Design and microfabrication of annular concentric capacitive micromachined ultrasonic trancducer

Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Electrical and Electronics Engineering, Turkey

Approval Date: 2014


Supervisor: BARIŞ BAYRAM


In this thesis, a novel concentric annular CMUT design has been fully designed. All design parameters are determined based on the analytical expressions based on CMUT theory. Each CMUT design consists of 4 concentric disc-shaped elements, having equal membrane areas. Each element features hexagonal CMUT cells having side lengths of 50 μm up to 80 μm. The CMUT design features fully-metallized membranes for robust operation. The novel CMUT design presented in this thesis is a state-of-the-art design based on microfabrication using direct wafer bonding technology. A complete microfabrication process flow has been developed for the realization of the CMUT design. Diamond coating on the silicon wafer has been characterized in terms of residual stress via Raman Spectroscopy measurements, and viability as a membrane material has been shown. . These measurements result in stresses between 385 compressive stresses up to 364 tensile stress regardless of the type of diamond and underlying silicon or silicon dioxide layer. An interlayer of PECVD-SiO2 is used on the diamond layer. Rapid thermal annealing (RTA) is used to improve the quality of the oxide layer, which is characterized thoroughly using a V-VASE ellipsometer. A surface activation for direct wafer bonding is developed based on a combination of UV/Ozone treatment followed by O2-plasma-activation using RIE. A successful bonding is achieved on two flat wafer surfaces. Due to lack of some essential cleanroom capabilities, a fully functional prototype could not be realized in our work. However, it is straightforward to microfabricate the CMUT based on the outcome of this thesis in the presence of a cleanroom having the essential equipment and setup.