Infoscience

Thesis

Carbon Nanotube Electromechanical Devices for Radio Frequency Applications

Carbon nanotubes (CNT) have been investigated extensively in the recent years for application in nano-electromechanical systems (NEMS). CNTs have extraordinary electrical and mechanical properties. They are stiff, with low density, high electrical conductivity and hence are the ideal candidates for NEMS. Beyond miniaturization, NEMS can also offer better performance than its predecessor micro-electro mechanical systems (MEMS). NEMS find application in ultra-sensitive mass and force detection, RF signal processing, low power operated switches with fast switching speeds, to name a few. Despite tremendous promise the main challenge in CNT research has been the fabrication technology to realize CNT based devices with precise control of location and properties of CNT. CNTs are one-dimensional structures with high aspect ratio. To realize devices for practical application with high signal-to-noise-ratio, it becomes necessary to operate many of them in parallel. To fabricate an array of CNTs in parallel posse's additional challenge to the fabrication technology but the resulting structures could provide benefits compared to both NEMS fabricated with one dimensional structures and MEMS. During the course of this thesis, different fabrication technologies were investigated to realize CNT array based devices. A self assembly method, dielectrophoresis and CVD growth were optimized to realize CNT based devices. A novel technique using a modified dielectrophoresis method was setup to be able to trap a single CNT. A CNT crossed junction was fabricated using the same. The electrical properties of the junction were studied. A Suspended Gate CNT array Field Effect Transistor (SG-CNT-FET) based resonator was demonstrated for the first time. SG-CNT-FET combines the excellent mechanical properties of CNTs with the gain of the solid state field effect transistor. The use of an array configuration ensures better signal-to-noise-ratio compared to device implemented with a single CNT. A novel process flow combining the fabrication of a FET and deposition of CNTs by dielectrophoresis was developed. Resonance frequency up to 150 MHz with a quality factor of approximately 50 was obtained. The resonance frequency is tuneable by modulating the tension in the CNT array by applying a DC bias. Dependence of the resonance frequency on the applied bias has been studied under the frame work of a continuum mechanics model. The small signal circuit parameters have been extracted, enabling future circuit design. Vertically grown CNT array were grown on metal lines to realize a tuneable MEM capacitor. The CNT array forms a membrane like structure which can be electro-statically actuated. Morphological and electrical characterizations were performed on the fabricated devices. DC characterization revealed the pull-in behavior like conventional MEMS. The Young's modulus of the grown vertical CNT array was extracted from the measured pull-in voltage. An effective Young's modulus of 30-100 MPa was obtained. An equivalent electrical model was developed to extract the capacitance from the S-parameters measurements, measured up to 6 GHz. The model was validated by ADS circuit simulations. A voltage controlled oscillator at 2.45 GHz has been implemented using the thus realized tuneable MEM capacitor.

Fulltext

Related material