Investigation of Element Variations in Ultrasound Transducer Arrays by Electrical Impedance Measurements
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The use of ultrasonic transducer array for medical applications has experienced a revolution in recent years. An ultrasonic transducer array is a transducer that contains a number of individually connected elements. Variations of the individual elements from their ideal state have significant effects on the performance of the linear transducer arrays. Due to practical requirements, the study in this thesis mainly focused on investigating the variance in the fabricated transducer structures. Single-element transducers using different matching layer materials were fabricated and compared. Electrical impedance and pulse-echo test were performed. Linear arrays consisting of only piezoelectric material were fabricated and characterized. Linear arrays consisting of a piezoelectric material and a DML (Dematching Layer) substrate were also fabricated and characterized. The structure of transducer arrays varied from element to element as prior expected due to fabrication process. Different sources causing variance in electrical impedance of each transducer structures were extensively investigated. Modeling such as 1D and 2D FEM simulations were build and compared to the experimental observations. More intriguingly, a novel bonding technique named SLID (Solid-Liquid Interdiffusion) to assembly stacks of piezoelectric ultrasonic transducers was also implemented and characterized. The performance of the transducer using this novel bonding method will be compared to that of the transducer using conventional epoxy bonding method. Advantages and disadvantages of both bonding methods were presented with simulations and characterization results. Based on these measurement results, essential recommendations was pointed out that SLID bonding method would be a potential bonding one for implementing high frequency transducers.