Nano-Electro-Mechanical Switches Based on Carbon Nanotubes Arrays

In the last forty years the semiconductor industry focused on the downscaling process of the CMOS (Complementary Metal-Oxide-Semiconductor) technology, trying to follow as much as possible the empirical Moore's Law. In the last five years this trend has been facing technology limits that have led to the development of a new research field, named Beyond CMOS, which investigates new electron devices concept and materials that can replace or co-integrate the standard technology, to make large scale integrated (LSI), high performance, low power devices. Carbon nanotubes (CNTs) are considered one of the most promising materials for Beyond CMOS devices, both at front-end and back-end level. The development of the research on this new organic material is due to its nanometer scale dimensions and to its outstanding electrical (metallic and semiconducting behavior, ballistic conduction, current density > 109 A/cm2) and mechanical (Young's modulus ∼1 TPa) properties, nevertheless due to its low mass (mass density ρ = 1300 kg/m3). Among the several CNT applications already reported in literature (interconnections, sensors, transistors, memory, etc.), a particular interest is on their use to build Nano-Electro-Mechanical (NEM) devices, such as resonators and switches. NEM relays based on carbon nanotubes show large-current density drive and high speed operation compared to metal-based NEMS counterparts, low actuation voltage and low power, performances which are promising for radio-frequency (RF) applications, where RF MEM devices are considered as replacement solution for their off-chip and/or solid-state counterparts in front-end transceivers. Most of the previously reported CNT NEM switches concerns relay configuration based on one single movable nanotube and its DC characterization. Despite individual proof-of-concept merits, an electromechanical switch based on a single CNT usually presents an output signal level that is too low for practical applications and a poor reliable fabrication process, which opposes the precise control at wafer level achieved by standard technology. Alternatively, a very large-scale integration of NEM devices requires the exploitation of high-density aligned arrays of nanotubes, grown in precise locations and that could be patterned into well-defined and controlled configurations. The objective of this research work was to develop LSI NEM switches based on high dense carbon nanotube arrays in the back end of the line (BEOL), with conductive and capacitive contacts and tested from DC up to high frequency (RF), in order to evaluate their performances in a wide range of operations. The synthesis process of carbon nanotubes was performed by thermal CVD and temperature, gas concentration and catalyst pretreatment were optimized to achieve high dense aligned CNT arrays. A study on the catalyst geometries and on the substrate materials were performed in order to obtain horizontal aligned arrays grown directly on metallic substrate without density change. We developed a novel surface micromachining process to suspend CNT arrays. The DC characterization of CNT NEM switches was exploited to extract the equivalent electromechanical properties of the CNT membranes, since they behave as a new body material. We extracted an equivalent flexural modulus of 8.5 GPa and a resistivity of 0.0077 Ωcm. The devices, in both configurations, present a very low actuation voltage, below 10 V, lower than typical metal NEM switch for the same dimensions and a comparable switching time. The RF characterization was performed in the telecommunication frequency range (up to 6 GHz) showing reasonably high isolation and low insertion loss. The relay shows a lower reliability than the capacitive switch, due to the elevated drive current which induces the CNTs burning at the metal-to-metal interface. The RF measurements were used to extract the parameters of the proposed devices equivalent lumped model. Several implementations of CNT NEM switches, both for DC (reconfigurable interconnects, logic functions, memories) and RF (signal routing purposes in RF system Front-Ends, phase shifting networks or digitalized capacitor banks) applications, were reported.


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