 1.1: Schematic diagram of single beam acoustic trapping model. An ultrasonic transducer having Gaussian intensity profile produces two rays (denoted as P a and P b ) on the sphere with the corresponding exiting rays of P a ’ and P b ’. The momentum transfers from those rays are defined by dP a and dP b , which determines the rate of the momentum change and resultant forces on the sphere. Acoustic radiation force is induced based on Newton’s third law and leads to the trapping where a red thick arrow indicates the trapping direction. (Lee, J. et al., 2011) .............................................................................. 6 Figure 1.2: (a) Direct piezoelectric effect in which a stress induces a charge separation. (b) Reverse piezoelectric effect in which a potential difference across the electrodes induces a strain. (Shung, 2006) ........................................................................................................ 8 Figure 1.3: Comparison of non-focused and focused acoustic beams from the ultrasound transducer. (Shung et al., 1992) ........................................................................................ 11 Figure 1.4: A cross-sectional drawing of the typical single element focused ultrasound transducers.(Shung, 2006) ................................................................................................ 13 Figure 2.1: A schematic fabrication flow of press-focused PMN-PT needle transducer. 19 Figure 2.2: SEM images of a 0.4x0.4 mm 2 post for press-focused PMN-PT needle transducer fabrication. ....................................................................................................... 20 Figure 2.3: A photograph of the completed press-focused high frequency PMN-PT needle transducer. ......................................................................................................................... 21 Figure 2.4: Block diagram of a pulse-echo test set-up for ultrasonic transducers. ........... 24 Figure 2.5: Experimental arrangement for the measurement of lateral beam profile for ultrasonic transducers........................................................................................................ 25 Figure 2.6: Measured electrical impedance of press-focused PMN-PT needle transducer. ........................................................................................................................................... 27 Figure 2.7: Measured pulse-echo waveform (solid line) and normalized frequency spectrum (dashed line) of press-focused PMN-PT needle transducer. ............................. 28 Figure 2.8: Measured lateral beam profile of press-focused PMN-PT high frequency needle transducer. ............................................................................................................. 29 Figure 2.9: A diagram of the UBM set-up system. ........................................................... 30 Figure 2.10: In vitro UBM image of the anterior portion of a rabbit eye. ........................ 31 ix Figure 2.11: Block diagram of single beam acoustic trapping experiment using high frequency focused transducer. .......................................................................................... 33 Figure 2.12: Text procedure of maximum displacement measurement for an individual targeted particle with single focused acoustic beam. (Lee, C. et al., 2010) ...................... 35 Figure 2.13: Measured maximum displacements as a function of frequency with different excitation voltages. ........................................................................................................... 36 Figure 2.14: Example of microparticle manipulation in two dimensions using the press- focused needle transducer. A single 15 µm polystyrene microsphere was manipulated along the movement of the transducer. A trapped microparticle is present inside a blue circle while a red dot is given as a reference point to show the location change of the microsphere. ...................................................................................................................... 38 Figure 3.1: Design configuration for the ultrahigh frequency self-focused ZnO transducer (not to scale). ..................................................................................................................... 44 Figure 3.2: A photograph of the spherically polished end of an aluminum rod with the sputtered ZnO layer on the top surface. ............................................................................ 45 Figure 3.3: SEM images of the sputtered ZnO film on the curveted aluminum (Al) backing (a) and a close-up view of the ZnO film (b). ....................................................... 46 Figure 3.4: A photograph of the fabricated ultrahigh frequency self-focused ZnO transducer. ......................................................................................................................... 47 Figure 3.5: Measured pulse/echo response (solid line) and normalized frequency spectrum (dashed line) for ultrahigh frequency self-focused ZnO transducer. ................ 49 Figure 3.6: Measured lateral beam profile of the ultrahigh frequency self-focused ZnO transducer. ......................................................................................................................... 50 Figure 3.7: A single 10 µm polystyrene microsphere was manipulated effectively using the self-focused ZnO ultrahigh frequency transducer. A red dot is given as a reference point to show the change in the particle location, and the trapped particle is present inside a blue circle. ...................................................................................................................... 52 Figure 4.1: Process steps used to fabricate the silicon lens. ............................................. 57 Figure 4.2: Effect of etching parameters (Table 4.1) on surface roughness of etched cavity: (a) Recipe A, (b) Recipe B, (c) Recipe C, (d) Recipe D and (e) Recipe E. .......... 59 Figure 4.3: SEM images of the etched cavities for two different mask opening size: (a) cavity diameter of 110 µm with depth of 50 µm via a mask opening diameter of 60 µm; (b) cavity diameter of 640 µm with depth of 140 µm via a mask opening diameter of 400 µm. .................................................................................................................................... 60 x Figure 4.4: Schematic fabrication flow of silicon lens ultrahigh frequency transducer. .. 63 Figure 4.5: Produced silicon etched lenses on the front of the silicon wafer (top picture), and backside of the ZnO layer with Cr/Au electrodes (bottom picture) after dicing. ...... 64 Figure 4.6: A photograph of the finished ultrahigh frequency silicon lens transducer. .... 65 Figure 4.7: Measured pulse-echo waveform (solid line) and normalized frequency spectrum (dashed line) for ultrahigh frequency silicon lens transducer. .......................... 66 Figure 4.8: A trapped 5 µm polystyrene microsphere motion is handled by the silicon lens ultrahigh frequency transducer. The trapped particle is present inside a blue circle while a red dot is given as a reference point to show the change in the particle location along with the movement of the transducer. ....................................................................................... 69 Figure 5.1: Fabrication procedure of the xPMN-PT-(1-x)PZT composite films using composite sol-gel method. ................................................................................................ 75 Figure 5.2: Fabrication process for the PMN-PT-PZT composite film transducer. Step1: A Cr/Au electrode was sputtered on the composite film and E-solder 3022 was cast on the electrode side as the backing. Step 2: The sample was diced into small posts and the substrate was removed by dipping in KOH solution at 80°C. Step 3: A lead wire was connected to the backing layer with conductive epoxy. Placing a brass housing concentrically with the element and then filling the insulating epoxy between the housing and the element. Step 4: Sputtering a Cr/Au electrode across the transducer face to form the ground plane connection. A vapor-deposited parylene thin layer was used as the matching layer. Finally, the transducer was housed in the SMA connector. ................... 77 Figure 5.3: XRD patterns of xPMN-PT-(1 −x)PZT composite films while x is (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7 and (e) 0.9. ......................................................................................... 79 Figure 5.4: SEM cross-sectional micrograph of the PMN-PT/PZT composite film. ...... 80 Figure 5.5: Frequency dependence of dielectric constants of xPMN-PT-(1-x)PZT composite films where x is 0.1, 0.3, 0.5, 0.7 and 0.9. ....................................................... 81 Figure 5.6: Polarization-electric field hysteresis loops of xPMN-PT-(1-x)PZT composite films where x is 0.1, 0.3, 0.5, 0.7 and 0.9. The insert: The remnant polarization (Pr) as a function of compositions................................................................................................... 82 Figure 5.7: A photograph of the 0.9PMN-PT-0.1PZT composite film transducer. .......... 83 Figure 5.8: Measured pulse-echo waveform (solid line) and normalized spectrum (dashed line) for 0.9PMN-PT-0.1PZT composite film transducer. ................................................ 84 xi Abstract Contactless particle trapping and manipulation have found many potential applications in diverse fields, especially in biological and medical research. Among the various methods, optical tweezers is the most well-known and extensively investigated technique. However, there are some limitations for particle manipulation based on optical tweezers. Due to the conceptual similarity with the optical tweezers and recent advances in high frequency ultrasonic transducer, a single beam acoustic tweezer using high frequency ( ≥ 20 MHz) focused transducer has recently been considered, and its feasibility was theoretically and experimentally investigated. This dissertation mainly describes the development of high frequency focused ultrasonic transducers for single beam acoustic tweezers applications. Three different types of transducers were fabricated. First, a 60 MHz miniature focused transducer (<1 mm) was made using press-focusing technique. The single beam acoustic trapping experiment was performed to manipulate 15 μm polystyrene microspheres using this transducer. In vitro ultrasonic biomicroscopy imaging on the rabbit eye was also obtained with this device. Second approach is to build a 200 MHz self-focused ZnO transducer by sputtering ZnO film on a curved surface of the aluminum backing material. An individual 10 μm microsphere was effectively manipulated in two dimensions by this type of transducer. Another ultrahigh frequency focused transducer based on silicon lens design has also been developed, where a 330 MHz silicon lens transducer was fabricated and evaluated. Microparticle trapping experiment was carried out to demonstrate that silicon lens transducer can manipulate a single microsphere as small as 5 µm. The realization of xii single beam acoustic tweezers using high frequency focused transducers can offer wide range of applications in biomedical and chemical sciences including intercellular kinetics studies and cell stimulation. Additionally, we propose a simple and efficient approach to prepare xPMN-PT- (1-x)PZT (where x is 0.1, 0.3, 0.5, 0.7 and 0.9) composite films with controllable dielectric constant that offers better performance for high frequency ultrasonic transducer applications. A 200 MHz single element transducer utilizing 0.9PMN-PT-0.1PZT thin film was built. This type of xPMN-PT-(1-x)PZT film transducers may satisfy current needs of very high frequency biomedical applications, such as ultrasonic biomicroscopy or acoustic tweezers. 1 Chapter 1 Introduction 1.1 In general Particle trapping and manipulation using contact free techniques have numerous applications in chemical, physical, biological and medical research. For instance, the ability to manipulate a cell with precise control facilitates the study of cell-cell interaction, cell adhesion, and cell sorting (Macdonald et al., 2003; He et al., 2005; Fritzsch et al., 2012). The manipulation techniques also might aid the investigation of molecular dynamics and mechanisms (Bausch et al., 1999; Bustamante et al., 2003; Huang, H. D. et al., 2004; Noom et al., 2007). Moreover, these techniques could provide tools for tissue engineering and regenerative medicine (Titushkin et al., 2006; Zhang et al., 2008; Berthiaume et al., 2011). A variety of contactless manipulation techniques have been developed over many decades, such as optical (Ashkin et al., 1986; Grier, 2003; Moffitt et al., 2008), electrostatic (Pohl, 1951; Karnik et al., 2005; Krishnan et al., 2010), magnetic (Crick et al., 1950; Gosse et al., 2002; Ong et al., 2008) and acoustic tweezers (Wu, 1991; Lee, J. et al., 2009; Shi et al., 2009). Among these techniques, optical tweezers is the most well- known and extensively investigated technique. Optical tweezer was introduced by Ashkin et al. (Ashkin, 1970; Ashkin et al., 1986), also known as single beam optical trap, in which a strongly focused beam of light was used to generate forces to allow particle trapping. High numerical apertures of microscope objectives are usually selected to produce tightly focused light with high intensity gradient for optical trapping. 2 Optical tweezers have shown the capability to trap and manipulate many kinds of small particles, including dielectric spheres, metal particles, cells, DNA, bacteria, and molecular motors (Ashkin et al., 1987; Svoboda et al., 1994; Bustamante et al., 2003; Grier, 2003; Zhang et al., 2008). While optical tweezers provide a wide range of applications with excellent precision, ultimately their use is limited by: 1) requirement of optically purified objects or shallow regions in a medium; 2) photodamage of targeted particles often occurs due to the high energy produced by focused lasers; 3) the apparatus used is quite complicated and expensive optical setups making it difficult to miniaturize and maintain. (Neuman et al., 2008; Rasmussen et al., 2008; Jeong et al., 2011; Ding et al., 2012) The electrostatic or magnetic tweezers, where the trapping forces based on electric or magnetic fields, may provide alternative methods to manipulate particles without the shortcomings from the optical tweezers. These two techniques are able to be miniaturized and achieve high throughput, but they have limited flexibility (Shi et al., 2009; Jeong et al., 2011). For example, electrostatic tweezers need extremely low salt concentrations in the particle-carrying solution, this requirement would undoubtedly limits many biological applications (Eijkel et al., 2010). The limitation of the magnetic tweezers is that the targets need to be pre-labeled with magnetic materials, a procedure that affects cell viability (Shi et al., 2009; Ding et al., 2012). In this regard, the acoustic tweezers that generate pressure gradient and acoustic forces for particles manipulation could serve as better solution (Wu, 1991; Lee, J. et al., 2009; Wang, Z. et al., 2011; Ding et al., 2012). Compared to the optical, electrical or magnetic tweezers, the acoustic counterpart requires no pretreatment of the particles and work for most particles 3 regardless of their optical, electrical or magnetic properties. Additionally, the acoustic tweezers are suitable for biomedical applications because the acoustic forces are relative noninvasive to biological objects. 1.2 Acoustic tweezers In acoustics, particle manipulation by employing various modes of operation have been reported including standing waves and Bessel beams (Woodside et al., 1997; Marston, 2006; Shi et al., 2009; Yanyan et al., 2009; Choe et al., 2011). Among them, the first study of acoustic tweezers was pioneered by Wu et al. (Wu, 1991) using two opposing sound beams to capture latex spheres and frog eggs. The tweezers utilized two counteracting 3.5 MHz ultrasonic transducers to create a force potential well for particle manipulation, an approach based on acoustic standing wave. The use of two ultrasonic transducers was necessary because relatively broad beams could not produce the required sharp intensity gradient for single beam acoustic trapping (Lee, J., Teh, S.-Y., et al., 2010). Acoustic standing wave field arises from the interference between two waves of the equal wavelength and amplitude propagating in opposite directions to form parallel stationary planes of node (minimum pressure) planes and anti-node (maximum pressure) planes (Woodside et al., 1997). The standing waves approach has shown some feasibility for manipulation of a group of biological particles than an individual biological particle by using multiple transducers or a transducer-reflector pair. Acoustic tweezers based on Bessel beams could generate a region of a negative axial radiation force for particles 4 manipulation (Marston, 2006). For instance, the Bessel beams approach has been able to trap microparticles via a multi-focui Fresnel lens (Choe et al., 2011). Due to the conceptual similarity with the optical tweezers and recent advances in high frequency ultrasonic transducers, a single beam acoustic tweezer using high frequency ( ≥ 20 MHz) focused ultrasound transducer has recently been considered, and its feasibility was theoretically and experimentally investigated (Lee, J. et al., 2005; Lee, J. et al., 2009; Jeong et al., 2011). This single beam acoustic trap is similar to its optical counterpart, and a strongly focused single beam acoustic wave is used to generate forces to allow particle trapping. It can provide higher trapping force in the range of tens of nanonewtons (nN) than those of optical tweezers in the order of one to hundreds piconewtons (pN) (Lee, J., Lee, C., et al., 2010). The larger trapping forces show the capability of manipulating larger particles and provide deeper penetration than optical counterparts (Lee, J., Teh, S.-Y., et al., 2010; Jeong et al., 2011). Additionally, the single beam acoustic tweezer can handle an individual particle effectively rather than a group of particles as in the standing wave case. 1.3 Fundamental physics of single beam acoustic tweezers The single beam acoustic trapping model using the ray acoustics approach is illustrated in Figure 1.1. (Lee, J. et al., 2005; Lee, Jungwoo et al., 2006; Lee, C. et al., 2010; Lee, J. et al., 2011) As an acoustic beam impinges on an interface, the momentum transfer occurs from the redirection of an incident beam. A sound beam can be approximated as a group of rays, so a net amount of momentum transfer is acquired by 5 integrating such individual changes over the entire beam. Suppose a pair of rays represented by ‘‘P a ’’ and ‘‘P b ’’ acting on a spherical particle in a sound field of Gaussian intensity distribution, and the corresponding exiting rays are denoted as P a ’ and P b ’. We assume there is no multiple internal scattering here for the sake of simplicity. As the incident rays and the existing rays interact with the spherical particle, the momentum transfers from those rays are defined by dP a and dP b , where dP a = P a - P a ’ and dP b = P b - P b ’. The rate of the momentum change determines the net force acted on the rays and thus induces an equal and opposite force of the trapping force based on Newton’s third law. Since P a that locates in higher intensity region is stronger than P b , the net force will direct the spherical particle toward the beam axis. In order to form a strong acoustic trap, a sharp intensity gradient at the focus is essential. Additionally, the acoustic impedance difference should be minimal at the interface to accomplish acoustic traps (Lee, Jungwoo et al., 2006). 6 Figure 1.1: Schematic diagram of single beam acoustic trapping model. An ultrasonic transducer having Gaussian intensity profile produces two rays (denoted as P a and P b ) on the sphere with the corresponding exiting rays of P a ’ and P b ’. The momentum transfers from those rays are defined by dP a and dP b , which determines the rate of the momentum change and resultant forces on the sphere. Acoustic radiation force is induced based on Newton’s third law and leads to the trapping where a red thick arrow indicates the trapping direction. (Lee, J. et al., 2011) 7 1.4 Ultrasonic transducers The underlying principles of ultrasonic transducers are discussed in this section. Ultrasonic transducers are acoustic devices that convert electrical energy into acoustic or ultrasonic energy and vice versa, at which the piezoelectric effect occurs from a piezoelectric element. The direct and reverse piezoelectric effects are illustrated in Figure 1.2. The direct piezoelectric effect is defined as a change in electric polarization with a change in applied stress, and the reverse effect is the change of strain or stress in a material due to an applied electric field. The simplest ultrasonic transducer is a single element transducer, which incorporates a piezoelectric element, matching and backing layers, electrical impedance matching, as well as, sometimes focusing. The piezoelectric element, a most important component of such a device, converts electrical energy to acoustical energy and vice versa. The piezoelectric element is a resonant material, and its thickness determines the resonant frequency. The most popular piezoelectric material is lead zirconate titanate (PZT) ceramics which have very strong piezoelectric properties following polarization. The other commonly used materials include lithium niobate (LiNbO 3 ) single crystals, and piezoelectric polymers such as polyvinylidene fluoride (PVDF). The fire-on silver or sputtered chrome-gold are applied on the surfaces of the piezoelectric elements as electrodes. The negative electrode is usually connected to a metal transducer housing that provides RF shielding to the device when connecting to an electrical adapter. The acoustic isolating material, such as epoxy, is often placed between the piezoelectric element and the housing to prevent ringing of the housing that follows the vibration of the piezoelectric element itself (Shung et al., 1996). 8 Figure 1.2: (a) Direct piezoelectric effect in which a stress induces a charge separation. (b) Reverse piezoelectric effect in which a potential difference across the electrodes induces a strain. (Shung, 2006) 9 In order to increase the conversion efficiency between the electrical energy and acoustical energy, the impedance of the piezoelectric element must be electrically matched to the electronics driving the element, as well as, acoustically matched to the loading medium, such as tissue. The performance of a transducer can be improved by using a matching layer in the front. Matching layers are inserted in between piezoelectric materials (typically ~30 MRayls) and loading medium (tissues~2 MRayls) to compensate for their acoustic impedance mismatch. For a monochromatic plane wave, 100% acoustic transmission between two media with impedances of Z 1 and Z 2 occurs when a matching layer has the thickness of λ m /4 and its acoustic impedance Z m (Kinsler et al., 1982; Shung et al., 1996) : = (1.1) where Z 1 and Z 2 are acoustic impedance of the piezoelectric materials and loading medium, respectively. For a pulsed wave of wideband transducers applications, however, DeSilets et al., (Desilets et al., 1978) demonstrated that a single matching layer should be modified to: = (1.2) For a two matching layer scheme, the acoustic impedances of the inner layer (Z m1 ) and outer layer (Z m2 ) should be respectively equal to = (1.3) = (1.4) 10 Typical matching materials include ceramic-loaded epoxy, glass, parylene et al. The backing materials serve as several purposes. It can be used to provide a rigid support for the fragile piezoelectric element, as well as, to offer electrical connections to the piezoelectric element and to dissipate heat. To avoid image artifacts caused by backing echos, there is an important feature of signal absorption using the backing materials. It also can serve to damp out the ringing and to increase bandwidth as the absorptive backing materials with an acoustic impedance similar to that of the piezoelectric element. However, the achievement of suppression of ringing or shortening of pulse duration sacrifice sensitivity due to a large portion of the energy is absorbed by the backing material (Shung et al., 1996). The commonly used backing materials are silver-loaded epoxy and tungsten-loaded epoxy. Transducers are often electrically tuned to maximize energy transmission and improve the bandwidth. The electrical matching circuit components may be placed between the transducer and external electrical devices (Desilets et al., 1978). Ideally, for maximal energy transmission, the transducer input impedance should be real and the input resistance should match that of the source. Since most transducers are coupled to a 50 ohm electronic load on transmit and receive, the piezoelectric element with the proper dielectric constant, area, and thickness has to be selected to reach a nominal 50 ohm real impedance. The clamped capacitance can be tuned out by adding an inductance either in series or in parallel with the transducer, and then a transformer may be used to match the remaining real device impedance to 50 ohm. 11 The focal point of a flat unfocused transducer occurs at the near-field/far-field transition point. However, the beam width at this point is typically quite wide and thus limits its applications. In order to rectify this issue, it is important to focus the ultrasonic beam. For ultrasound imaging applications, the primary function of acoustic focusing is to improve the lateral resolution at a certain axial range (Shung et al., 1996). More importantly, a highly focused ultrasonic transducer with a sharp intensity variation in the lateral direction is the requirement for single beam acoustic trapping (Lee, J. et al., 2005). The profile of non-focused and focused beams from a transducer is shown in Figure 1.3. Figure 1.3: Comparison of non-focused and focused acoustic beams from the ultrasound transducer. (Shung et al., 1992) 12 In single element transducers, focusing can be utilized either by using a lens (lens- focused) or by using a curved transducer (self-focused). Figure 1.4 illustrates the internal construction of lens-focused and self-focused transducers. Acoustic lens can be made of polyurethane, polymethylpentene (TPX), silicon rubber, quartz, sapphire, polystyrene or silicon. Based on Snell’s Law, a convex lens is used as the sound velocity in the lens material is less than in the loading medium, such as polyurethane or silicon rubber. Otherwise, a concave lens is selected while the sound velocity in the lens material is higher than in the loading medium, such as quartz or polystyrene. Self-focused transducers could be fabricated by shaping the piezoelectric element via hard pressing (Lockwood et al., 1994), pressure defection (Fleischman et al., 2003), mechanical dimpling (Lam et al., 2012), or coating piezoelectric films on curved surfaces (Robert et al., 2004) . It is noted that the beam width at the focal point of a transducer, W b , can be approximated as following equation (Shung, 2006). ≈ # λ (1.5) where f # is the f-number defined as the ratio of focal distance to aperture dimension. From this relationship, it is clear that higher frequency transducers have narrower beams at the focal point than lower frequency transducer. 13 Figure 1.4: A cross-sectional drawing of the typical single element focused ultrasound transducers.(Shung, 2006) 14 1.5 Outline of this dissertation In Chapter 1, the introduction of contactless particle trapping, acoustic tweezers, as well as basic concepts of the single beam acoustic trapping and ultrasonic transducers are demonstrated. Chapter 2 reports the design, fabrication and characterization of the miniature (<1 mm) press-focused ultrasonic transducer. In vitro ultrasonic biomicroscopy imaging on the rabbit eye and the single beam acoustic trapping experiment are present to demonstrate the capabilities of this type of transducer. Chapter 3 describes the development of the self-focused ultrahigh frequency ZnO transducer, where the transducer is made by sputtering ZnO film on a curved surface of the aluminum backing material. The performance of the fabricated transducer is evaluated, and its ability for single beam acoustic trapping is demonstrated. In Chapter 4, the development of ultrahigh frequency focused transducer based on silicon lens design is discussed. The fabrication and evaluation of the silicon lens transducer are described. The application of single beam acoustic trapping is also performed. In Chapter 5, we focus on the development of PMN-PT-PZT composite films with tunable dielectric constant for high frequency ultrasonic transducer applications. The dissertation is concluded in Chapter 6 with a brief summary and suggestion of future work. 15 Chapter 2 Press-focused High Frequency Needle Transducer A miniature focused needle transducer (<1 mm) was fabricated using the press- focusing technique. The measured pulse-echo waveform showed the transducer had center frequency of 57.5 MHz with 54% bandwidth and 14 dB insertion loss. To evaluate the performance of this type of transducer, in vitro ultrasonic biomicroscopy imaging on the rabbit eye was obtained. Moreover, a single beam acoustic trapping experiment was performed using this transducer. Trapping of targeted particle size smaller than the ultrasonic wavelength was observed. Potential applications of these devices include minimally invasive measurements of retinal blood flow and single beam acoustic trapping of microparticles. 2.1 Introduction Recent advances in high frequency ultrasonic transducer have made further applications such as high frequency imaging and acoustic microparticle trapping possible (Foster et al., 2000; Shung et al., 2009). Higher center frequency can achieve better spatial resolution of ultrasonic imaging as well as smaller particle manipulation. For a number of applications, the size of the transducer is crucial and there is a need to miniaturize the transducer size. A small diameter unfocused needle transducer has been reported for ultrasonic imaging and blood flow measurements in the eye (Zhou, Q. F. et al., 2007). In this chapter, focused needle transducers that will achieve better spatial resolution than unfocused transducers will be reported. In addition, a focused needle 16 ultrasonic transducer offers more flexibility in carrying out single beam acoustic trapping experiments given the extremely congested environment in the small area within which the experiments are performed (Lee, J. et al., 2005). 2.2 Materials and methods For a miniaturized transducer, piezoelectric materials with high dielectric constant are more desirable since the electrical impedance of a transducer is inversely proportional to the dielectric constant of the piezoelectric material (Cannata et al., 2003). Comparing with other materials, Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) single crystal is a promising candidate to build small transducers because of its high dielectric constant (700~5000). Additionally, it has higher piezoelectric mechanical coupling coefficient (0.58), which enhances energy conversion and improves the sensitivity of the transducer (Park et al., 1997). The high piezoelectric constant (1000~2000) and low dielectric loss also make it suitable for high frequency transducer applications. Table 2.1 shows a brief list of the material properties of the PMN-PT single crystal (HC Materials Corp., Urbana, IL). Traditionally, two types of focused transducers can be fabricated via lens-focusing and self-focusing techniques. Comparing with lens-focused transducers, self-focused transducers that typically yield higher sensitivity can be fabricated quicker and the devices produced are more consistent (Cannata et al., 2003). Hence, PMN-PT single crystal press-focused needle transducers were developed in this study to meet the demands of further applications. 17 Table 2.1: Material properties of PMN-PT single crystal. Property Value Electromechanical coupling coefficient (K t ) 0.58 Clamped dielectric constant 797 Dielectric loss 0.0036 Piezoelectric constant (d 33 , pc/N) 1430 Density (g/cm 3 ) 8 Acoustic impedance (MRayl) 36.9 Longitudinal sound velocity (m/s) 4608 Curie Temperature (°C) 131 Figure 2.1 illustrates the fabrication process flow of a PMN-PT press-focused needle transducer. The process details are described as follows. First, the PMN-PT single crystal was lapped to 23 µm, and then Cr/Au (500 Å /1000 Å) electrodes were sputtered on both sides of the PMN-PT by an NSC-3000 automatic sputter coater (Nano-Master, Austin, TX). The silver epoxy matching layer, made from a mixture of Insulcast 501 epoxy (American Safety Technologies, Roseland, NJ) and 2-3 µm silver particles (Aldrich Chemical Co., Milwaukee, WI) with the mass ratio of 1.25 to 3 (particles: epoxy), was cast with the aid of an adhesion promoter (AP-131, Lord Corp., Erie, PA) on the PMN-PT and lapped to 7 µm. A conductive silver epoxy (E-solder 3022, Von Roll Isola Inc., New Haven, CT) was cast onto the opposite side as the backing material and lapped to 2 mm. Table 2.2 shows the material properties of the E-solder backing layer as well as the properties of all other passive materials used in the design (Cannata et al., 2003). The thickness of the sample was measured by thickness gage (Sylvac Ultra Digit Mark IV) and Heidenhain CT25/ND281B thickness measuring system during the lapping process. After lapping and polishing, the sample was then diced to 0.4x0.4 mm 2 posts and 18 housed inside a polyimide tube with an inner diameter of 0.57 mm. Figure 2.2 shows the SEM (JSM-6610, JEOL, Peabody, MA) pictures of a dicing post, where the thickness of PMN-PT and silver matching layer is confirmed as 23 µm and 7 µm, respectively. A lead wire was connected to the backing layer with an additional amount of conductive epoxy. The polyimide tube provided electrical isolation from the 20-gauge needle housing with an outer and inner diameter of 0.9 mm and 0.7 mm, respectively. The device was press focused by a steel ball bearing at 75°C to obtain a 0.4 mm focus and an f-number of 1. Next, a layer of Cr/Au (500 Å /1000 Å) was sputtered across the transducer face to form the ground plane connection. A 4 µm parylene layer was vapor-deposited by a PDS 2010 Labcoator (Specialty Coating Systems, Indianapoils, IN) on the front face of the transducer, serving as an acoustic matching layer and a protection layer. Lastly, the transducer was housed in a subminiature version A (SMA) connector. A fabricated press- focused needle transducer is shown in Figure 2.3. Table 2.2: Passive material properties used in the transducer designs. Material Use Density (g/cm3) Sound Velocity (m/s) Acoustic Impedance (MRayl) E-solder 3022 (centrifuged) Conductive backing 3.20 1850 5.92 Insulcast 501 and 2-3 μm silver particles Matching layer 3.86 1900 7.3 Parylene Matching layer 1.18 2200 2.6 EPO-TEK 301 Insulating epoxy 1.15 2650 3.05 19 Figure 2.1: A schematic fabrication flow of press-focused PMN-PT needle transducer. 20 Figure 2.2: SEM images of a 0.4x0.4 mm 2 post for press-focused PMN-PT needle transducer fabrication. 21 Figure 2.3: A photograph of the completed press-focused high frequency PMN-PT needle transducer. 2.3 Transducer evaluation The pulse-echo response and insertion loss of the transducer were measured at room temperature in a de-ionized water bath using a pulse-echo arrangement by reflecting the transmitted signal from a polished x-cut quartz target placed at the focus of the transducer (Cannata et al., 2003). The transducer was excited by a pulser/receiver (Panametrics model 5900PR, Panametrics Inc., Waltham, MA) with an electrical impulse at 200 Hz repetition rate and 50 Ω damping. The reflected echo was received by the transducer and then digitized and displayed via a 1GHz oscilloscope (LC534, LeCroy Group., Chestunt Ridge, NY) with 50 Ω coupling. Figure 2.4 shows the block diagram of 22 a normal pulse-echo measurement. The frequency spectrum was generated via Fast Fourier Transform (FFT) of the received waveform. The characteristics of the transducers were determined from the measured frequency spectrum (Erikson et al., 1982). (a) Lower and upper -6 dB frequencies (f l and f u ): the frequencies at which the magnitude of the amplitude in the spectrum are 50% (-6 dB) of the maximum. (b) Center frequency (f c ): 2 u l c f f f + = (2.1) (c) -6 dB bandwidth (BW): % 100 × − = c l u f f f BW (2.2) (d) -20 dB pulse length (PL): the time duration it takes the oscillation to reduce to 10% (- 20 dB) of its maximum peak amplitude. In order to measure the insertion loss (IL), an arbitrary function generator (AFG 3252, Tektronix Inc., Richardson, TX) with 50 Ω output impedance was used to generate multicycle sine wave burst to excite the transducer. The received echo voltage amplitude was measured by the oscilloscope in 50 Ω coupling mode. Two-way insertion loss was 23 calculated using the ratio of the frequency spectrum of the transmitted and received responses. Due to transmission into the quartz crystal, the 1.9 dB lost was compensated in the final insertion loss calculation. The signal loss resulting from attenuation in the water bath was also compensated using an attenuation of 2.2 x 10 -4 dB/mm-MHz 2 (Lockwood et al., 1994). The equation used was = 20 +1.9 +2.2 ∙10 ∙2 ∙ (2.3) where V T and V R are the transmitting and receiving amplitudes, respectively; d is the distance from the transducer to the target; f c is the center frequency. A tungsten wire with diameter of 6 µm (California Fine Wire Co., Grover Beach, CA) was linearly scanned with an in-house computer-controlled exposimetry system to determine the lateral beam profile of the transducer. The wire target was mounted to an automated three-axis positioning system (Inchworm actuator, Burleigh Inc., Fishers, NY), and the transducer was hold by a dual rotation gimbal fixture. The system scanned the wire across the transducer sound field at a step size of 0.5 µm, The pulse intensity integral (PII), determined as the time integral of the intensity of an echo taken over the time where the acoustic pressure is non-zero (Raum et al., 1997), was calculated from the received ultrasonic echoes reflected back from the wire target. As a result, the lateral intensity distribution (beam profile) was estimated (Huang, B. et al., 2004). The experimental set-up is schematically illustrated in Figure 2.5. 24 Figure 2.4: Block diagram of a pulse-echo test set-up for ultrasonic transducers. 25 Figure 2.5: Experimental arrangement for the measurement of lateral beam profile for ultrasonic transducers. 26 The frequency dependence of the electrical impedance and phase of the transducer are displayed in Figure 2.6, which was measured by Agilent 4292A impedance analyzer (Agilent Technologies, Santa Clara, CA). It shows that the electrical impedance at resonant peak is 35 Ω at 63.5 MHz. The series resonant frequency is 59 MHz, where the minimum impedance occurs. The parallel resonant frequency that has maximum impedance is 67 MHz. The electromechanical coupling coefficient k t can be determined according to ) 2 tan( 2 p s p p s t f f f f f k − ⋅ ⋅ = π π (2.4) where f s and f p are series and parallel resonant frequencies. The k t value was calculated to be about 0.51. Compared to pure PMN-PT single crystal disk (k t =0.58), k t decreases around 12 % after fabricating a press-focused needle transducer. The degradation of k t may be due to the external stresses from the lapping, dicing and press-focusing. Earlier literature also reported that the electromechanical coupling coefficient k t degraded about 3.6 % on a press-focused LiNbO 3 single-crystal transducer (Cannata et al., 2003). 27 Figure 2.6: Measured electrical impedance of press-focused PMN-PT needle transducer. Figure 2.7 shows the measured pulse-echo waveform and normalized frequency spectrum of the fabricated transducer. We can find that the center frequency is 57.5 MHz with a -6 dB bandwidth of 54 %. The insertion loss was measured and compensated to be 14 dB, which is comparable to that of the previous reported 44 MHz flat needle PMN-PT transducer (15 dB) (Zhou, Q. F. et al., 2007). The measured lateral beam profile of the press-focused needle transducer (f # =1) is given in Figure 2.8. The -6 dB beam width was determined to be 24 µm by scanning a 6 μm diameter tungsten wire as pulse-echo target at the focus, where translational scans at the focus of the transducer yield the lateral 28 profile of projections of the beam. A commonly used estimation of beam width at focal point is defined by the Equation (1.5). The measured beam width (24 µm) is in approximate agreement with the theoretically predicted width of 26 µm. Figure 2.7: Measured pulse-echo waveform (solid line) and normalized frequency spectrum (dashed line) of press-focused PMN-PT needle transducer. 29 Figure 2.8: Measured lateral beam profile of press-focused PMN-PT high frequency needle transducer. 2.4 In virto ultrasonic biomicroscopy imaging An excised normal rabbit eyeball was imaged with this 60 MHz press-focused needle transducer as part of an ultrasonic bio-microscope (UBM) system. A UBM system acquires adjacent A-scan lines to obtain high resolution, two-dimensional image of tissue. The set-up of the UBM system (see Figure. 2.9) was based on the design previously described by Turnbull et al.(Turnbull et al., 1995). 30 Figure 2.9: A diagram of the UBM set-up system. tr im co u th le dy o o to F A line ransducer w mprove ima ollecting pul ltrasound pe he rabbit eye evels. The R ynamic rang f the UBM s f the pulse ( o obtain an im igure 2.10: I ear mechani as used in t age grayscal lse-echo line enetration de e. The final RF data used ge. The imag system, sinc 0.8 mm). Fu mage at the In vitro UBM ical scan (Se the system. le visualizat es spacing a epth, which w image was f d to form thi ge was not o ce the focus urther improv focus with b M image of t ervo Motor, A logarithm tion. One f t 10 µm. Th was set to be formed by c s image was obtained at t of the transd vement of U better spatial the anterior p , LAR37; SM mic compres frame of im he depth of i e 4 mm to im converting th s logarithmi the focus du ducer (0.4 m UBM system l resolution. portion of a MAC, Carls ssion algorit mage data w image was d mage the ant he scan line ically compr ue to limitati mm) actually m electronics rabbit eye. sbad, CA) o thm was us was obtaine determined b terior chamb data to 256 ressed to a 5 ions of the p y fell in the r should enab 31 of the ed to ed by by the ber of 6 gray 55 dB pulser range ble us 32 2.5 Microparticles manipulation The experimental configuration of acoustic trapping is shown in Figure 2.11 where the press-focused needle transducer was used in a microparticle immobilization experiment. Specifically, the microparticles were suspended in a designed chamber filled with water, sitting on top of a microstage. The chamber had an opening with an acoustically transparent mylar film at its bottom, through which particle motions were detected by an inverted microscope (IX-71, Olympus, Japan). The transducer was mounted on a three-axis motorized linear stage (LMG26 T50MM, OptoSigma, Santa Ana, CA) and interrogated the microparticles from above. The position of the transducer could be controlled by a computer with a customized LABVIEW code. The movement of the position could be varied from 1 µm to 10 µm by programmed increments. Prior to the particle trapping experiment, a pulse-echo test was performed to ensure that the targeted particles were located on the focal plane. The transducer was driven in a sinusoidal burst mode whose waveforms were generated from a function generator (AFG3251, Tektronix, Anaheim, CA) and then amplified by a 50 dB power amplifier (525LA, ENI, Rochester, USA) to achieve desirable peak-to-peak voltage amplitude. The pulse repetition period was set to 1 ms and its duty factor was 0.03 %. The trapping of the microparticles was observed through a CMOS camera (ORCA-Flash2.8, Hamamatsu, Japan) attached to the microscope, and the images as well as videos captured by the CMOS camera were recorded with a computer. 33 Figure 2.11: Block diagram of single beam acoustic trapping experiment using high frequency focused transducer. 34 To perform microparticle manipulation, polystyrene microspheres (Polysciences, Warrington, PA) of 15 µm mean diameter were loaded into the chamber as targeted particles to be trapped. The maximum displacement of trapped particles (Lee, J. et al., 2009) was measured and quantified to determine how far the trapping force could attract the particle from the center of the trap by this press-focused needle transducer. Figure 2.12 illustrates the test procedure. Specifically, when a particle was stably trapped, the transducer was subsequently turned off and moved by a programmed distance of the motorized stages. The transducer was then turned back on to observe whether the particle could be attracted. If so, the distance was further increased and the above procedure was repeated, until the particle could no longer be attracted. The final distance was identified as the maximum displacement. As shown in Figure 2.13, the measurement was carried out under various excitation conditions. The frequency of the transducer was varied from 52.5 to 62.5 MHz and its peak-to-peak voltages were applied at 19, 25 and 32 Vpp. Generally, at each voltage, the displacement reached its maximum value when operating at resonance frequency of 57.5 MHz where the maximum peak pressure occurs. Meanwhile, the displacements were increased with the higher excitation voltages. The above phenomena was also observed and described in previous lipid droplets trapping experiments (Lee, J. et al., 2009). 35 Figure 2.12: Text procedure of maximum displacement measurement for an individual targeted particle with single focused acoustic beam. (Lee, C. et al., 2010) 36 Figure 2.13: Measured maximum displacements as a function of frequency with different excitation voltages. 37 The trapped particle could be moved in different paths by moving the transducer with the programmed movement. Figure 2.14 depicts a single trapped microsphere transportation using this press-focused needle transducer. The camera system captured a series of images to illustrate a transporting of an individual immobilized particle in the focus plane. Note that the bright circular structure in the background is the projection of the transducer. A red dot is given as a reference point to show the location change of the microsphere, and inside the blue circle is a trapped microsphere. It shows the feasibility of two-dimensional manipulation of an individual microparticle with this single acoustic focused beam. While we have previously demonstrated that the single beam acoustic trapping in Mie regime (Lee, J. et al., 2005; Lee, J. et al., 2009), where the trapped particle size (D) is larger or close to λ, it is worth noting that the trapped particle size (15 µm) is smaller than the ultrasonic wavelength (26 µm) in this study. Even with the diameter of particle, strictly speaking, still being in Mie region, these experimental results indicate that single beam acoustic tweezers are capable of manipulating particles of a size not only larger but also smaller than wavelength. In case of optical tweezers, Ashkin et al. had reported the ability of trapping occurred over full size range from Mie to Rayleigh particles (Ashkin et al., 1986). The observation of current single beam acoustic trapping experiments confirm similar capability can be achieved by acoustic tweezers as well. 38 Figure 2.14: Example of microparticle manipulation in two dimensions using the press- focused needle transducer. A single 15 µm polystyrene microsphere was manipulated along the movement of the transducer. A trapped microparticle is present inside a blue circle while a red dot is given as a reference point to show the location change of the microsphere. 39 2.6 Conclusion In summary, using press-focusing technique, we have fabricated a miniature press-focused needle transducer (<1 mm) with a 0.4 mm focus and f-number equal to 1. The measured center frequency is 57.5 MHz with a bandwidth of 54 % and insertion loss is 14 dB. In vitro UBM image of the rabbit eye has been obtained using this press- focused needle transducer. The results on polystyrene microparticle trapping by this type of transducer is exciting, since it shows that it is possible to acoustically trap particles with a size smaller than the ultrasonic wavelength. The results indicate the Rayleigh particles trapping may be feasible by single beam acoustic transducers. 40 Chapter 3 Self-focused Ultrahigh Frequency ZnO Transducer Ultra high frequency ( ≥ 200M Hz) focused transducers are required to generate tightly focusing acoustic microbeams for effectively trapping of small microparticles. In this study, the self-focused ZnO transducer with ultrahigh center frequencies was built using a simple fabrication technique. The measured pulse-echo waveform of the fabricated transducer shows a center frequency of 204 MHz and a -6 dB bandwidth of 27%. The -20 dB pulse length and the measured insertion loss of this transducer are 26 ns and 61 dB, respectively. The transducer was shown to be able to manipulate a single 10 µm polystyrene microsphere in two dimensions demonstrating the capability of this type of ultrahigh frequency transducer. 3.1 Introduction With the successful demonstration of particle trapping using high frequency focused ultrasounds, the desire to use single beam acoustic tweezers for more practical applications (ex. to examine cell mechanics) drives the ultrasound frequency higher to manipulate particles or cells with a size of a few microns, because the beam width is inversely proportional to the transducer frequency. Additionally, in order to handle an individual particle with very small size efficaciously, the beam width should not be too wide to achieve this purpose. Hence, an ultra-high frequency ( ≥ 200M Hz) focused transducers which can generate tightly focusing acoustic microbeam is required for effectively trapping of an individual small microparticle. 41 One of the technical challenges for building ultra-high frequency (UHF) transducers is the preparation of piezoelectric elements with a thickness on the order of micrometers. Lapping down and mechanically machining traditional piezoelectric ceramics or crystals to very thin thickness is quite a challenge and time-consuming. Piezoelectric thin film technology is a viable alternative to solve this problem with simplifying the fabrication process and allowing very high frequency transducers to be produced in a precisely controlled, repeatable manner. Zinc oxide (ZnO) and aluminum nitride (AlN) sputtered thin films are two of the most studied and common used piezoelectric films in ultrasonic transducers (Martin et al., 2000; Trolier-Mckinstry et al., 2004). They are interesting piezoelectric films for very high frequency transducer designs because the sputtering process can be precisely controlled and is compatible with various substrates including metal and silicon. Both thin films have quite similar piezoelectric properties, and they are ideal for fabricating large aperture single element transducers due to their low dielectric constant (~ 9). Comparing with AlN, ZnO is the better choice for ultrasonic transducer designs operating in thickness mode since it has a larger electromechanical coupling coefficient (k t =0.28) than AlN (k t =0.17) (Kino, 1987; Cannata et al., 2008). In the past, the design of focused ZnO thin film ultrasound transducers and arrays were based on lens structure. Generally, a concave lens was formed by polishing the other end of the substrate spherically or cylindrically, where the polished flat sapphire or fused silica served as the substrate (Ito et al., 1995; Yokosawa et al., 1997; Martin et al., 2000). Compared to the lens transducer design, a lensless transducer with a shaped piezoelectric element should be relatively simple and cost effective. Moreover, the 42 transducers with spherically shaped active elements have been proved to be more effective in producing high-sensitivity devices because the installation of extra lens would cause attenuation (Cannata et al., 2000; Snook et al., 2002) which can be deleterious especially at very high frequencies ( ≥100 MHz). It is expected that the lensless transducer design should be better suited for ultrahigh frequency acoustic tweezers application. The purpose of this work is to introduce lensless ultrahigh frequency microbeam device for acoustic tweezers application, where the self-focused ZnO UHF transducer was built by sputtering ZnO film on a spherically shaped surface of the aluminum backing material. The performance of the fabricated transducer was evaluated, and the capability of microparticles trapping using this type of the transducer was also demonstrated. 3.2 Materials and method In this work, the ultrahigh frequency self-focused transducer was fabricated by a simple technique that was utilized previously to fabricate transducers at lower frequencies (Robert et al., 2004; Cannata et al., 2008). Figure 3.1 shows the schematic design of a UHF self-focused ZnO transducer. Specifically, a 3.2 mm diameter aluminum (Al) rod was selected as the backing substrate. Note that several reasons to choose aluminum as the backing material (Cannata et al., 2008): it is conductive with low acoustic impedance (19 MRayls), its melting point (660°C) is much higher than the operating temperature required for the ZnO sputtering process, and it is relatively soft and 43 easy to machine. After machining and polishing process, we were able to obtain approximately a length of 15 mm and well-polished flat surface on one end of the Al rod. A 2 mm highly polished chrome/steel ball bearing (Bal-Tec, Los Angeles, CA) was then used to press the spherical shape into the polished end. Next, a 1 mm diameter and 5 mm long of the Al rod was machined using a lathe. After fabrication of the Al backing material, ZnO piezoelectric films were deposited on the spherical polished surface of the Al rod using an RF sputtering technique. A photograph of the sputtering ZnO film on the spherically polished end of an aluminum backing rod is present in Figure 3.2. The thickness of sputtered ZnO layer was 14.5 µm (see Figure 3.3), which was obtained under O 2 : Ar (1:1) gas pressure of 10 mTorr at 300°C with an RF power of 300 W by using a MRC 822 Sputtersphere sputtering system (Materials Research Corp., Orangeburg, NY). The deposition rate was around 0.6 µm/h. Once cooled to room temperature, the lead wire was connected to the backing layer with a small amount of conductive epoxy (E- Solder 3022). The device was housed in the center of a brass ring, and an insulating epoxy (EPO-TEK 301) was then poured into the void between the housing and the device. After curing overnight, a thin layer of Cr/Au (500 Å /1000 Å) was sputtered across the device surface and brass ring to form the ground plane connection and to provide RF shielding. A 2 μm parylene layer was vapor deposited on the front face of the transducer, serving as an acoustic matching layer and also a protection layer. Finally, the transducer was assembled with a SMA connector. A completed UHF self-focused ZnO transducer is shown in Figure 3.4. 44 Figure 3.1: Design configuration for the ultrahigh frequency self-focused ZnO transducer (not to scale). 45 Figure 3.2: A photograph of the spherically polished end of an aluminum rod with the sputtered ZnO layer on the top surface. 46 Figure 3.3: SEM images of the sputtered ZnO film on the curveted aluminum (Al) backing (a) and a close-up view of the ZnO film (b). 47 Figure 3.4: A photograph of the fabricated ultrahigh frequency self-focused ZnO transducer. 3.3 Transducer evaluation The performance of self-focused ZnO transducer was evaluated in a room temperature de-ionized water bath from pulse-echo response measurements. During the measurement, the transducer was connected to a Panametrics 5910PR pulser/receiver (Panametrics Inc., Waltham, MA) and excited by an electrical impulse at 200 Hz repetition rate and 50 Ω damping. An x-cut quartz was used as a target. The testing distance was at the focal length of the transducer. The reflected waveform was received 48 by the transducer and displayed by a 1GHz LC534 oscilloscope (LeCroy Group., Chestunt Ridge, NY) with 50 Ω coupling. The frequency spectrum was generated via Fast Fourier Transform (FFT) of the received waveform. The center frequency and the –6 dB bandwidth were determined from the measured frequency spectrum. The insertion loss was calculated by comparing the spectra of the transmitted and received responses. Compensation was applied for the attenuation in the water bath and the loss caused by the imperfect reflection from the quartz target (Cannata et al., 2003). The lateral beam profile of the transducer was evaluated with a wire target. Wire scanning was performed using a three-axis positioning system (Inchworm, Burleigh Inc., Fishers, NY) with an in-house computer-controlled exposimetry system at a resolution of 0.5 μm. A 6 μm diameter tungsten wire (California Fine Wire, Grover Beach, CA) was used as a target. The pulse intensity integral (PII) was calculated from the received echoes that were reflected from the wire target. The measured pulse-echo waveform and normalized frequency spectrum of the UHF self-focused ZnO transducer is shown in Figure 3.5. The lower and upper -6 dB frequencies of f l and f u are found to be 177 and 231 MHz, respectively. According to Equation (2.1) and (2.2), the center frequency is 204 MHz and the –6 dB bandwidth is around 27 %. The transducer is focused at 1 mm with an f-number of 1. Additionally, the -20 dB pulse length and the measured insertion loss of this transducer are 26 ns and 61 dB, respectively. Figure 3.6 shows the lateral beam profile where the 6 µm diameter tungsten wire is placed at the focus of the self-focused ZnO transducer. Because the acoustic energy is 49 mostly concentrated within the -6 dB contour of the beam profile, the ultrasonic beam width is conventionally determined at -6 dB. As a result, a measured beam width was estimated to be 8.5 µm by detecting spatially a point where its PII is reduced by 6 dB from the maximum. The result is in approximate agreement with the theoretically predicted width of 7.5 µm (beam width=f-number x wavelength) (Shung, 2006). Figure 3.5: Measured pulse/echo response (solid line) and normalized frequency spectrum (dashed line) for ultrahigh frequency self-focused ZnO transducer. 50 Figure 3.6: Measured lateral beam profile of the ultrahigh frequency self-focused ZnO transducer. 3.4 Microparticles manipulation The capability of this 200 MHz microbeam device in manipulating a single microsphere was performed via the experimental setup for acoustic trapping as shown in Figure 2.11, where the UHF self-focused ZnO transducer was used for single beam acoustic trapping. Specifically, polystyrene microspheres (Polysciences, Warrington, PA) of 10 µm mean diameter were loaded into the designed chamber filled with water as targeted particles to be trapped. The chamber had an opening with an acoustically transparent mylar film at its bottom. The transducer was mounted with a three-axis 51 motorized linear stage above the mylar film, so the focused ultrasound wave was emitted from the top to the bottom. The control of the position was carried out by a customized LABVIEW code. The trapping of the microparticles was monitored and recorded with a CMOS camera which was assembled with an inverted microscope. The images as well as videos captured by the CMOS camera were recorded with a computer. Prior to the particle trapping experiment, a pulse-echo test was performed to ensure that the targeted particles were located on the focal plane. In order to achieve desirable peak-to-peak voltage amplitude for particle trapping, the transducer was driven in a sinusoidal burst mode whose waveforms were generated from a function generator and then amplified by a 50 dB power amplifier. The sinusoidal burst was set at the following parameters to trap an individual microparticle effectively: the operating frequency was 204 MHz with the pulse repetition period of 1 ms, the duty factor of 0.1 % and the peak-to-peak voltage amplitude of 32 V. A single microsphere was then targeted within the field of view of the microscope. The transducer with the trapped particle was moved in a random path by the motorized stage to demonstrate the capability of two-dimensional single microparticle manipulation. Figure 3.7 illustrates the transporting motion of a trapped particle in the focus plane using this self-focused transducer where the transducer’s aperture is shown in the background. Owing to the tightly focused microbeam, the polystyrene particle was firmly immobilized to the spot and held stationary. It was observed that a single 10 µm microsphere was manipulated along with the movement of this self-focused ZnO transducer. Additionally, the trapped particle could be moved over a wide range by moving the transducer. Note that there was no contact between the transducer and microparticles. 52 Figure 3.7: A single 10 µm polystyrene microsphere was manipulated effectively using the self-focused ZnO ultrahigh frequency transducer. A red dot is given as a reference point to show the change in the particle location, and the trapped particle is present inside a blue circle. 53 3.5 Conclusion In summary, we have designed and fabricated ultrahigh frequency self-focused ZnO transducer by employing a simple and cost-effective fabrication technique. The transducer has center frequency at 204 MHz with a -6 dB bandwidth of 27 %. The measured beam width of this device is 8.5 µm, and it provides the tightly focusing microbeam needed for acoustic trapping of microparticles. An individual 10 µm microsphere was effectively manipulated in two dimensions using this type of transducer. The realization of microbeam acoustic tweezers paves the way for exploring new biomedical applications of acoustic tweezers including intercellular kinetics studies and cell fusion control. 54 Chapter 4 Silicon Lens Ultrahigh Frequency Transducer The ultrahigh frequency focused transducer based on silicon lens design has been developed for single beam acoustic tweezer application. A 330 MHZ silicon lens transducer was fabricated and evaluated. Microparticle trapping experiment was carried out to demonstrate that the device can manipulate a single microsphere as small as 5 µm. The result suggests that this silicon lens ultrahigh frequency ultrasonic transducer is capable of handling particles at the cellular level and that single beam acoustic tweezer may be a useful tool to manipulate single cells or molecules for a wide range of biomedical applications. 4.1 Introduction Similar to optical tweezers, a tightly focused ultrasound microbeam is needed to manipulate microparticles in acoustic tweezers. The development of ultrahigh frequency ultrasonic transducers is necessary allow these devices to be utilized to trap particles or cells with a size of a few microns. Tightly focused ultrasound microbeams at 96 MHz and 30 MHz have been reported to be capable of trapping small lipid particles of a size at 50 µm and 125 µm in distilled water, respectively (Lee, J. et al., 2009; Lee, C. et al., 2010). To allow more practical biomedical applications at the cellular level, the size of particles that can be trapped must be further reduced. This goal cannot be met without the development of ultrahigh frequency ultrasonic transducers, since the beam width is inversely proportional to the transducer frequency. More recently, a 200 MHz lens- 55 focused transducer based on a traditional design with a sapphire lens was shown to be capable of trapping cell of a size of 10 μm diameter (Lee, J. et al., 2011). Typically, a sapphire lens is made via the grinding method. With the increasing operation frequency of the transducer, the demand for small radius and high sphericity of the lens also increases. The traditional grinding method to make a concave radius in sapphire becomes much difficult as the radius decreases. Comparing with the sapphire lens fabrication, the small silicon lens can be made using the etching technique without machining. Several advantages can be expected using silicon as the lens material (Hashimoto et al., 1991): (1) Many lenses in a batch process with good uniformity can be utilized using lithography and etching techniques, (2) it is possible to make multi lens on a silicon lens body for advanced transducer configurations, (3) the silicon wafer is cheaper than the sapphire crystal. Therefore, in this work, we introduce a silicon lens ultrahigh frequency focused transducer design for microparticles manipulation. 4.2 Materials and methods 4.2.1 Silicon lens The isotropic chemical wet etching technique is selected to make silicon lens in this study. It is one of the standard techniques in micro-electro-mechanical systems (MEMS) micromachining, able to produce a spherical cavity with excellent sphericity, minimal surface roughness and a high degree of uniformity (Albero et al., 2009). The most common isotropic chemical etchant for silicon etching is HNA solution, a mixture of hydrofluoric acid (HF), nitric acid (HNO 3 ), and acetic acid (CH 3 COOH) 56 (Bogensch¨Utz et al., 1967; Schwartz et al., 1976; Williams et al., 1996; Kovacs et al., 1998). The HNO 3 in the solution oxides the silicon, while fluoride ions from the HF reacts with the silica to form the soluble silicon compound H 2 SiF 6 . The acetic acid helps prevent the dissociation of the nitric acid, thereby preserving the oxidizing power. The overall reaction of silicon in HNA etchant can be simplified as + +6 → + + + (4.1) The fabrication process flow of the silicon lens (Figure 4.1) is simply composed of five steps: hard mask preparation, photolithography, RIE etching, HNA etching, and removal of mask layers. The process details are as follows. First, a ~ 1 µm low-stress silicon nitride (SiN x ) layer was deposited on a double-side polish (100) silicon wafer by low pressure chemical vacuum deposition (LPCVD, Tystar Torrance, CA) as the hard mask for the HNA etching of silicon. The hard mask was used to protect the areas of the silicon wafer that should not be attacked by the HNA etchant. It should be noted that the low stress silicon nitride is required to avoid breaking of the mask in the zone of under- etching. The wafer was then patterned to open the areas for the etching via the photolithography technique. Next, the silicon nitride was selectively etched off for silicon etching using reactive ion etching (RIE). Once the exposed silicon nitride is etched, acetone is used to remove residual photoresist. Subsequently, the wafer was immersed in the HNA solution to form the silicon lens structure. The final step is to remove the residual silicon nitride mask by RIE. 57 Figure 4.1: Process steps used to fabricate the silicon lens. 58 Table 4.1: Effect of etching recipes on etching rate and surface roughness of etched cavity. HF (ratio) HNO3 (ratio) HAC (ratio) Temp. (°C) Etching rate (µm/min) Surface smoothness A 2 3 6 25 2~3 Poor B 5 10 16 25 4~5 Poor C 1 5 3 25 2~3 Fair D 1 5 3 50 3~4 Better E 4 7 11 50 3~5 Best To study the effect of etching parameters for lens structure, various etchants and conditions were taken to find out the optimized parameters for making a silicon lens with high sphericity and minimal surface roughness. Table 4.1 lists the etching results with different etching recipes. The surface roughness of the etched cavity is present in Figure 4.2. As shown in Figure 4.2, the optimum etching parameters (recipe E) to achieve smooth surface of the etched cavity is at an HF concentration of 20 vol%, HNO 3 concentration of 35 vol% and CH 3 COOH concentration of 55 vol% as well as operating temperature is at 50°C. The earlier literature also reported that mirror-like surface could be obtained in high etching temperature (Hashimoto et al., 1991). By patterning different opening size of the mask, we are able to make different cavity geometry for silicon lens UHF transducer applications. For instance, a cavity size of around 110 μm with depth of 50 µm was fabricated via a mask opening of about 60 µm diameter; another cavity size of 59 around 640 µm with depth of 160 µm was fabricated via a mask opening of about 400 µm diameter (see Figure 4.3). Figure 4.2: Effect of etching parameters (Table 4.1) on surface roughness of etched cavity: (a) Recipe A, (b) Recipe B, (c) Recipe C, (d) Recipe D and (e) Recipe E. 60 Figure 4.3: SEM images of the etched cavities for two different mask opening size: (a) cavity diameter of 110 µm with depth of 50 µm via a mask opening diameter of 60 µm; (b) cavity diameter of 640 µm with depth of 140 µm via a mask opening diameter of 400 µm. 61 4.2.2 Silicon lens transducer After preparing the silicon lens using isotropic etching technique, the UHF silicon lens transducers were fabricated as the following steps (see Figure 4.4). First, a thin layer of Cr/Au (500 Å /1000 Å) was sputtered on the opposite side of the lens part as the bottom electrodes. The ZnO piezoelectric thin film was then deposited on the bottom electrodes using RF sputtering machine for the desired thickness. The sputtering conditions were set to O 2 : Ar (1:1) gas pressure of 10 mTorr at 300°C with an RF power of 300 W; as a result, the deposition rate was around 0.6 μm/h. Next, the top Cr/Au electrodes was patterned and deposited on ZnO layer by a combination of lift-off process and sputtering technique. Lift-off is a simple, easy approach for making metallic patterns on a substrate. Generally, the lift-off process starts with patterning photoresist on a substrate by photolithography to make desired area without photoresist, and then metal thin film is deposited all over the substrate, finally the photoresist under the film is removed with solvent (usually Acetone) and taking the film with it to leave the residual metal film on the desired area. Subsequently, the wafer was diced to small chips (4x4 mm 2 ) by the dicing saw (TCAR864-1, Thermocarbon Inc. Casselberry, FL). Figure 4.5 shows a silicon wafer with the etched lenses on the front and backside of the ZnO layer with Cr/Au electrodes after dicing. A diced chip was then electrical connected with the lead wires to bottom and top electrodes by conductive epoxy (E-solder). To provide RF shielding, the device was encased in a brass tube. The gap between the cylindrical brass housing and the device was filled by insulating Epotek 301 epoxy. A 1 µm parylene layer was vapor deposited on the front face of the transducer, serving as an acoustic matching layer and also a 62 protection layer. Lastly, the silicon lens transducer was housed in a SMA connector. Figure 4.6 shows a picture of a completed silicon lens UHF transducer. 63 Figure 4.4: Schematic fabrication flow of silicon lens ultrahigh frequency transducer. 64 Figure 4.5: Produced silicon etched lenses on the front of the silicon wafer (top picture), and backside of the ZnO layer with Cr/Au electrodes (bottom picture) after dicing. 65 Figure 4.6: A photograph of the finished ultrahigh frequency silicon lens transducer. 4.3 Transducer evaluation The performance of the silicon lens ultrahigh frequency transducer is discussed in this section. The center frequency, bandwidth, pulse length, and insertion loss were measured by a typical pulse-echo test. A Panametrics 5910PR pulser/receiver (Panametrics Inc., Waltham, MA) was used to excite each transducer with an electrical impulse at 200 Hz repetition rate and 50 Ω damping. A flat x-cut quartz was used as a target and placed at the focus of the transducer. The echoes were received and digitized by a 1 GHz oscilloscope (LC534, LeCroy Corp., Chestnut Ridge, NY). Fast Fourier transform (FFT) of the received waveform yielded the frequency response of the echo. The characteristics of the transducer, such as center frequency and bandwidth, were determined from the measured frequency spectrum. The insertion loss was determined by comparing the spectra of the transmitted and received responses. The loss due to 66 absorption in the quartz crystal as well as the attenuation in the water bath was compensated in the final insertion loss calculation (Cannata et al., 2003). The single element silicon etched lens transducer consisted of cavity diameter of 645 µm and piezoelectric layer of 4 µm ZnO film produced by the fabrication procedure as described previously. The measured pulse-echo waveform and normalized frequency spectrum of the fabricated transducer is shown in Figure 4.7. We can find that the center frequency of the transducer is 330 MHz, and -6 dB bandwidth is determined to be 21 % as well as -20 dB pulse length is 19 ns. The transducer is focused at 410 µm and its f- number is 0.64. Additionally, the insertion loss is measured and compensated to be 68 dB. Figure 4.7: Measured pulse-echo waveform (solid line) and normalized frequency spectrum (dashed line) for ultrahigh frequency silicon lens transducer. 67 It is noted that the length of coaxial cable affects the measurement of the pulse- echo response because it limits the performance of high frequency transducer. In order to avoid the cable loading effect (Choi et al., 2011), we did not use the cable in our test. However, it is good to know how the cable length affects the measurements. Table 4.2 lists the measured results of the silicon lens transducer using different cable length. When we use longer cable, basically the measured center frequency becomes lower. Additionally, the bandwidth and pulse length also change with different cable length. Table 4.2: Effect of coaxial cable length on the measurements of the pulse-echo response. Cable length (cm) f c (MHz) BW (%) PL (ns) 0 330 21 19 11 212 24 26 22 148 37 31 4.4 Microparticles manipulation Since the transducer has ultrahigh center frequency and tightly focused, it is perfectly suited for acoustic trapping. To observe the manipulation of a single microparticle using silicon lens UHF transducer, the experimental system for targeted particle trapping was applied as shown in Figure 2.10. In this work, the polystyrene microspheres (Polysciences, Warrington, PA) with mean diameter of 5 µm were used as the targeted particle. Microspheres were loaded into the water chamber with an 68 acoustically transparent mylar film at its bottom. The transducer was mounted on a three- axis motorized linear stage, and the stage was controlled by a customized LABVIEW program. A function generator and a 50 dB power amplifier were used to generate sinusoidal burst waveforms for driving transducer. The motion of the trapped particle was recorded via a CMOS camera connected to the microscope. In order to trap a single 5 µm microsphere effectively, the sinusoidal burst was set as following parameters: the excitation frequency was 330 MHz with the pulse repetition period of 1 ms, the duty factor of 0.1 % and the peak-to-peak voltage amplitude of 32 V. After driving by the sinusoidal burst, the transducer is able to trap a single microsphere within the field of view of the microscope. As shown in Figure 4.8, the motion of an individual trapped 5 µm polystyrene particle could be manipulated by the silicon lens UHF transducer without any contact. The result recommends that this silicon lens ultrahigh frequency transducer is capable of manipulating particles at the cellular level. The device could be a useful tool to manipulate single cells or molecules for a wide range of biomedical applications. 69 Figure 4.8: A trapped 5 µm polystyrene microsphere motion is handled by the silicon lens ultrahigh frequency transducer. The trapped particle is present inside a blue circle while a red dot is given as a reference point to show the change in the particle location along with the movement of the transducer. 70 4.5 Conclusion In summary, we have demonstrated the fabrication of ultrahigh frequency silicon lens transducer and its application for single beam acoustic tweezer. The silicon lens is made based on isotropic wet etching technique without machining. Using optimum etching parameters and opening size of mask, we are able to make a silicon lens having deserved geometry with smooth surface of the etched cavity. It shows that a fabricated silicon lens transducer has center frequency of 330 MHz with -6 dB bandwidth of 21 %, and insertion loss is 68 dB. The capability of this silicon lens ultrahigh frequency transducer in manipulating a single 5 µm microsphere has been described. As the size of the trapped particle is at the cellular level, this 330 MHz ultrasonic silicon lens transducer has great potential to be a single cell manipulator for wide range of applications in biomedical and chemical sciences including investigating intercellular adhesion and cell stimulation. 71 Chapter 5 PMN-PT-PZT Composite Films for High Frequency Transducer Applications We have successfully fabricated xPMN-PT-(1-x)PZT, where x is 0.1, 0.3, 0.5, 0.7 and 0.9, thin films with a thickness of approximately 9 μm on platinized silicon substrate by employing a composite sol-gel technique. X-ray diffraction analysis and scanning electron microscopy revealed that these films are dense and creak-free with well- crystallized perovskite phase in the whole composition range. The dielectric constant can be controllably adjusted by using different compositions. Higher PZT content of xPMN- PT-(1-x)PZT films show better ferroelectric properties. A representative 0.9PMN-PT- 0.1PZT thin film transducer is built. It has 200 MHz center frequency with a -6 dB bandwidth of 38 %. The measured two-way insertion loss is 65 dB. 5.1 Introduction A number of significant advances in high frequency ultrasonic transducers have been made in recent years. To achieve very high spatial resolution in non-destructive evaluation of materials and non-invasive imaging of superficial anatomic structures in humans as well as small animals, very high frequency (>100 MHz) ultrasonic transducers have received growing attention lately (Shung et al., 1996; Lukacs et al., 2000; Coleman et al., 2004). Moreover, very high frequency (VHF) transducers can be also applied to the manipulation of microparticles and biological cells (Takeuchi et al., 1994; Lee, J. et al., 2005; Lee, J. et al., 2009). However, it is challenging and time consuming to build such 72 high frequency transducers with traditional top-down approach in which piezoelectric ceramics or crystals are lapped down and mechanically machined to the order of micrometers. Therefore, piezoelectric thin film technology provides an alternative solution in simplifying the fabrication process of high frequency transducers. Piezoelectric thin film techniques (Zhou, Q. F. et al., 2011), such as screen- printing (Thiele et al., 2001; Kuscer et al., 2009), tape-casting (Levassort et al., 2003), aerosol deposition (Lebedev et al., 2002; Lau et al., 2010), composite sol-gel (Barrow et al., 1995; Dorey et al., 2004), electrophoretic deposition (Chen et al., 2009), and ink-jet printing (Wang, T. M. et al., 2005) have been shown to be capable of producing piezoelectric films (<50 μm) without compromising the piezoelectric properties in comparison to those of bulk materials. Among these techniques, the composite sol-gel approach has been successfully used to fabricate several different types of piezoelectric thin films from which high frequency ultrasound transducers are built. They include lead zirconate titanate (PZT) (Zhu et al., 2008), lead magnesium niobium-lead titanate (PMN- PT) (Zhu et al., 2010) and potassium sodium niobate/bismuth sodium titanate (KNN/BNT) ceramics (Sien Ting et al., 2011). The composite sol-gel technique is a modified version of the sol-gel method, which is developed by dispersing ceramic powder into sol-gel solution and followed by optimized pyrolysis and annealing steps (Barrow et al., 1995; Zhou, Q. F. et al., 2011). This approach has been verified to produce piezoelectric films in the range of 1 to 50 μm without cracks due to strongly bonded network between the sol-gel and the ceramic particles. The composite sol-gel technique is unique and desirable. It not only is capable of producing crack-free thin films, but also has a number of advantages, i.e., cost-effective, low annealing temperature, flexible and 73 good control of film stoichiometry. Therefore, composite sol-gel approach was selected for this work. It is known that the dielectric constant of the piezoelectric material plays an important role in electrical impedance matching of transducers to the electronic components. Thus it would be desirable if the dielectric constant of a piezoelectric film can be tuned without sacrificing its piezoelectric performance. Here, we propose a simple and efficient approach to prepare x(0.65PMN-0.35PT)-(1-x)PZT (xPMN-PT-(1-x)PZT) composite films with controllable dielectric constant. The strategy is to mix PMN-PT (65-35) and PZT materials in different ratios to synthesize xPMN-PT-(1-x)PZT piezoelectric films with the composite sol-gel technology. This approach should allow the dielectric constant to be tunable since the intrinsic dielectric constants of PMN-PT and PZT phases are different. It has been reported that PMN-PT/PZT composite films have a higher dielectric constant than PZT films because of the high permittivity of the PMN-PT phase (Dorey et al., 2004). However, there has been no report to date either on making tunable dielectric constant with different composition of PMN-PT/PZT ceramic films or on using xPMN-PT-(1-x)PZT films as the active element for high frequency ultrasonic transducer fabrication. In this study, we demonstrate the dielectric and piezoelectric properties of xPMN- PT-(1-x)PZT films when x= 0.1, 0.3, 0.5, 0.7 and 0.9 prepared with the composite sol-gel technique. The results show that the dielectric constant can be controllably adjusted with different compositions of PMN-PT/PZT composite films. The performance of a high frequency ultrasonic transducer fabricated from such a xPMN-PT-(1-x)PZT film as the active element has been evaluated. The results are reported. 74 5.2 Experiment 5.2.1 Preparation and characterization of xPMN-PT-(1-x)PZT composite films In this work, a sol-gel composite method was used to prepare PMN–PT/PZT composite ceramics. Figure 5.1 shows the flowchart of the fabrication procedure. Specifically, the PMN-PT as well as PZT precursor sol-gel solution was synthesized using lead acetate trihydrate (Aldrich Chemical Co., Milwaukee, WI), zirconium n- propoxide (Aldrich Chemical Co.), titanium isopropoxide (Aldrich Chemical Co.), magnesium ethoxide (Aldrich Chemical Co.), and niobium ethoxide (Chemat Technology, Northride, CA) as the raw materials with 2-methoxyethanol (Aldrich Chemical Co.) as the solvent. The xPMN-PT-(1-x)PZT composite solution (where x is 0.1, 0.3, 0.5, 0.7 and 0.9) were prepared by ball milling a mixture of PMN-PT as well as PZT powder and precursor solution for 24 hours. The mass ratio of powder to solution was 1:4. Next, the prepared composite solution was deposited on a Pt(111)/Ti/SiO 2 /Si(100) substrate by spin coating at 2000 rpm for 30 seconds. After deposition, each layer was subjected to a two- stage pyrolysis sequence to drive out the solvent and decompose organic compounds. A 1.5-minute heat treatment at 200°C was followed by another 1.5-minute heat treatment at 400°C in air. Subsequently, each layer was annealed at 700°C for 60 seconds with rapid thermal annealing. The above process was repeated until the desired film thickness was achieved. Finally, the xPMN-PT-(1-x)PZT thin films were sintered at 750°C for one hour at air atmosphere to increase the film crystallinity. The crystallized structure of PMN-PT/PZT composite films was examined by X- ray diffractometry (XRD, Ultima IV, Rigaku, Tokyo, Japan). The thickness and 75 morphology of the composite film were observed by a scanning electron microscope (SEM, S-3500N, Hitachi, Tokyo, Japan). The chromium/gold top electrodes with a dimension of 0.5x0.5 mm 2 were sputtered through a shadow mask onto the films to evaluate electrical properties. The dielectric properties were measured with an Agilent 4292A impedance analyzer. A ferroelectric test system was used to acquire hysteresis loops. Figure 5.1: Fabrication procedure of the xPMN-PT-(1-x)PZT composite films using composite sol-gel method. 76 5.2.2 Fabrication and evaluation of transducer The xPMN-PT-(1-x)PZT composite film transducer was fabricated from a combination of the silicon etching process and conventional transducer technology (Cannata et al., 2003; Sien Ting et al., 2011). Figure 5.2 illustrates the fabrication procedures. The procedures details are described as follows. First, a Cr/Au (500 Å /1000 Å) thin layer was sputtered on the top surface as the electrode. A very lossy conductive epoxy (E-solder 3022) was then applied on the films with acoustic impedance around 5.5 MRays as backing materials with the aid of an adhesion promoter (AP-131). This backing layer was centrifuged at 3000 rpm for 15 minutes to increase acoustical impedance and ensure conductivity over the entire active element. After centrifugation, the acoustic impedance can be increased to 5.9 MRayls because of the improvement of density of E- solder 3022. After curing at room temperature overnight, the backing layer was lapped to around 1 mm. The sample was then diced into 0.25x0.25 mm 2 squares. Each square carries a single thin film element. The film with the support of the backing layer was then peeled off from the silicon substrate by dipping into a 20% concentrated KOH solution at 80°C for 5 to 10 minutes. After the peeling off process, the lead wire was connected to the backing layer with an additional amount of conductive epoxy. A brass housing was placed concentrically with the single element device, and an insulating epoxy was then poured into the void between the housing and the device. Next, a layer of Cr/Au was sputtered across the transducer face to form the ground plane connection. A 1.5 µm thick parylene layer with acoustic impedance of 2.6 MRayls was vapor-deposited on the front face of the transducer, serving as an acoustic matching layer and also a protection layer. Lastly, the transducer was housed in a SMA connector. 77 Figure 5.2: Fabrication process for the PMN-PT-PZT composite film transducer. Step1: A Cr/Au electrode was sputtered on the composite film and E-solder 3022 was cast on the electrode side as the backing. Step 2: The sample was diced into small posts and the substrate was removed by dipping in KOH solution at 80°C. Step 3: A lead wire was connected to the backing layer with conductive epoxy. Placing a brass housing concentrically with the element and then filling the insulating epoxy between the housing and the element. Step 4: Sputtering a Cr/Au electrode across the transducer face to form the ground plane connection. A vapor-deposited parylene thin layer was used as the matching layer. Finally, the transducer was housed in the SMA connector. 78 The fabricated transducer was tested in a de-ionized water bath at room temperature using a pulse-echo arrangement by reflecting the transmitted signal off a polished x-cut quartz target placed at the far field of the transducer. A Panametrics model 5910PR pulser/receiver was used to excite the transducer with an electrical impulse at 200Hz repetition rate and 50 Ω damping. The reflected waveform was received and digitized by a 1GHz LC534 LeCroy oscilloscope with 50Ω coupling. The frequency spectrum can be generated via Fast Fourier Transform (FFT) of the received time-domain signal. The center frequency and the bandwidth at -6dB were determined from the frequency spectrum. The insertion loss was calculated by comparing the frequency spectrum of the transmit and receive responses. Compensation was applied for the loss due to the transmission into the quartz and the attenuation in the water bath. 5.3 Results and discussion The XRD pattern of xPMN-PT-(1-x)PZT composite films on a Pt/Si wafer where x is equal to 0.1, 0.3, 0.5, 0.7 and 0.9 are shown in Figure 5.3. It demonstrates that pure perovskite phase of PMN-PT/PZT ceramic films were synthesized successfully in the whole composition range from the composite sol-gel method. There is no trace of pyrochlore phase presented in the XRD patterns. The well-crystallized perovskite phase indicates a production of high-quality film with good piezoelectric properties. Figure 5.4 shows the SEM micrograph of the PMN-PT/PZT composite film. It is observed that the film is dense and crack-free on the substrate. The film thickness is around 9 μm and the agglomerated size of the grains is in the range of 200 nm to 500 nm. 79 Figure 5.3: XRD patterns of xPMN-PT-(1 −x)PZT composite films while x is (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7 and (e) 0.9. 80 Figure 5.4: SEM cross-sectional micrograph of the PMN-PT/PZT composite film. The dielectric constant and dielectric loss of the composite films were measured using an impedance analyzer. The measured dielectric losses of xPMN-PT-(1-x)PZT films are below 0.05 between 1 kHz and 1 MHz in the whole compositions. Figure 5.5 shows the frequency dependence of dielectric constants for xPMN-PT-(1-x)PZT films of different compositions. It is worth noting that the dielectric constant is adjustable with different compositions; for instance, the dielectric constant increases with higher PMN- PT content. The change in dielectric constant can be attributed to different intrinsic 81 permittivities of PMN-PT and PZT phases which have different polarisability in PMN- PT/PZT composite films (Bellaiche et al., 1999). Since PMN-PT has higher dielectric constant, the polarisability is enhanced with high content of PMN-PT which leads to an increase of dielectric constant in xPMN-PT-(1-x)PZT films. These results suggest that tunable dielectric constant can be realized by controlling different compositions of PMN- PT/PZT ceramics via the composite sol-gel method. Figure 5.5: Frequency dependence of dielectric constants of xPMN-PT-(1-x)PZT composite films where x is 0.1, 0.3, 0.5, 0.7 and 0.9. 82 The ferroelectric hysteresis loops of xPMN-PT-(1-x)PZT thin films of different compositions are shown in Figure 5.6, and the insert is the remnant polarization (Pr) as a function of compositions. The ferroelectric hysteresis loops exhibit a good symmetry for all compositions. Higher Pr are observed at higher PZT contents of these xPMN-PT-(1- x)PZT ceramic systems. Due to the weak polarization of PMN-PT phase in PMNPT/PZT composite films (Dorey et al., 2004), we propose that it slows down the ferroelectric transition and kinetics; as a result, the ferroelectric switching is hindered. Hence, the decrease in polarization can be attributed to the increasing portion of PMN-PT, which restricts or hinders the ferroelectric contribution from the PZT content. Figure 5.6: Polarization-electric field hysteresis loops of xPMN-PT-(1-x)PZT composite films where x is 0.1, 0.3, 0.5, 0.7 and 0.9. The insert: The remnant polarization (Pr) as a function of compositions. 83 A single element transducer was fabricated from a 0.9PMN-PT-0.1PZT thin film as described (see Figure 5.7). The fabricated transducer was poled at 100°C for 10 minutes at 120 V in air to elicit the piezoelectric response. Figure 5.8 shows the measured pulse-echo waveform and normalized frequency spectrum of the transducer. The central frequency is 200 MHz and the -6 dB bandwidth is around 38%. The PMN-PT-PZT composite films exhibit good dielectric and ferroelectric properties with comparable transducer’s performance to other reported composite sol-gel films transducers (see Table 5.1). The measured insertion loss of this transducer is 65 dB. The results show the capability of this tunable dielectric constant xPMN-PT-(1-x)PZT composite film for high frequency (>100MHz) biomedical applications, such as ultrasonic biomicroscopy or acoustic tweezer. Figure 5.7: A photograph of the 0.9PMN-PT-0.1PZT composite film transducer. 84 Figure 5.8: Measured pulse-echo waveform (solid line) and normalized spectrum (dashed line) for 0.9PMN-PT-0.1PZT composite film transducer. 85 Table 5.1: Measured properties and performance of composite sol-gel films very high frequency transducers. Transducer f c (MHz) BW (%) P r (µc/cm 2 ) ε r (at 1KHz) 9 μm PMN-PT-PZT 200 38 27 2534 10 μm PZT (Zhu et al., 2008) 156 50 42 1925 12 μm PMN-PT (Zhu et al., 2010) 110 64 30 3326 5 μm KNN/BNT (Sien Ting et al., 2011) 193 34 24 848 5.4 Conclusion In summary, we have fabricated xPMN-PT-(1-x)PZT (where x is 0.1, 0.3, 0.5, 0.7 and 0.9) thin films with a thickness around 9 μm on platinum-buffered Si substrate via the composite sol-gel method. The results show that these films are crack-free and dense with well-crystallized perovskite phase in the whole composition range. We can controllably adjust the dielectric constant of PMN-PT/PZT films by varying the composition. Additionally, we verified that higher PZT content yields better ferroelectric properties in these xPMN-PT-(1-x)PZT composite films. A high frequency single element transducer utilizing 0.9PMN-PT-0.1PZT thin film was built. The measured center frequency is 200 MHz with a -6 dB bandwidth of 38 % and the insertion loss is 65 dB. This type of xPMN-PT-(1-x)PZT film transducers may satisfy current needs of VHF biomedical applications. 86 Chapter 6 Summary and Future Work 6.1 Summary Three different types of high frequency focused transducers were developed and applied for single beam acoustic tweezer applications. First, a 60 MHz focused needle transducer (<1 mm) was fabricated using press-focusing technique. In vitro UBM image of the rabbit eye has been obtained using this press-focused needle transducer. Additionally, a single beam acoustic trapping experiment was performed to manipulate 15 μm polystyrene microsphere using this type of transducer. Trapping of targeted particle size smaller than the ultrasonic wavelength was observed, which indicates the Rayleigh particles trapping may be feasible by single beam acoustic transducers. Potential applications of these devices include minimally invasive measurements of retinal blood flow and single beam acoustic trapping of microparticles. Ultra high frequency ( ≥ 200M Hz) focused transducers are required to generate tightly focusing acoustic microbeams for effectively trapping of particles or cells with a size of a few microns. The ultrahigh frequency self-focused ZnO transducer was developed and fabricated using a simple technique, where the transducer is made by sputtering ZnO film on a spherically shaped surface of the aluminum backing material. The transducer has center frequency at 204 MHz with -6 dB bandwidth of 27 % and insertion loss of 61 dB. An individual 10 μm microsphere was effectively manipulated in two dimensions using this type of transducer. Another ultrahigh frequency focused transducer based on silicon lens design has been developed for single beam acoustic tweezer application. The silicon lens is made using isotropic wet etching technique, 87 where the smooth surface as well as deserved geometry of a lens cavity can be obtained through suitable lithography and etching parameters. A 330 MHz silicon lens transducer was fabricated. The -6 dB bandwidth and the measured insertion loss of this transducer are 21 % and 68 dB, respectively. Micro-particle trapping experiment was carried out to demonstrate that the transducer can manipulate a single microsphere as small as 5 µm. As the size of the trapped particle is at the cellular level, the realization of microbeam acoustic tweezers paves the way for exploring new biomedical applications of acoustic tweezers including intercellular kinetics studies and cell fusion control. Additionally, we propose a simple and efficient approach to prepare xPMN-PT- (1-x)PZT composite films with controllable dielectric constant for high frequency ultrasonic transducer applications. It is known that the dielectric constant of the piezoelectric material plays an important role in electrical impedance matching of transducers to the electronic components. Thus it would be desirable if the dielectric constant of a piezoelectric film can be tuned without sacrificing its piezoelectric performance. We have fabricated xPMN-PT-(1-x)PZT (where x is 0.1, 0.3, 0.5, 0.7 and 0.9) thin films on platinum-buffered silicon substrate by employing a composite sol-gel technique. These films are crack-free and dense with well-crystallized perovskite phase in the whole composition range. The dielectric constant can be controllably adjusted by using different compositions. A representative high frequency single element transducer utilizing 0.9PMN-PT-0.1PZT thin film was built. The measured center frequency is 200 MHz with a -6 dB bandwidth of 38 % and the insertion loss is 65 dB. This type of xPMN-PT-(1-x)PZT film transducers may satisfy current needs of very high frequency biomedical applications, such as ultrasonic biomicroscopy or acoustic tweezer. 88 6.2 Future work 6.2.1 High frequency focused transducers In this dissertation, we have proposed three methods to make high frequency focused transducers that can apply for single beam acoustic tweezer applications. However, further investigation in device fabrication, materials and testing still are required for advanced acoustic tweezer applications. First of all, in order to trap much smaller particles effectively for further applications, the center frequency of the transducers still need to be increased. One of our future goals is to make focused transducers with center frequency higher than 500 MHz. To make such ultrahigh frequency focused transducers, we need to select proper materials and method to achieve the goal. Although we have demonstrated that ZnO thin film is suited for ultrahigh frequency transducers, the other piezoelectric films could provide better properties to make ultrahigh frequency transducers. For example, PZT and PMN-PT films have much better electromechanical properties and high dielectric constants, both of them are able to fabricate in the range of 1 µm to 10 µm using composite sol-gel technique. Hence, different silicon lens ultrahigh frequency transducers based on PZT and PMN-PT films will need to be studied. Additionally, the insertion loss of the UHF transducers reported in this dissertation are still high and should be reduced. The insertion loss will be further improved in the future for better performance by introducing a matching layer of more optimized acoustic impedance (Zhou, Q. et al., 2009). Moreover, we could focus on investigating new and novel design as well as fabrication techniques to make ultrahigh frequency focused transducers with better performance. For example, we could explore the polydimethylsiloxane (PDMS) concave microlens (Feng et al., 2012) for ultrahigh 89 frequency focused transducer applications, where mechanically flexible and biocompatible PDMS with low acoustic impedance (1~1.9 MRayls) (Zhuang et al., 2008) should be well-suited for ultrasound transducers. Besides the materials and fabrication issues, the testing instruments and methods to evaluate transducer performance may also need to be upgraded and modified. 6.2.2 Single beam acoustic tweezers Ray acoustic trapping model was proposed by Dr. Lee et al. (Lee, J. et al., 2005) to demonstrate the trapping mechanism of the single beam acoustic tweezers in Mie regime, where the particle size is greater or close to the ultrasound wavelength. However, we have shown that it is possible to acoustically trap particles with a size smaller than the ultrasonic wavelength, which indicates the Rayleigh particles (D<<λ) trapping may be feasible with single beam acoustic tweezers. The detailed mechanism should be studied in the future work. Ultrahigh frequency focused transducers can provide tightly focused microbeam and have shown the capability to trap and manipulate particles at the cellular level. Several potential applications using this microbeam technique can be considered. Basically, similar to optical tweezers, we can apply single beam acoustic tweezers to confine or constrain single cells, as well as to assemble, organize, locate, sort and modify them (Zhang et al., 2008). For example, one of the potential applications is to investigate intercellular adhesion processes. The microbeam acoustic tweezers can be exploited as a remote sensing tool to examine the adhesion process of leukocytes (or white blood cells) 90 to vascular endothelial cells because of its non-invasiveness and moderate energy use (Lee, J. et al., 2011). Another example is to incorporate with microfluidic devices as the cell sorting tool. Actually, in our center, we have demonstrated the sorting of microfluidic droplets with a size of 50 µm and 100 µm using a 30 MHz focused transducer recently (Lee, C. et al., 2012). 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Abstract (if available)
Abstract
Contactless particle trapping and manipulation have found many potential applications in diverse fields, especially in biological and medical research. Among the various methods, optical tweezers is the most well-known and extensively investigated technique. However, there are some limitations for particle manipulation based on optical tweezers. Due to the conceptual similarity with the optical tweezers and recent advances in high frequency ultrasonic transducer, a single beam acoustic tweezer using high frequency (≥ 20 MHz) focused transducer has recently been considered, and its feasibility was theoretically and experimentally investigated. ❧ This dissertation mainly describes the development of high frequency focused ultrasonic transducers for single beam acoustic tweezers applications. Three different types of transducers were fabricated. First, a 60 MHz miniature focused transducer (<1 mm) was made using press-focusing technique. The single beam acoustic trapping experiment was performed to manipulate 15 µm polystyrene microspheres using this transducer. In vitro ultrasonic biomicroscopy imaging on the rabbit eye was also obtained with this device. Second approach is to build a 200 MHz self-focused ZnO transducer by sputtering ZnO film on a curved surface of the aluminum backing material. An individual 10 µm microsphere was effectively manipulated in two dimensions by this type of transducer. Another ultrahigh frequency focused transducer based on silicon lens design has also been developed, where a 330 MHz silicon lens transducer was fabricated and evaluated. Microparticle trapping experiment was carried out to demonstrate that silicon lens transducer can manipulate a single microsphere as small as 5 µm. The realization of single beam acoustic tweezers using high frequency focused transducers can offer wide range of applications in biomedical and chemical sciences including intercellular kinetics studies and cell stimulation. ❧ Additionally, we propose a simple and efficient approach to prepare xPMN-PT-(1-x)PZT (where x is 0.1, 0.3, 0.5, 0.7 and 0.9) composite films with controllable dielectric constant that offers better performance for high frequency ultrasonic transducer applications. A 200 MHz single element transducer utilizing 0.9PMN-PT-0.1PZT thin film was built. This type of xPMN-PT-(1-x)PZT film transducers may satisfy current needs of very high frequency biomedical applications, such as ultrasonic biomicroscopy or acoustic tweezers.
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Creator
Hsu, Hsiu-Sheng
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Core Title
Development of high frequency focused transducers for single beam acoustic tweezers
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
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Materials Science
Publication Date
11/20/2012
Defense Date
10/18/2012
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acoustic tweezers,focused,microparticles manipulation,OAI-PMH Harvest,sol-gel,transducers,ultrahigh frequency,ultrasound
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English
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Shung, Kirk Koping (
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), Goo, Edward K. (
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), Yen, Jesse T. (
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), Zhou, Qifa (
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hsiusheh@usc.edu,hsiusheng@gmail.com
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Tags
acoustic tweezers
focused
microparticles manipulation
sol-gel
transducers
ultrahigh frequency
ultrasound