Development of High Frequency Miniature Ultrasound Transducers

Abstract

Small, high frequency (≥ 10MHz) broadband ultrasound transducers are required in modern medical imaging systems to provide short range, high resolution images for studying of microstructures in soft tissues, such as the composition of small tumors or a vessel wall. The manufacturing of these probes using conventional methods, i.e. lapping and dicing, becomes difficult and expensive for high frequency applications and there is a need to produce small ultrasound transducers with low cost and high reliability. Being identified as one of the key technologies that has produced μ- scale, high volume products in a wide variery of markets such as electronics, telecommunications, automobiles, etc..., Micro-Electro-Mechanical-Systems (MEMS) technology seems to deliver a good solution.

This thesis presents the development of novel small-sized, high frequency broadband ultrasound transducers based on silicon processing borrowed from the MEMS industry. We have categorized the work in this thesis within two main themes:

1. The modeling, fabrication and characterization of silicon-polymer composite acoustic matching layers formed by MEMS techniques. Two different micromachining approaches have been employed: Deep Reactive Ion Etch (DRIE) and Anisotropic Wet Etch. The modeling of the micromachined composite materials was carried out both by analytical and finite element method (FEM) simulations. The properties of the fabricated composites were extracted via electrical impedance and pulse-echo measurements of air-backed transducers.

2. The design, fabrication and characterization of high frequency (15 MHz) broadband ultrasound transducers, where the composite materials described above were used as an intermediate layer in the stack of multiple matching layers.

The main contributions are:

1. Fabrication and testing of silicon-polymer composites made by an advanced technique, DRIE, used as matching layers for 15 MHz ultrasound transducers. A method for extracting the composite properties via inversion scheme from electrical impedance measurements of air-backed transducers with composites as single matching layers was proposed.

2. Modeling of silicon-polymer composite materials and ultrasound transducers under different circumstances, i.e. in contact with fluid/solid media. This provides the understanding of the dynamic behavior of the silicon-polymer composite, helps to define the maximum allowable period sizes and properties of the composites without influence the transducer’s performance. The modeling was carried out by analytical calculations and FEM simulations.

3. Development of an in-house anisotropic wet etching process of silicon to form well-defined size, high aspect ratio, vertical sidewall trenches. This process is benificial for constructing of many MEMS devices for various applications such as biological, optical and energy harvesting sensors.

4. Development of an in-house fabrication process to manufacture high frequency broadband ultrasound transducers based on micromachining techniques, i.e. photolithography and anisotropic wet etch of (110)-oriented silicon wafers.