Vertically aligned carbon based varactors

This paper gives an assessment of vertically aligned carbon based varactors and validates their potential for future applications. The varactors discussed here are nanoelectromechanical devices which are based on either vertically aligned carbon nanofibers or vertically aligned carbon nanotube arrays. A generic analytical model for parallel plate nanoelectromechanical varactors based on previous works is developed and is used to formulate a universal expression for their voltage-capacitance relation. Specific expressions for the nanofiber based and the nanotube based varactors are then derived separately from the generic model. This paper also provides a detailed review on the fabrication of carbon based varactors and pays special attention to the challenges in realizing such devices. Finally, the performance of the carbon based varactor is assessed in accordance with four criteria: the static capacitance, the tuning ratio, the quality factor, and the operating voltage. Although the reported performance is still far inferior to other varactor technologies, our prognosis which stems from the analytical model shows a promise of a high quality factor as well as a potential for high power handling for carbon based varactors

Direct measurement of bending stiffness and estimation of Young's modulus of vertically aligned carbon nanofibers

The bending stiffness of individual, as-grown, vertically aligned carbon nanofibers was measured using a custom-built atomic force microscope placed inside a scanning electron microscope. The internal structure of the nanofiber was best modeled as dual-phase, composed of an inner graphitic core covered with a tapered amorphous carbon shell. It was found that the fibers have a relatively low bending stiffness, with Young's modulus values of about 10 GPa for the inner core and 65 GPa for the outer shell. The low Young's modulus of the inner core is attributed to a non-zero angle between the graphitic sheets and the nanofiber axis. The weak shear modulus between graphitic sheets thereby dominates the mechanical behaviour of the fibers

Synthesis and characterization of vertically aligned carbon nanofibers for nanoscale devices

The synthesis of vertically aligned carbon nanofibers (VACNFs) by direct current plasma enhanced chemical vapor deposition (dc PECVD) has presented a unique opportunity to realize nanoscale three-dimensional devices at a reasonable cost. The determinism offered by the synthesis process in terms of control over the spatial and the geometrical properties of the resulting nanofibers provides a powerful tool to implement a wide range of applications from nanoelectromechanical systems to biological devices. However, the use of VACNFs as building blocks for nanoscale devices has not been a trivial task. The work presented in this thesis aims at incorporating the synthesis of VACNFs as an integral part of the nanofabrication process. The dc PECVD synthesis process is scrutinized by dividing it into three phases. The effect of growth parameters on each phase is investigated independently. Special attention is paid to the choice of materials involved in the synthesis process. Reactively sputtered TiN is chosen as the growth underlayer and a detailed discussion is given on its optimal deposition conditions. The material and process optimizations manifest themselves by a successful fabrication of individually addressable arrays of VACNFs with a sub-micrometer pitch between the adjacent nanofibers. The potential applications of such three-dimensional nanoscale arrays as well as a suitable measurement scheme are also discussed. As an important parameter for designing VACNF based devices, the bending stiffness of as-grown nanofibers is directly measured. It is shown that the assumption of a uniform internal structure is inadequate in describing nanofibers’ mechanical properties and that a dual phase model is needed in which different Young’s moduli are assigned to the inner graphitic core and the outer amorphous carbon shell. The potential of integrating the VACNF synthesis with CMOS technology is also assessed. The level of deterioration in the basic functionality of individual transistors on ASIC chips fabricated in standard 130 nm bulk CMOS technology are compared when the chips are subjected to three disparate CVD techniques with relatively low processing temperatures

Toward Carbon based NEMS

A systematic analysis and assessment of carbon nanotube (CNT) based NEMS switches is presented and their features are compared to typical complementary metal-oxide-semiconductor (CMOS) performance parameters. It is shown that CNT-based switches with considerably smaller leakage current compared to CMOS switches can be realized. These switches demonstrate very small standby-power dissipation.This thesis also pays special attention to the future integration of carbon based NEMS with mainstream circuitry. It is our belief that the adoption of novel nanotechnologies are closely tied to their successful integration with CMOS technology, not only to benefit from its versatility and maturity but also to be able to present an added value to the full-fledged platform. This thesis demonstrates a relatively low temperature direct current plasma enhanced chemical vapor deposition (dc PECVD) process capable of growing vertically aligned carbon nanofiber-like structures with negligible deterioration of bulk CMOS transistors’ functionality.A main feature of this thesis is the toolbox composed of analytical and computational components to design and simulate a single-pair VACNF based system. Nanoelectromechanical devices based on this building block have been fabricated. The inherent discharging problem of dc PECVD synthesis method is addressed and resolved. Moreover, two different methods are proposed to extract the Young’s modulus of the synthesized vertically aligned carbon nanofibers

Synthesis and characterization of vertically aligned carbon nanofibers for nanoscale devices

The synthesis of vertically aligned carbon nanofibers (VACNFs) by direct current plasma enhanced chemical vapor deposition (dc PECVD) has presented a unique opportunity to realize nanoscale three-dimensional devices at a reasonable cost. The determinism offered by the synthesis process in terms of control over the spatial and the geometrical properties of the resulting nanofibers provides a powerful tool to implement a wide range of applications from nanoelectromechanical systems to biological devices. However, the use of VACNFs as building blocks for nanoscale devices has not been a trivial task. The work presented in this thesis aims at incorporating the synthesis of VACNFs as an integral part of the nanofabrication process. The dc PECVD synthesis process is scrutinized by dividing it into three phases. The effect of growth parameters on each phase is investigated independently. Special attention is paid to the choice of materials involved in the synthesis process. Reactively sputtered TiN is chosen as the growth underlayer and a detailed discussion is given on its optimal deposition conditions. The material and process optimizations manifest themselves by a successful fabrication of individually addressable arrays of VACNFs with a sub-micrometer pitch between the adjacent nanofibers. The potential applications of such three-dimensional nanoscale arrays as well as a suitable measurement scheme are also discussed. As an important parameter for designing VACNF based devices, the bending stiffness of as-grown nanofibers is directly measured. It is shown that the assumption of a uniform internal structure is inadequate in describing nanofibers’ mechanical properties and that a dual phase model is needed in which different Young’s moduli are assigned to the inner graphitic core and the outer amorphous carbon shell. The potential of integrating the VACNF synthesis with CMOS technology is also assessed. The level of deterioration in the basic functionality of individual transistors on ASIC chips fabricated in standard 130 nm bulk CMOS technology are compared when the chips are subjected to three disparate CVD techniques with relatively low processing temperatures

An easy-to-implement method for evaluation of capacitive resonant sensors

A novel method that can be used to characterize resonant capacitive sensors is presented. The method is based on measurement of the third harmonic of the current through the capacitor; thus the detection signal is frequencyseparated from the excitation signal. The measurement is time-continuousand only requires a very simple resonant structure where a single electrode is simultaneously used for excitation and detection. The readout circuit iseasily implemented with a single operational amplifier and standard equipment such as a lock-in amplifier or a function generator and a spectrum analyzer. Measurements of a resonant structure confirm the feasibility of the concept

Growth characterization of vertically aligned carbon nanofibers on top of TiN buffer layer for nanoelectromechanical devices

Initial growth of vertically aligned carbon nanofibers (VACNFs) from Ni catalyst seeds in the range of 40 to 100 nm as fabricated using hole-mask colloidal lithography on top of reactively sputtered TiN is studied. We observe that the initial growth conditions could cause a growth mode transition from base-type to tip-type. We attribute this transition to a change in the crystallographic orientation of the Ni catalyst seeds induced by initial growth conditions. A convenient method to deposit stoichiometric TiN films is also presented

Growth characterization of vertically aligned carbon nanofibers on top of TiN buffer layer for nanoelectromechanical devices

Initial growth of vertically aligned carbon nanofibers (VACNFs) from Ni catalyst seeds in the range of 40 to 100 nm as fabricated using hole-mask colloidal lithography on top of reactively sputtered TiN is studied. We observe that the initial growth conditions could cause a growth mode transition from base-type to tip-type. We attribute this transition to a change in the crystallographic orientation of the Ni catalyst seeds induced by initial growth conditions. A convenient method to deposit stoichiometric TiN films is also presented

CMOS considerations in nanoelectromechanical carbon nanotube-based switches

In this paper, we focus on critical issues directly related to the viability of carbon nanotube-based nanoelectromechanical switches, to perform their intended functionality as logic and memory elements, through assessment of typical performance parameters with reference to complementary metal-oxide-semiconductor devices. A detailed analysis of performance metrics regarding threshold voltage control, static and dynamic power dissipation, speed, and integration density is presented. Apart from packaging and reliability issues, these switches seem to be competitive in low power, particularly low-standby power, logic and memory applications