Development and microfabrication of capacitive micromachined ultrasound transducers with diamond membranes


Thesis Type: Postgraduate

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

Approval Date: 2011

Student: MEHMET CEZAR

Supervisor: BARIŞ BAYRAM

Abstract:

This thesis presents the development and microfabrication of capacitive micromachined ultrasonic transducers (CMUT) with diamond membranes for the first time in the literature. Although silicon and silicon nitride (Si3N4) membranes have been generally used as the membrane material in CMUTs. These membrane materials have moderate properties that can cause damage during the operation of CMUTs. In this thesis, a new material for the membrane is introduced for CMUTs. Diamond has exceptional potential in the area of micro-nano technologies due to unrivalled stiffness and hardness, excellent tribological performance, highly tailorable and stable surface chemistry, high thermal conductivity and low thermal expansion, high acoustic velocity of propagating waves, and biocompatibility. Based on these excellent material properties, diamond is employed in the new generation CMUT structures for more robust and reliable operations. The microfabrication process of CMUT has been generally performed with either sacrificial release process or wafer bonding technique. High yield and low cost features of wafer bonding process makes it preferable for CMUT devices. In this thesis, plasma-activated direct wafer bonding process was developed for the microfabrication of 16-element 1-D CMUT arrays with diamond membranes. They were designed to operate at different resonance frequencies in the range of 1 MHz and 10 MHz with different cell diameters (120, 88, 72, 54, 44 μm) and element spacing (250, 375 μm). 1-D CMUT array devices can be used for focusing ultrasound applications. The electronic circuit for 1-D CMUT devices with diamond membranes was designed and implemented on PCB for the ultrasound focusing experiment. This electronic circuit generates continuous or burst AC signals of ± 15 V with different and adjustable phase shifting options at 3 MHz frequency. 16 elements of 72 μm 1-D CMUT array were successfully tested. Fully functional 7 elements of 1-D CMUT array are focused at an axial distance of 5.81 mm on the normal to the CMUT center plane. The CMUT array was excited using 10 Vp−p with 10 cycles sinusoidal signals at 3 MHz. The microfabrication process and focusing ultrasound of 1-D CMUT devices with diamond membranes are done successfully in this thesis.