Introduction and Overview
1.5 Thesis Organization and Chapter Overview
Following the introduction and overview in this chapter, in Chapter 2 we lay the foundations of VHF, UHF and Microwave nanomechanical resonators. Both basic theoretical principles and experimental techniques are included, with an emphasis on transforming general fundamentals to the specific nanomechanical resonators operating in these high frequency① ranges.
Chapter 3 is on the development of a low-noise, stable, self-sustaining oscillator with a low-loss UHF vibrating NEMS resonator as its frequency-determining element. The self-sustaining oscillator is important because it demonstrates the feasibility of building active oscillators with passive nanomechanical vibrating resonators, and thus converting
①Throughout the text, “high frequency” is used in its literal meaning which can be taken as an inclusive but less formal designation of a loosely-defined wide range possibly covering several frequency bands in the radio frequency spectrum; while the high frequency band (3-30MHz) itself is referred as its technical term acronym, HF.
direct current (DC) power into radio-frequency (RF)② power utilizing NEMS resonators.
The NEMS oscillator operates in the UHF band, which is much higher than attained by the state-of-the art MEMS oscillators based on vibrating MEMS resonators. This technology readily demonstrates mass sensitivity in the zeptogram-scale, as well as its unique advantages.
In parallel to the self-sustaining oscillator operation, we present the development of the technologies of embedding UHF NEMS resonators into low-noise phase-locked loop (PLL) systems in Chapter 4. In this approach, a more stable frequency source is often used to work as a voltage-controlled oscillator (VCO) to drive a NEMS resonator and to lock to and track the resonance. This NEMS-PLL system-level operation represents another generic approach of real-time NEMS resonance locking and tracking. This technology has shown NEMS mass sensitivity that is sufficient for single-biomolecule sensing.
An important issue that has arisen in engineering NEMS is the trade-off between scaling devices (both sizes and operating frequency) and attaining high quality factors (low loss or dissipation). The dissipation issues have become especially keen for UHF NEMS resonators. Chapter 5 is dedicated to carefully exploring the dissipation mechanisms and limiting factors on the device quality factor. Important and dominant energy loss mechanisms have been identified and guidelines and possible solutions for quality factor engineering are discussed.
②Radio-frequency (RF) is the interesting portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current (AC) signals fed to an antenna. Broadly-defined RF usually covers from kHz to GHz ranges. The term RF used in the thesis complies with this convention but also has an emphasis on the ranges from HF band to 1GHz (beyond 1GHz we often use the term microwave), and implications on communications.
Besides pursuing the ultimate performance of the best top-down UHF NEMS resonators, we have been keeping an open eye to the possibilities of devices made by bottom-up chemical synthesis techniques. Chapter 6 presents our latest efforts with high-performance resonators based on Si nanowires (NWs). We demonstrate that these Si NWs are robust resonators that can operate in the VHF/UHF ranges. The Si NW resonators have been realized with both metallized and pristine (non-metallized) high-impedance NWs. Their wonderful piezoresistive effect offers very promising piezoresistive detection. With comprehensive characterizations of the basic specifications, frequency stability and dissipation issues, we show that the Si NWs have excellent performance comparable to that of the state-of-the-art top-down devices.
Finally in Chapter 7, the research effort toward the engineering of UHF NEMS resonators for ultimate sensitivity and low-noise applications is summarized, with major conclusions drawn and future interesting short-term and long-term research topics suggested and envisioned.
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