AbstractsComputer Science

Surface-micromachined silicon carbide structures and their application for microsystems

by Qing Zhang

Institution: McGill University
Department: Department of Electrical and Computer Engineering
Degree: PhD
Year: 2014
Keywords: Engineering - Electronics and Electrical
Record ID: 2034093
Full text PDF: http://digitool.library.mcgill.ca/thesisfile127007.pdf


The first part of this thesis presents the design, fabrication and testing of silicon carbide (SiC)-based capacitive micromachined ultrasonic transducers (CMUT). The use of DC-sputtered amorphous SiC (a-SiC) as a structural layer allows the fabrication process to limit the temperature to a thermal budget of 200ºC for 2 h, which is the lowest reported to date, making this technology ideally suited for above-IC integration. The high Young’s modulus of the deposited SiC film, along with its very low residual stress, result in high strength and resilient CMUT membranes. The placement of the suspended aluminum electrode directly at the bottom side of the membrane reduces the effective size of the electrostatic transduction gap, resulting in superior electro-mechanical coupling. Fabricated transducers are tested in air with both continuous-wave and pulsed signals, using a pitch-and-catch configuration. The transducer pair, composed of 110 µm-diameter membrane arrays, exhibits a resonant frequency of 1.75 MHz, a 3 dB-bandwidth of 0.15 MHz and a transmission gain of -38 dB. The CMUT prototypes showcase the versatility of low-temperature DC-sputtered SiC films applied in the field of microelectromechanical systems (MEMS).The second part of this thesis presents a low-cost and low temperature wafer-level packaging solution for MEMS devices. Heat-decomposable polymer poly(propylene carbonate) (PPC) is used as the sacrificial material to release the capping layer in a clean and fast manner. Free-standing caps made of a SiC films and as large as 450 µm in diameter are successfully fabricated. To demonstrate the validity of this technology, surface-micromachined Pirani vacuum gauges are fabricated as an example of MEMS devices and encapsulated. Capped Pirani gauges respond to pressure between 1 mtorr and 1 atm. The Pirani gauges are sealed with Parylene C films that exhibit near-hermetic properties and the initial sealing pressure for 300 µm-diameter cavities is characterized to be in the range of tens of torr. The developed silicon carbide-based encapsulation technology offers a practical solution to reduce cost in MEMS packaging therefore eliminating a large bottleneck for their commercialization. La première partie de cette thèse présente la conception, la fabrication et la qualification de transducteurs ultrasoniques micro-usinés capacitifs (CMUT) à base de carbure de silicium (SiC). L'utilisation de SiC amorphe déposé par pulvérisation cathodique comme couche structurelle permet aux températures du procédé de fabrication de se limiter à un budget thermique de 200˚C pendant 2 h, le plus faible rapporté à ce jour. Ainsi, la technologie est idéalement adaptée pour intégration directe sur circuits intégrés. Le module de Young élevé de la couche de SiC, ainsi que ses contraintes résiduelles très faibles, engendre des membranes CMUT très solides et résistantes. Le posit ionnement de l'électrode d'aluminium directement sous la membrane suspendue permet de réduire la taille effective de l'espacement de transduction électrostatique,…