Exploring the quality factor limits of room temperature nanomechanical resonators

Nanomechanical resonators have become integral to technological advancements and scientific progress in modern society. Not only do they enable exquisite precision in sensing applications, but they also allow for exploring quantum mechanics and the development of quantum technologies. These applications require resonators with high quality factors capable of isolating them from environmental noise, specifically crucial at room temperature.

The focus of this thesis is on investigating the limits of quality factors in nanomechanical resonators operating at room temperature. The study revolves around four main facets, addressing limitations in fabrication techniques, and design strategies, exploring the impact of aspect ratio on quality factor enhancement, and investigating the potential for temperature sensing.

Firstly, we address the limits imposed by current fabrication techniques to realize high aspect ratio resonators, such as stiction and collapse due to interfacial forces like capillary. To overcome these challenges, we develop and characterize an SF6 plasma etching technique which enables a quick and controllable release of nanomechanical resonators. The high fidelity achieved through this approach allows the use of advanced optimization strategies to design resonators with exceptional quality factors.

In doing so, we tackle the limits of design strategies, which have primarily relied on human intuition until now. By harnessing the power of Bayesian Optimization and inspired by nature, we discover a strategy to increase the quality factor at low order mode via a torsional soft-clamping mechanism. The experimental validation of the resulting spiderweb resonators confirms quality factors surpassing 1 billion at room temperature in the kHz frequency range. Notably, these resonators contain no features smaller than 1 micrometer, ensuring a fast and cost-effective fabrication.

Expanding on these findings, the thesis explores the limits of aspect ratio in quality factor enhancement. By bridging nanomechanics and macromechanics, we create nanomechanical resonators with centimeter-scale lateral sizes. Utilizing multi-fidelity Bayesian Optimization alongside stiction-free fabrication techniques, our strategy allows to reduce the computational cost and to suspend the fragile structures with a fabrication yield approaching 100%, leading to a quality factor above 6 billion.

Finally, the thesis investigates the potential of high quality factor nanomechanical resonators for temperature sensing. We develop a primary noise thermometer to detect temperature across a wide range. The elevated quality factor enables the detection of the effect of the Brownian motion on the resonator’s motion. However, it also poses limitations on the measurement scheme due to the narrow linewidth of the resonators.

Combining all these aspects, this thesis explores and pushes the boundaries of quality factors in nanomechanical resonators at room temperature. It presents novel fabrication techniques, advanced design strategies, and sensing capabilities of high quality factor resonators. The findings offer valuable insights and open up new possibilities for applications in precision sensing, quantum mechanics, and beyond.