Analog front end circuits for highly integrated MUT based ultrasound imaging systems

Over the last 20 years, MEMS technology (which stands for Micro-Electro-Mechanical Systems) has revolutionized the world of consumer electronics in many ways, enabling the so-called second silicon revolution. Its production process leverages the same batch fabrication techniques used in the integrated circuit industry, which translates into low per-device production costs; in addition, the fabrication of highly integrated systems often is able to outperform their competitors made using the most precise macroscale level machining technique. Today, the most notable example of MEMS are microsensors and microactuators since they are a primary players in many market segments such as automotive, industrial, and, more recently, smart medical care systems. In this regard, the so-called MUTs (Micromachine Ultrasound Transducers) are fundamental for the development of next generation ultrasound imaging systems since they enable the cost effective production of highly complex 2D imaging arrays capable of capturing 3D volumes. This, combined with efficient edge AI to guide data acquisition and interpretation, will enable the development of medical diagnostic appliances that can be operated by users with limited medical knowledge. It is important to note that the real potential of MUTs starts to become fulfilled only when these miniaturized transducers are able to be merged onto a common silicon substrate along with integrated circuits: while the electronics are fabricated using integrated circuit process sequences (e.g., CMOS), the micromechanical components are integrated using compatible processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. For this reason, research nowadays is heavily focused into the design and optimization of highly integrated front end electronic interfaces for both CMUT based (capacitive MUTs) and PMUT based (piezoelectric MUTs) ultrasound imaging systems. In this framework, compact size, low power and high dynamic range are fundamental design targets for the circuital front end since they are key enablers for a widespread deployment of next generation ultrasound smart probes. The objective of this dissertation is to analyze different ultrasound systems and, for each of them, to identify and design the best integrated solution in order to optimize the front end performances. The proposed circuital interfaces are then fabricated and electrically characterized. This thesis is organized as follows: • Chapter 1 introduces the MUTs operative principle, their fabrication process and circuital modelling, as well as describing a modern imaging system for medical ultrasound diagnostic. • Chapter 2 presents the design and fabrication of a transceiver front end for a 1D PMUT array to be used in low frequency sonography, which was measured and proved to achieve significant optimization features. • Chapter 3 shows a design solution of a bipolar 3-level high-voltage pulser for PMUT probes which is able to minimize the number of mosfets and, thus, limits its overall area. • Chapter 4 describes a novel Low Noise Amplifier (LNA) solution to be employed in a 2D PMUT matrix; the LNA was designed to achieve optimal performances in terms of noise-power tradeoff as well as a minimal area occupation able to be integrated successfully into the pitch of the matrix. The circuit was fabricated and electrically characterized, demonstrating outstanding performances with respect to what is reported in open literature. • Chapter 5 verifies via a feasibility study that the same LNA previously described can be employed successfully in the fabrication of the analog front end of a 2D CMUT matrix, proving that it is a versatile solution as well. • Chapter 6, lastly, draws the conclusions of this work. This work is part of the Moore4Medical project funded by the ECSEL Joint Undertaking under grant number H2020-ECSEL-2019-IA-876190.