Process development for microfabrication of phase reversal CMUT devices for structural health monitoring and development of dynamic characterization processes for MEMS applications

Abstract

If appropriately designed, Capacitive Micromachined Ultrasonic Transducers (CMUTs) offer advantageous properties such as low cost, small size, low impedance, and environmental friendliness, over piezoelectric transducers. These advantageous properties of CMUTs enable the CMUT devices to be employed in a large area of applications, such as medical applications and non-destructive testing (NDT) applications. CMUT devices and technologies that are heavily developed for medical applications also shed light on the development of CMUT devices to be used in Structural Health Monitoring (SHM) applications for civil infrastructures. Continuous monitoring of the signals produced by the sudden changes happening within civil infrastructures such as bridges or railways may give crucial information about the health of these structures. The rapid release of localized strain energy, which generates Acoustic Emission (AE) waves, is an important indicator of the state of the health of a structure. Detecting AE wave signals may give significant clues about damage formation such as impact, crack initiation, or crack growth. Because AE waves are scattered among a broad range of frequencies, sensing of such AE waves should also be done in broadband, and sensors are preferred to be highly sensitive among such band. For real-life applicable developments, it should be also considered that the environment of the real-life application may be very noisy due to many unrelated reasons, which makes employment of the CMUTs developed in a tightly controlled laboratory environment unpractical for the real-life applications. The noise may often be induced by the noise interferences that are produced by a variety of events that are not needed to be detected. To prevent misjudgments, it is important to differentiate between noise interferences and relevant AE signals, as the presence of significant noise can hinder the detectability of AE waves associated with structural damage. In this process development for CMUT prototype microfabrication study, we collaborated with a group of researchers who have introduced a new approach to designing broadband CMUTs, as well as a unique type of CMUT combination that uses phase-reversal (PR) of generated electrical current for detecting a wide range of mechanical vibration wave frequencies and reducing unwanted noise. By considering the simplest combination of two CMUT cells, the theoretical study, supported by FEM simulations, demonstrated that reversing the electrical current phase of one cell can create low-frequency and high-frequency stopbands for noise rejection, which is applicable for CMUTs operating in air damping. The primary objective of this thesis study is to develop microfabrication processes to microfabricate PR-CMUT devices to bridge the gap between theoretical design and real-world application of PR-CMUT devices. These PR-CMUT arrays that are designed for wafer-scale batch-compatible manufacturability have a flat passband in the 200-250 kHz and 200-300 kHz frequency ranges and two improved stopbands on both sides of the relevant frequency ranges. The photolithography masks, compatible material selections, and microfabrication process flows (integration processes) required for the microfabrication of these PR-CMUT devices were designed considering the capabilities of our cleanroom facility. Microfabrication of the devices was tried multiple times, and in line with the problems encountered in these processes, the microfabrication process flows were updated and the PR-CMUT devices were tried to be produced in multiple iterations. Unit processes, and multiple integration processes were developed and completed. Possible solutions to be implemented in the future microfabrication studies were determined. Additionally, dynamic characterization of individual circular geometry CMUT membranes were explored using a ZYGO Optical Profilometer. With this measurement tool (ZYGO), it is possible to measure CMUT device membrane displacements precisely when the membrane of the CMUT device is moving (vibrating) dynamically. Results obtained from ZYGO Optical Profilometer tool were compared with the impedance analyzer results. It was shown that the resonance frequency of a circular membrane CMUT device can be observed with the ZYGO Optical Profilometer. Furthermore, based on the conclusions from the studies in this thesis, future studies are suggested for further development towards realization and characterization of these PR-CMUT MEMS (MicroElectroMechanical System) devices.