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Abstract

This paper describes the multiphysic methodology developed to design carbon nanotubes (CNT) based variable capacitor (varactor). Instead of using classical RFMEMS design methodologies; we take into account the real shape of the CNT, its nanoscale dimensions and its real capacitance to ground. A capacitance-based numerical algorithm has then been developed in order to predict the pull-in voltage and the RF-capacitance of the CNT-based varactor. This software, which has been validated by measurements on various devices, has been used to design varactor device for which 20V of actuation voltage has been predicted. We finally extend the numerical modeling to describe the electromagnetical behavior of the devices. The RF performances has also been efficiently predicted and the varactor (in parallel configuration) exhibits predicted losses of 0.3dB at 5GHz and quality factor of 22 at 5 GHz, which is relevant for high quality reconfigurable circuits requirements where as the expected sub-microsecond switching time range opens the door to real time tunability.

Abstract

This paper describes a design methodology specifically developed for RF-MEMS based tuner, which translates into optimal performances in terms of both impedance coverage of the Smith Chart and ï“MAXú value. The design methodology is moreover associated with an RF-MEMS technology developed for medium power (in the watt range) applications and exhibiting a design robustness regarding the capacitive RF-MEMS contact quality. A tuner has been designed thanks to this efficient procedure, fabricated and measured. The circuit exhibits excellent measured RF-performances: a wide impedance coverage is reached from the first test with a value of [“MAX] of 0.82 at 14GHz.

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.

Mots-Clés / Keywords

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.

Mots-Clés / Keywords