Mechanical Engineering PhD Thesis Defense by Yasin Kılınç

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KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

PhD THESIS DEFENSE BY YASİN KILINÇ

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Title: A New Composite Resonator Architecture Based On Coupled Vertical NW Arrays

Speaker: Yasin Kılınç

Time: November 13, 2017, 16:30

Place: ENG Z27

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. B. Erdem Alaca (Advisor, Koç University)

Prof. Yusuf Leblebici (EPFL)

Assoc. Prof.  Demircan Canadinç (Koç University)

Prof. Dr. Alphan Sennaroğlu (Koç University)

Assoc. Prof. Arda D. Yalcinkaya (Boğaziçi University)

Abstract:

A new composite resonator architecture is developed at the nanoscale by stacking nanowires (NWs) perpendicular to the surface of a substrate and coupling the NWs in the resulting vertical array mechanically by means of a membrane-type coupling element called coupling membrane (CM). Employment of CMs corresponds to a significant expansion to rather limited mechanical coupling toolkit at the nanoscale due to connections even between non-adjacent resonators. Electromechanical characterization of resonators is performed using electrostatic actuation and piezoresistive readout to ensure the best integration possibility with on-chip electronics in the future. Electrical measurements are performed not only for coupled vertical NW arrays but also for single clamped-clamped NWs to examine effects of mechanical coupling and multiple resonators in an array on frequency spectrum characteristics. Coupled NW arrays consisting of 8 NWs designed based on finite element modeling (FEM) modal analysis simulations are fabricated using the developed top-down fabrication flow from the 1 mm thick device layer of a silicon-on-insulator (SOI) wafer. Effects of resonator and coupling element on frequency spectrum response are studied separately by changing the lengths of NW and CM systematically. Two piezoresistive frequency downmixing techniques of the two-source/1ω and two-source/2ω schemes are compared in terms of detected signal amplitudes and their dependence on gate voltages. In general, frequency spectrum measurements provide mechanical modes up to 130 MHz in high vacuum. Electromechanical characterization of piezoresistive single Si NW samples fabricated in the 10 mm thick device layer of a SOI substrate is demonstrated for the first time in the literature. Detected mechanical modes from 18 MHz to 101 MHz for single NW samples are successfully associated to the first flexural in-plane and out-of-plane modes using FEM simulations thanks to the determination of an actual NW cross section by TEM measurements. Moreover, NW array measurements result in multiple mechanical modes both dispersed in a wide frequency range and in close proximity such that being reminiscent of typical spectrum response of a bandpass filter. However, inevitable nonuniformity of NWs in an array and absence of accurate spatial measurements complicate implementation of FEM simulations in the NW array case. Thus, FEM simulations are conducted to investigate more fundamental issues such as proving existence of mechanical coupling between NWs in a stack by distinguishing collective array modes from an individual NW resonance. To this end, experimental resonance frequency shifts of single NW and NW array samples with respect to electrode voltages are compared between each other and with FEM results. These comparisons between simulations and measurements reveal similar and distinct characteristics simultaneously for different spectrum modes.