Local Piezoelectric Behavior in PZT-Based Thin Films for Ultrasound Transducers
Open Access
- Author:
- Griggio, Flavio
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 11, 2011
- Committee Members:
- Susan E Trolier Mckinstry, Dissertation Advisor/Co-Advisor
Susan E Trolier Mckinstry, Committee Chair/Co-Chair
Thomas Nelson Jackson, Committee Member
Clive A Randall, Committee Member
Thomas R Shrout, Committee Member - Keywords:
- PZT
thin films
ultrasound transducer
piezo-force microscopy - Abstract:
- Piezoelectric microelectromechanical systems (MEMS) are currently used in inkjet printers and precision resonators; numerous additional applications are being investigated for sensors, low-voltage actuators, and transducers. This work was aimed at improving piezoelectric MEMS by taking two approaches: 1) identifying factors affecting the piezoelectric response of ferroelectric thin films and 2) demonstrating integration of these films into a high frequency array transducer. It was found that there are several key factors influencing the piezoelectric response of thin films for a given material composition. First, large grain size improves the piezoelectric response. This was demonstrated using chemical solution deposited lead nickel niobate – lead zirconate titanate (0.3)Pb(Ni<sub>0.33</sub>Nb<sub>0.67</sub>)O<sub>3</sub> - (0.7)Pb(Zr<sub>0.45</sub>Ti<sub>0.55</sub>O<sub>3</sub>), (PNN-PZT) ferroelectric thin films. It was shown that this composition allows greater microstructural control than does PZT. Dielectric permittivities ranging from 1350 to 1520 and a transverse piezoelectric coefficient e<sub>31,f</sub> as high as – 9.7 C/m<sup>2</sup> were observed for films of about 0.25 μm in thickness. The permittivity and piezoelectric response as well as extrinsic contributions to the dielectric constant increased by 14 and 12 % respectively for samples with grain sizes ranging from 110 to 270 nm. A second factor influencing the piezoelectric response is film composition with respect to the morphotropic phase boundary (MPB). The composition dependence of the dielectric and piezoelectric nonlinearities was characterized in epitaxially grown (0.3)Pb(Ni<sub>0.33</sub>Nb<sub>0.67</sub>)O<sub>3</sub>-(0.7)Pb(Zr<sub>x</sub>Ti<sub>1-x</sub>O<sub>3</sub>) thin films deposited on SrTiO<sub>3</sub> to minimize the influence of large-angle grain boundaries. Tetragonal, MPB and rhombohedral films were prepared by changing the Zr/Ti ratio. The largest dielectric and piezoelectric nonlinearities were observed for the rhombohedral sample; this resulted from a higher domain wall mobility due to a smaller ferroelectric distortion and superior crystal quality. Thirdly, changes in the mechanical boundary conditions experienced by a ferroelectric thin film were found to influence both the properties and the length scale for correlated motion of domain walls. Microfabrication was employed to release the PZT films from the Si substrate. Nonlinear piezoelectric maps, by band excitation piezoforce microscopy, showed formation of clusters of higher nonlinear activities of similar size for clamped PZT films with different microstructures. However PZT films that had been released from the Si substrate showed a distinct increase in the correlation length associated with coupled domain wall motion, suggesting that the local mechanical boundary conditions, more than microstructure or composition govern the domain wall dynamics. Release of both the local and the global stress states in films produced dielectric nonlinearities comparable to those of bulk ceramics. The second research direction was targeted at demonstrating the functionality of a one dimensional transducer array. A diaphragm geometry was used for the transducer arrays in order to benefit from the unimorph-type displacement of the PZT-SiO<sub>2</sub> layers. For this purpose, the PZT and remaining films in the stack were patterned using reactive ion etching and partially released from the underlying silicon substrate by XeF<sub>2</sub> etching from the top. Admittance measurements on the fabricated structures showed resonance frequencies at ~40 MHz for a 80 μm diameter-wide diaphragms with a PZT thickness of 1.74 μm. In-water transmit and receive functionalities were demonstrated. A bandwidth on receive of 80 % centered at 40 MHz was determined during pitch-mode tests.