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

Capacitive Micromachined Ultrasonic Transducers (CMUTs) for Humidity Sensing

In the last two decades, capacitive micromachined ultrasonic transducers (CMUTs) have proven themselves to be promising for various ultrasound imaging and chemical sensing applications. Although holding many benefits for ultrasound imaging, CMUTs have certain weaknesses such as the relatively low output pressure at transmission, which hinder their development in the diagnostic imaging application. In the sensing area, CMUTs have shown attractive features such as high mass sensitivity, miniaturized array configuration, and ease of functionalization. However, their potential for humidity sensing is less explored. The objectives of this thesis lie in two aspects. One is to offer a solution to overcome the limitation of low output pressure, and the other is to develop CMUTs as resonant gravimetric humidity sensors. The major efforts are made on the second task. For the first objective, a novel dual-element ultrasonic transducer is proposed. It incorporates two transducer technologies by using a circular piezoelectric element for ultrasound transmission and an annular CMUT element for reception. The hybrid transducer combines the broad bandwidth and high receive sensitivity of the CMUT and the high output power of the piezoelectric transducer to improve the overall sensitivity and axial resolution. The annular CMUT is designed, fabricated, and concentrically aligned with the piezoelectric probe via a custom housing. Immersion measurements show that the hybrid dual-element transducer improves the axial resolution by 25.58% and the signal-to-noise ratio by 8.55 dB over the commercial piezoelectric probe. For the second objective, a CMUT-based resonant humidity sensor is first developed with the direct wafer bonding technique. Graphene oxide (GO) is employed as the sensing material. Due to combination of the mass-sensitive CMUT and the moisture-sensitive GO, the sensor exhibits rapid response/recovery, good repeatability, and higher sensitivity than most of its competitors. The second generation of CMUT-based humidity sensors aims to further improve the relative humidity (RH) sensing performance by adopting the nitride-to-oxide wafer bonding technology for CMUT fabrication. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the new process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. It provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Additional benefits of the nitride-to-oxide wafer bonding technique are the reduced fabrication complexity and more controllable membrane thickness. Finally, a dual-frequency (10/14 MHz) CMUT is developed using this fabrication technique. It generates two RH response curves and can provide more accurate RH sensing. Due to the independence of the two resonance frequencies, the dual-frequency CMUT also shows great potential for identification of different chemicals. This thesis demonstrates that CMUT sensors can be strong candidates for miniaturized, highly sensitive, and reliable humidity sensors

A Feasibility Study of Micromachined Ultrasonic Transducers Functionalized for Ethanol Dectection

The chemical sensing system plays an important role in medical and environmental monitoring. Gases exhaled by humans include nitrogen, oxygen, water vapor, carbon dioxide and volatile organic compounds (VOCs). The VOCs are important and provide valuable information for non- invasive diagnosis. For instance, ethanol detection is beneficial for checking blood alcohol. In time blood alcohol level checking before checking can prevent a person from unsafe driving. Due to the extremely low concentration of the target gases, a gas sensor with high sensitivity, selectivity and low detection limit is required. There is a high demand for low cost, fast, accurate and easy-to-use self-check diagnosis devices. With low cost and high portability, micro-electromechanical systems (MEMS) sensors have been extensively studied for chemical sensing, which provide a cheap self-diagnosis solution. Capacitive Micromachined Ultrasonic Transducers (CMUTs) and Piezoelectric Micromachined Ultrasonic Transducer (PMUTs), which both work based on the mass-loading effect, are considered as the promising types of MEMS sensors for gas sensing. Since they are fabricated in a batch manner with the similar process of silicon-based integrated circuits, CMUTs and PMUTs are able to provide massive parallelism, easy integration with microelectronic circuits, and a higher quality factor. In this research, studied the feasibility of using PMUTs and CMUTs fabricated by our lab for ethanol detection through simulation and experiments. Models for are built via COMSOL for PMUT and CMUT respectively. The simulation results of a single sensing element demonstrated that both CMUTs and PMUTs show great potential for gas sensors. The chemical experiments through frequency response measurement exhibit that both the PMUTs and CMUTs are effective for ethanol detection based on the mass-loading effect. When the gas analyte is attached to the sensing layer, a higher resonance frequency of the transducer induces a higher frequency shift, which means the higher resonance frequency of transducer, the higher sensitivity of a gas sensor is and the lower concentration of ethanol can be detected. Additionally, a CMUT array is also applied to ethanol detection. It provides a good preliminary study of the CMUTs functionalized with more sensing materials for chemical detection in future