Mechanical Engineering PhD Thesis Defense by Mohammad Nasr Esfahani








Title: Integration of Silicon Nanowires with 3D Devices: Fabrication and Modeling


Speaker: Mohammad Nasr Esfahani


Time: June 16, 2017, 9:30 AM


Place: ENG B29

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

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

Assoc. Prof.  Murat Sozer (Koç University)

Asst. Prof. Şükrü Ekin Kocabaş (Koç University)

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

Asst. Prof. Selim Hanay (Bilkent university)




Silicon nanowires as fundamental building blocks in many nanoelectromechanical systems find applications in sensors and electronic devices. Extensive use of silicon nanowires in modern electronic devices faces integration challenges with the high-order architectures such as three-dimensional integrated circuits and microelectromechanical systems. A monolithic integration of silicon nanowires with microstructures can address the shortcoming of available techniques marred by the lack of batch techniques compatible with semiconductor manufacturing. In this study, a monolithic incorporation of silicon nanowires in bulk silicon with an etch depth of 40?m is demonstrated through a combination of high-resolution lithography and deep etching technology. This technique opens up new possibilities for modern sensors by replacing capacitance-based motional detection by high-sensitivity piezoresistive system via silicon nanowire use. Such monolithic approach to downscaling silicon piezoresistive sensors is presented in this study by spanning an electrostatic comb-drive actuator and a micromechanical amplifier by a single nanowire. Similar contribution of piezoresistive silicon nanowire electromechanical resonators to 3D devices is depicted based on frequency down-mixing approach as well. As this multi-scale integration is accomplished in a batch-compatible fashion, the presented technique carries significant implications for physical and biological miniaturized piezoresistive sensor technologies.

In addition to integration challenges, the full potential of the miniaturized silicon nanowires in the field of physical sensors is yet to be realized. Mechanical modeling is one of the main challenges in design of nanowires with scale dependence of transport properties. Surface stress is considered as the major effect of size dependence in mechanical properties of nanowire.  A sound guideline in mechanical modeling can address the successful engineering design of such unique architecture. In this work, a guideline for selecting the optimum engineering tool for the frequency response modeling of flexural nanowire resonators is introduced through a benchmarking study among analytical and numerical techniques. The limited applicability due to the associated computational cost in available approaches to analyze thermomechanical behavior of nanowires with surface stress contribution are addressed in this study by using a temperature-dependent interatomic potential in the standard Cauchy-Born theory. In this respect a comprehensive study on crystal orientation and geometrical parameters can shed light on the role played by the surface stress in determining silicon nanowire mechanical behavior. This is accomplished by linking the local surface stress at the atomic scale to the overall behavior of the continuum system. Addressing the existing controversy regarding the contribution of surface stress in the mechanical behavior of silicon nanowires, results in this work can provide a guideline for the interpretation of existing and the design of future experimental investigations.