Micromachined Integrated Transducers for Ultrasound Imaging

The purpose of this project is to develop capacitive micromachined ultrasonic transducers (CMUTs) for medical imaging. Medical ultrasound transducers used today are fabricated using piezoelectric materials and bulk processing. To fabricate transducers capable of delivering a higher imaging resolution it is however necessary to develop new fabrication methods that allows fabrication of transducer elements with smaller dimensions.

By using microfabrication technology it is possible to push the dimensions down and provide higher design flexibility. This project is part of a large ultrasound project and collaboration with a lot of partners to improve medical ultrasound imaging. The focus in this part of the project is to design, fabricate and characterize 1D CMUT arrays.

Two versions of 1D transducers are made, one at Stanford University and one at DTU. Electrical and acoustical characterizations are carried out successfully for both types of arrays. The arrays made at Stanford is found to suffer from low breakdown voltage of the supporting oxide and was not useful for medical imaging.

The arrays made at DTU are used for various tests, both of the design, performance, possible packaging, and post-processing. The electrical characterization shows serious charging effects in the device which is shown to be reduced by reversing the bias polarity. Furthermore, the wirebonding and glob top packaging scheme shows to cause loose connection for several elements.

This is still under investigation, but two possible solutions are suggested. Two devices are assembled into probe handles and initial acoustical characterizations are promising. Even though the sensitivity is currently low, images are produced with recognizable features both on phantoms and volunteers.

It can be mentioned that a -6 dB fractional bandwidth of 100-110 % is measured.