Engineering at the limits of the nanoscale

Abstract

At the nanoscale, unique properties and phenomena emerge that can lead to scientific and technological paradigms beyond those classically envisioned. Exploring these opportunities at the few-nanometer regime requires unprecedented precision, resolution, control and uniformity, not readily feasible through conventional fabrication and metrology techniques. In particular, the dynamic, reliable and reversible structural tuning of such small dimensions remains a great challenge, yet a promising platform to enable devices of new and improved functionalities. To overcome these challenges, alternative techniques are necessary to push the frontiers of nanoscale processing. In this thesis, the challenges and prospects of engineering active devices at the limits of the nanoscale are evaluated using a case study that focuses on developing a new platform for nanoelectromechanical (NEM) switches. The proposed NEM switches that rely on electromechanical modulation of the tunneling current in <5 nm switching gaps possess the potential to overcome the limitations of the conventional counterparts - minimizing stiction and lowering the actuation voltage. Combined top-down and bottom-up fabrication methodologies are introduced for achieving active structures of the desired complexity with nanometer precision, resolution and control. Integration of device engineering and physics with chemistry and materials science leverages an understanding of material synthesis, surfaces and interfaces to achieve manipulation of matter in the nanometer regime with a precision and control otherwise not feasible. Accordingly, two example hybrid fabrication techniques are introduced allowing precise fabrication of electrically-active nanogaps. Molecules are proposed as nanoscale structural components which can also control surface interactions and forces utilizing their chemical and mechanical properties. When used as interconnects between neighboring surfaces, they can precisely define nanoscale spacings. Uniquely, the mechanics of the molecular layer can be used to allow controlled and reversible tuning of the spacing where the elastic restoring force of the molecules balances the dominating surface adhesive forces to allow for stable yet mechanically active structures. Feasibility of molecules as nanoscale scaffolds and springs are demonstrated in this work in an electromechanically tunable molecular tunneling junction. In such a junction, changes in the tunneling gap leads to an exponential modulation of the tunneling current. If sufficiently large, this modulation can serve as a NEM switching mechanism. The molecules provide precision in defining small switching gaps necessary to reduce the actuation voltage while the force control provided through the molecular layer's mechanics helps control the surface adhesion. These proposed tunneling-based switches, referred to as "squitches", form a promising platform towards a more energy-efficient operation. Two- and multi-terminal designs of squitches are proposed and experimentally demonstrated with example devices showing actuation voltages <2 V and current modulations >10⁴. The design of squitches pushes the limits of nanoscale processing and broadly helps reveal the challenges and prospects of engineering at dimensions few nanometers in size. By implementing a multidisciplinary approach, one can gain access to the limits of the nanoscale to investigate the emerging physical phenomena and develop next generation nanodevices beyond squitches. The key is continuous development of versatile processing techniques allowing nanoscale manipulation and characterization with high precision and control.