Integration of Electronics and Mechanics in Next Generation Ultrasound Transducers in Medical Imaging

Abstract

Capacitive micromachined ultrasonic transducer (CMUT) has been emerging technology in the field of medical imaging after its first demonstration in early 90s. CMUTs are fabricated on silicon substrates using micromachining techniques. They can generate high frequencies due to the smaller dimensions feasible with microfabrication. CMUTs for immersion applications could be easily designed to provide fractional bandwidths exceeding 100%. Therefore, they are promising for applications like intravascular ultrasound (IVUS) where high resolution is required at short range. In this domain it may perform better than the existing piezoelectric ultrasound transducers. But there are certain issues that need to be taken care of before they can be used as commercial ultrasound transducers. One is echoes due to the acoustic wave propagation in the silicon substrate, and another is resonances caused by cross coupling between the neighboring elements in the CMUT arrays.

In this thesis we investigate an acoustic backing structure that is to be added on the back side of the CMUT. The backing should absorb incident acoustic waves across a wide frequency band, and it must effectively suppress structural resonances and vibrations. To achieve this, the backing of a CMUT should have an acoustic impedance that matches with the silicon substrate, and it should be lossy. A good candidate for the backing is a tungsten filled epoxy. If the backing structure is thick enough, it will absorb the acoustic wave reflected back to the transducer and thus will remove any trailing echoes. In many cases the transducer is intended for applications where there is little room for a thick backing. Various approaches has been suggested to reduce the extra volume and weight added by a thick absorbing backing, one approach is to use a grooved backing structure. The grooves at the bottom of the backing provide an extra attenuation by scattering the waves in different directions so that a thinner backing would be sufficient. The scattering removes power from the specular reflection from the back surface which otherwise degrades the image quality. It has been shown that this type of structure reduces the specular reflection for a range of frequencies. When CMUTs are used in practical applications, the propagation of waves from a fluid medium into the backing or vice versa is blocked to some degree by total reflection, except for a range of steering angles around broadside. This is due to the difference in acoustic velocities of silicon and the fluid medium. This blocking is accompanied by the generation of surface waves in the silicon substrate, which also may impact the imaging and therefore must be controlled. In the present work we investigate the acoustic signal transmitted into the backing relative to the signal transmitted into the fluid medium when CMUT arrays on top of the silicon substrate are excited. This gives us an estimation of how much attenuation is needed from the acoustic backing layer.

The purpose of the grooves on the backing structure is to provide an attenuation due to scattering which is in addition to the absorption loss in the epoxy tungsten composite. Experiments performed on the grooved backing structures show that such structures provide a satisfactory performance for the waves traveling normal to the bottom of the backing structure. But the acoustic waves propagating into the backing structure may not always be normal to the bottom of the structure. Therefore, the performance of the grooved backing structure is also studied for the waves traveling in oblique directions to the bottom surface of the backing. From our simulations we know that the performance of grooved structures for oblique incidence is not significantly different from that of normal incidence when the incident angle is within ±30◦ to the surface normal to the bottom of the structure. We have also shown that for a CMUT-backing stack immersed in a fluid medium, waves that reach into the backing from the fluid medium with significant power mostly lie within this range of angles. It means that a CMUT transducer with grooved backing structure always provide extra attenuation for the waves propagating into the substrates when it is used in practical applications.

In this work we have also investigated acoustic properties of some polymers. The polymers, RTV 615 and Sylgard 160, have acoustic properties such that a thin layer of these polymers could be used to minimize the acoustic cross coupling in the CMUT arrays significantly. These materials could also be used as lens material for CMUT transducers together with stiff polymers such as TPX.