Performance of Switched Mode Arbitrary Excitation using Harmonic Reduction Pulse Width Modulation (HRPWM) in Array Imaging Applications

Switched excitation allows the miniaturisation of excitation circuitry for transducer integrated front ends, high channel count and portable ultrasound systems. Harmonic Reduction Pulse Width Modulation (HRPWM) provides a method to design five level switched mode excitation signals with control of instantaneous amplitude, frequency and phase plus minimised third harmonics for advanced ultrasound applications. This paper details the application of HRPWM using commercial transmit front end integrated circuits and linear array transducers. The ability of HRPWM to control the pressure of the ultrasound wave is investigated. A full scale error between desired and measured pressure of 3.5% at 4.1 MHz is demonstrated. The temporal windowing of linear frequency modulated excitation signals using HRPWM is demonstrated. Pulse compression linear imaging of a tissue phantom is demonstrated where an improvement in the -20 dB axial resolution of a nylon mono-filament target from 2.14 mm using bipolar excitation to 1.88 mm using HRPWM is shown

Capacitive Micromachined Ultrasonic Transducers for Non-destructive Testing Applications

Ultrasound is a popular technique for industrial non-destructive testing (NDT) applications. By sending ultrasonic waves into an object and observing the amplitude and the delay of the reflected or transmitted waves, one can characterize the material, measure the thickness of the object, and detect discontinuities (flaws) as well as the size, location, and orientation of the defects in the object. Traditionally, ultrasonic transducers for NDT are made with piezoelectric crystals. Meanwhile, another class of ultrasonic transducers known as capacitive micromachined ultrasonic transducers (CMUTs) have become popular in medical ultrasound research because of their large bandwidths and other attributes that allow them to be integrated into the tip of a catheter. However, CMUTs have not been widely adopted in ultrasonic NDT applications. In this thesis, three important CMUTs characteristics that could potentially make them attractive for NDT applications are introduced and demonstrated. First, CMUTs can be beneficial to NDT because the fabrication techniques of CMUTs can easily be used to implement high-frequency, high-density phased arrays, which are essential for high resolution scanning. Surface scanning using a 2-D row-column addressed CMUT array was demonstrated. Secondly, CMUTs can be integrated with supporting microelectronic circuits, thus one can implement a highly integrated transducer system, which can be useful in structural health monitoring NDT applications. Front-end microelectronic circuits that include a transmit pulser and a receive amplifier were designed, tested, and characterized. Thirdly, CMUTs are suitable for air-coupled applications because of their low acoustic impedance at resonance. Air-coupled CMUTs fabricated in a standard RF-MEMS process were characterized and tested. This thesis concludes with an analysis of the potential usefulness of CMUTs for ultrasonic NDT. While many ultrasonic NDT applications are better off being performed using conventional piezoelectric transducers, CMUTs can and should be used in certain NDT applications that can take advantage of the beneficial characteristics of this exciting transducer technology

Piezoelectric Micromachined Ultrasound Transducer (PMUT) Arrays for Integrated Sensing, Actuation and Imaging

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed

Fabrication of CMUTS based on PMMA adhesive wafer bonding

Capacitive Micromachined Ultrasonic Transducers (CMUTs) are the potential alternatives for the conventional piezoelectric ultrasonic transducers. CMUTs have been under an extensive research and development since their first development in the mid- 1990s. Initially developed for air-coupled applications, CMUTs have shown far better acceptability in immersion-based applications (i.e. medical ultrasonic imaging, medical therapy, and underwater imaging) when compared to the piezoelectric ultrasonic transducers. CMUTs are parallel-plate capacitors fabricated using the Micro Electro Mechanical Systems (MEMS) technology. Despite of the fact that various CMUT fabrication methods have been reported in the literature, there are still many challenges to address in CMUTs design and fabrication. Standard fabrication techniques are further sub-divided into the Sacrificial Layer Release Process and the Wafer Bonding methods. A number of complications are associated with these techniques, such as optimization of the design parameters, process complexity, sacrificial layer material with the corresponding etchant selection, wafer cost and selection. In particular, the sacrificial release methods consist of complex fabrication steps. Furthermore, structural parameters like gap height and radius have optimization issues during the sacrificial release process. On the other hand, the wafer bonding techniques for the CMUTs fabrication are simple and have a great control over the structure parameters in contrast to the sacrificial release methods. At the same time, the wafer-bonded CMUTs require very high quality wafer surface and have a very high contamination sensitivity. For this purpose, this dissertation aims to develop a simple, low cost and lower constraint thermocompression-based technique for the CMUT fabrication. The proposed wafer bonding technique for the CMUT fabrication in the dissertation uses Polymethyl methacrylate (PMMA) adhesive as an intermediate layer for the thermocompression wafer bonding. The advantages associated with the PMMA adhesivebased wafer bonding over the other wafer bonding methods include low process temperature (usually 200 C or less), high wafer surface defects and contamination tolerance, high surface energy and low bonding stresses. These factors will add cost effectiveness and simplicity to the CMUTs fabrication process. Furthermore, the achieved receive sensitivity with the reported CMUT is found comparable to the commercially available ultrasonic transducer

MEMS Technology for Biomedical Imaging Applications

Biomedical imaging is the key technique and process to create informative images of the human body or other organic structures for clinical purposes or medical science. Micro-electro-mechanical systems (MEMS) technology has demonstrated enormous potential in biomedical imaging applications due to its outstanding advantages of, for instance, miniaturization, high speed, higher resolution, and convenience of batch fabrication. There are many advancements and breakthroughs developing in the academic community, and there are a few challenges raised accordingly upon the designs, structures, fabrication, integration, and applications of MEMS for all kinds of biomedical imaging. This Special Issue aims to collate and showcase research papers, short commutations, perspectives, and insightful review articles from esteemed colleagues that demonstrate: (1) original works on the topic of MEMS components or devices based on various kinds of mechanisms for biomedical imaging; and (2) new developments and potentials of applying MEMS technology of any kind in biomedical imaging. The objective of this special session is to provide insightful information regarding the technological advancements for the researchers in the community