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Abstract

This publication deals with the design of carbon nanotubes (CNT) based nano-electromechanical system (NEMS) consisting in a variable capacitor working at microwave frequencies. This device is based on an array of moveable CNT cantilever electrostatically actuated over a ground plane (figure 1 and 2). To design this component, a time-efficient numerical algorithm for the prediction of CNT electromechanical behavior has been developed. This numerical tool permits to calculate the pull-in voltage and flexion of the CNT's tip for various devices' parameters like CNT's diameter and length, initial air gap (g) ...Our software also takes into account the Van der Waals (VdW) forces and the fringing field effects. The results demonstrate that, unlike for RF-MEMS, fringing field effects are preponderant for CNT-based NEMS. This paper also discusses on the accuracy of the developed software. In order to validate our prediction, we used finite element simulation software : COMSOL® and experimental results found in literature and compare them to our prediction. Results prove that we obtain, for a decrease of the simulation time by two orders of magnitude, a maximum error on the pull-in voltage of 7% for various kinds of structures and dimensions. These results were finally used for the design of NEMS demonstrators. The microwave behavior of the varactors, over a large range of frequency, is presented. Simulations with 3D finite-element-method electromagnetic software were performed to optimize the structure and predict its microwave performances, which conclude the design of our microwave carbon nanotubes (CNT) based nano-electromechanical system (NEMS) variable capacitor.

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Abstract

This paper deals with the development of a micro-interconnection technology suitable for the elaboration of RF-NEMS (Nano-ElectroMechanical Systems) varactors. It aims to present an extension of RF MEMS concept into nano-scale domain by using multi-walled carbon nanotubes (MWCNT) as movable part instead of micrometric membranes into reconfigurable passive circuits for microwave applications. For such a study, horizontal configuration of the NEMS varactors has been chosen and is commented. The technology is established to fulfill several constraints, technological and microwave ones. As far as technological requirements are concerned, specific attentions and tests have been carried out to satisfy: " Possible and later industrialization. No e-beam technique has been selected for RF NEMS varactor elaboration. Lateral MWCNT growth performed on a Ni catalyst layer, sandwiched between two SiO2 layers, showed feasibility of suspended MWCNT beam. " High thermal budget, induced by the MWCNT growth by CVD (Chemical Vapor Deposition), at least to 600°C. All the dielectric and metallic layers, required to interlink the nano world with the micrometric measurements one, have been studied accordingly. Consequently, the order of the technological steps has been identified. About microwave and actuation specifications (targeted close to 25V), the minimization of losses and actuation voltage implies large layer's thicknesses compared to the CNT diameter. Several specific technological issues are presented in this paper, taking care of both technological and microwave compatibility to go toward RF NEMS varactor's elaboration.

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In this paper, the impact of double-wall carbon nanotubes (DWCNTs) concentration within BenzoCycloButen (BCB) composites on electromagnetic properties is evaluated and modeled with coplanar test structures. It is demonstrated that only 0.37wt. % of DWCNT concentration increases the line losses level by more than a decade over a large frequency range. The resulting engineerable composite brings to RF-designer a novel degree of freedom to design and optimize microwave innovative components.

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This paper introduces the use of germanium as resistive material in RF MicroElectroMechanical (MEMS) devices. Integrated resistors are indeed highly required into RF MEMS components, in order to prevent any RF signal leakage in the bias lines and also to be compatible with ICs. Germanium material presents strong advantages compared to others. It is widely used in microtechnologies, notably as an important semi-conductor in SiGe transistors as well as sacrificial or structural layers and also mask layer in various processes (Si micromachining especially). But it also presents a very high resistivity value. This property is particularly interesting in the elaboration of integrated resistors for RF components, as it assures miniaturized resistors in total agreement with electromagnetic requirements. Its compatibility as resistive material in MEMS has been carried out. Its integration in an entire MEMS process has been fruitfully achieved and led to the successful demonstration and validation of integrated Ge resistors into serial RF MEMS variable capacitors or switches, without any RF perturbations.

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A brushlike array of metallic carbon nanotubes is integrated in a trench etched in a silicon substrate and terminated with two coplanar lines. When the incoming microwave signal frequency is equal to the mechanical resonance frequency of the nanotubes forming the array, a deep and narrow notch is formed in the microwave frequency response of the array, indicating that the array is acting like a resonator. The quality factor of the array at the fundamental resonance frequency of 1.4 GHz is around 800 at room temperature.

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