Micromachined Integrated 2D Transducers for Ultrasound Imaging

Conventional ultrasound imaging produces 2D images of the human body, and these images are today routinely used for diagnostics purposes. The twodimensi images can efficiently be displayed on a computer screen and are easy to interpret, whereas 3D images are more difficult to interpret but provide additional information about the imaged objects. The added dimension in a 3D image can provide vital information and can be used for new innovative imaging techniques. However, the complexity of the underlying transducer technology increases enormously when the imaging capabilities go from 2D to 3D. New advances in the transducer technology , such as row-column addressed arrays, drastically reduce the complexity of ultrasound systems capable of 3D imaging. The row-column technology reduces the required number of electrical interconnection from N2 to 2N, where N is the number of channels in the ultrasound system. The main goal of this thesis is to develop the row-column addressed transducer array. The technology of capacitive Micromachined Ultrasound Transducers (CMUT) is used for the development platform for the row-column arrays due to the high flexibility and desired imaging properties offered by this technology.
Theoretical analysis of a CMUT is presented in the first part of thesis, including investigation of the static and dynamic properties. Multiple CMUT fabrication methods are developed, demonstrated, and used to fabricate various row-column arrays of different sizes. The area of row-column addressed arrays can be larger than conventional ultrasound transducers which introduces new challenges that has to be addressed. These challenges included increased resistance in the electrodes, and a design criterion for sufficient resistance of long row-column elements is derived and experimentally documented.
Two 92+92 4.5 MHz row-column addressed CMUT arrays are assembled into ultrasound probes. The CMUT probes are electrically characterized, long term stability test during 16 h showed no sign of dielectric charging and the electromechanical coupling coefficient is estimated to be 18.3%. The acoustical performance of the probes are characterized and 3D imaging capabilities are demonstrated at a depth of 7 cm in a cyst phantom.