Mechanical Engineering PhD Thesis Defense by Mustafa Yılmaz

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

PhD THESIS DEFENSE BY MUSTAFA YILMAZ

Title: Silicon-Based Nanowires: Top-Down Fabrication and Mechanical Behavior

Speaker: Mustafa Yılmaz

Time: September 04, 2018, 15:00

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

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

Prof. Dr. Metin Muradoğlu (Koç University)

Prof. Dr. Arda D. Yalçınkaya (Boğaziçi University)

Prof. Dr. Mehmet H. Omurtag (Istanbul Technical University)

Prof. Dr. İsmail Lazoğlu ( Koç University)

Abstract:

To keep the pace of technological advancement and meet requirements in electronic/electromechanical systems and sensors, device dimensions constantly evolve and the integration of nanoscale building blocks with microelectromechanical systems (MEMS) become crucial. These integrated nanoscale building blocks add new functionalities as well as improve the existing performance of MEMS technologies. A number of candidate materials emerged for this purpose, where one-dimensional nanostructures of Silicon (Si) and Silica (SiO₂) attract increasing interest. These building blocks along with their batch-compatible manufacturing and mechanical response are studied as the subject of this thesis.

In the case of Si, a batch-compatible fabrication technology is developed in this study. For this purpose, a monolithic integration methodology of silicon nanowires with a 10-mm-thick silicon-on-insulator (SOI) MEMS device is proposed through the combination of high-resolution lithography and a two-step Si etching approach. This technique leads to an assembly of ultimate precision, where a 2-order-of-magnitude scale difference is spanned. The success of integration is evaluated. The reported methodology allows nanowires to be placed on the top surface of the MEMS device providing ease of access for further processes. As the first reported study in literature demonstrating such monolithic integration in SOI MEMS devices, this work presents a way for scaling down to nano in MEMS devices for future.

In addition to the aforementioned fabrication studies, mechanical characterization of suspended Si nanowires with different cross-sectional geometries with a minimum dimension of 28 nm is carried out through bending tests within a scanning electron microscope. Both analytical and numerical computational analyses are carried out to determine the geometry-dependent loss of stiffness (down to a minimum value of 62 GPa in elastic modulus) and increase in the fracture strength (in a range of 10 to beyond 20 GPa). Both results are compared and validated through literature.

Furthermore, in the cases of SiO₂, an affordable and reliable methodology for fabricating silica nanowires from silsesquioxane-based precursors by e-beam patterning is investigated and reported in this study. SiO₂ nanowires are released through a deep reactive ion etching process and can be easily be integrated with a MEMS device following a methodology such as the introduced in the case of silicon. In this study, the main focus was placed on achieving a reliable fabrication of different nanowire geometries and mechanical characterization of resulting silica nanowires. Superplastic behavior with enormous deflections reaching 10-mm is achieved without fracture. Mechanical characterization through bending tests along with a large-deformation elastic model is utilized for predicting the deviation from the elastic behavior. A critical stress level of 0.1-0.4 GPa is observed for the onset plasticity. The reported fabrication methodology is compatible with semiconductor manufacturing technologies for possible utilization of silica nanowires in electromechanical devices.

The Si fabrication technique demonstrated in this thesis is the first example in literature of monolithically integrating Si nanowires on MEMS devices where nanowire surface processes are viable. The associated Si nanowires are also the first reported samples in the literature within the given geometric scale that are fabricated by dry etching whose fracture strength data is provided. Another study in this thesis results in a novel approach to obtain silica nanowires that are highly align and can sustain large deformations without fracture by using affordable precursors.