(Invited) Highly Functional Graphene Nano-Electromechanical (GNEM) Devices for Advanced Switch and Sensor Applications

Graphene, a true two-dimensional material with extraordinary mechanical- and electronic properties, is thought to be ideal for nanoelectromechanical systems (NEMS), like mass- and force sensors. Here, we present two different ways to fabricate suspended graphene for the intended use in future NEMS applications. The fabrication schemes do not require transfer of graphene from a catalyst where the graphene is grown on to another supporting substrate. The transfer is a source of several issues causing irreproducibility in large-scale production of graphene devices. We obtain suspended graphene membranes by locally removing the copper thin film on top of which the graphene is catalytically grown. The membranes are uniform and exhibit mechanical properties similar to those of exfoliated graphene. Also, suspended graphene beams with electrical interconnects are fabricated from non-catalytically grown graphene on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Both approaches represent the first steps towards transfer-free fabrication of suspended graphene for NEMS applications.

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.

Graphene is a wonderful material with outstanding properties which is ideal for micro-electro-mechanical systems (MEMS) and also nano-electro-mechanical systems (NEMS) like sensors and switches including strain gauges, mass and force sensors. Graphene is single atom thick layer of graphite which consists of carbon atoms that are bonded together in a hexagonal honeycomb lattice. A way of fabricating suspended graphene is presented. Graphene grown on copper (Cu) foil using chemical vapour deposition (CVD) method is used for transference on silicon wafer with etched holes on top. Graphene is transferred on the silicon using Poly (methyl methacrylate) (PMMA) method and the analysis is done using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The suspended graphene is quite uniform and the results show that it is a promising approach for suspended graphene, however, considerable effort is needed for the development of the transference and fabrication process.

The trend of miniaturization has led to the development of graphene nanoelectromechanical system (NEMS) switch due to high demand for low power consumption device applications. One of the important step before start the fabrication process is to model and simulate the graphene NEMS switch in order to get the optimum device dimension with low pull-in voltage applications. The present work represents simulations of suspended graphene length and air beam effects on the pull-in voltage for NEMS switch application. The analysis is done by 3D simulation using COMSOL Multiphysics software under electromechanics interface based on Finite Element Method (FEM). Comparable values with the conventional semiconductor switch of the pull-in voltage can be achieved at ratio below 10 to 0.5 value of the graphene beam length and air gap, respectively. This article is expected to estimate the operational parameter and dimension that can produce low pull-in voltage for a graphene NEMS switch operations.

Graphene-based thermal rectification was investigated by measuring the thermal transport properties of asymmetric suspended graphene nanomesh devices. A sub-10 nm periodic nanopore phononic crystal structure was successfully patterned on the half area of the suspended graphene ribbon by helium ion beam milling technology. The ‘differential thermal leakage’ method was developed for thermal transport measurement without disturbance from the leakage of electron current through the suspended graphene bridge. A thermal rectification ratio of up to 60% was observed in a typical device with a nanopore pitch of 20 nm. By increasing the nanopore pitch in a particular range, the thermal rectification ratio showed an increment. However, this ratio was degraded by increasing the environmental temperature. This experiment suggests a promising way to develop a high-performance thermal rectifier by using a phononic crystal to introduce asymmetry on homogeneous material.

Graphene nanoelectromechanical (NEMS) systems offer great promise for ultrasensitive force, mass, and pressure sensors due to their extremely low mass and electronic integrability. Crucial to these applications is the ability to tune the shape of the mechanical modes and their corresponding resonance frequencies. Here, we show that Focused Ion Beam (FIB) milling is a high-yield means to engineer the shape of graphene NEMS, which in turn modifies their mode shape. We describe the specialized cutting methods necessary to achieve complex geometries such as beams, tapered beams, and trampolines. We transduce the mode shapes using optical interferometry and show that they are consistent with membrane theory. This work is an enabling step for future work which relies on well-defined mode shapes, such as point mass-sensing.

The direct growth of graphene on insulating substrate is highly desirable for the commercial scale integration of graphene due to the potential lower cost and better process control. We report a simple, direct deposition of nanocrystalline graphene (NCG) on insulating substrates via catalyst-free plasma-enhanced chemical vapor deposition at relatively low temperature of ∼800 °C. The parametric study of the process conditions that we conducted reveals the deposition mechanism and allows us to grow high quality films. Based on such film, we demonstrate the fabrication of a large-scale array of nanoelectromechanical (NEM) switches using regular thin film process techniques, with no transfer required. Thanks to ultra-low thickness, good uniformity, and high Young's modulus of ∼0.86 TPa, NCG is considered as a promising material for high performance NEM devices. The high performance is highlighted for the NCG switches, e.g. low pull-in voltage <3 V, reversible operations, minimal leakage current of ∼1 pA, and high on/off ratio of ∼10(5).

Design and Simulation of RF MEMS Switch for X-band Applications

We report the first ever silicon carbide (SiC) nano-electro-mechanical systems (NEMS) switches capable of operation from 25 to 600degC. We have developed both laterally- and vertically-actuated designs with threshold voltages of less than 5 V. Switches have been verified to cycle more than 3 billion times at room temperature and more than 2 million times at 600degC. Plausible models to describe the contact resistance in two actuation voltage regimes are also suggested: (i) at low actuation voltage, the resistance is dominated by the native oxide which forms on the SiC surface; and (ii) at higher actuation voltages, the resistance can be described by Sharvin's model.

A study has been made of the rate coefficients and product ion distributions for the reactions at 300 K of the ions N+, N2+, N3+, N4+, O+, O2+, and NO+ with CH3NH2, NH3, H2S, CH3OH, H2CO, COS, O2, H2O, CH4, CO2, CO, H2, and N2 molecules listed in increasing order of their ionization energies. These measurements are intended as a contribution to stratospheric chemistry. In the binary reactions of the ions of large recombination energy with molecules of low ionization energy, multiple ion products generally result and the rate coefficients are close to gas kinetic. Conversely, the low recombination energy ions NO+ and O2+ generally undergo ternary association reactions with the large ionization energy molecules. The reactions of N2+ and N4+ are very similar, the most common mechanism apparently being direct charge transfer usually followed by fragmentation, the nitrogen–nitrogen bonds in the reacting ions remaining intact. The N+ and N3+ reactions differ from the N2+ and N4+ reactions in that they show a greater propensity to form N–X bonds, X=O, C, S, H, etc. The O+ and O2+ reactions generally proceed via direct charge transfer where energetically possible.

Graphene nanoelectromechanical (GNEM) devices functionalized by using helium ion beam for nanoscale thermal engineering

A DFT study of transition metal (Fe, Co, Ni, Cu, Ag, Au, Rh, Pd, Pt and Ir)-embedded monolayer MoS2 for gas adsorption

Inductively coupled plasma nanoetching of atomic layer deposition alumina

In the present work, a bare (6,0) single-walled carbon nanotube (SWCNT)is chosen to be surface functionalized with a single-stranded DNA (ssDNA)oligomer in order to facilitate a transition in the intrinsic electronic property of the former entity i.e. shift from a metallic to semiconducting nature. The coupled system has been designed with the help of ATK-VNL simulation tool. Induction of a bandgap in SWCNT on account of ssDNA functionalization is observed. The bandgap value is obtained to be 0.016 eV, Transmission spectra and Eigenstate analyses support relevant changes in the bandstructure. I-V characteristic plots also elucidate the semiconducting behaviour of the SWCNT-ssDNA hybrid model. This study could be helpful for opening newer avenues for designing Nano Electromechanical Systems (NEMS)for biomedical and healthcare applications.

Nanoelectromechanical (NEM) switches have the advantages of zero leakage current, abrupt switching characteristics, and harsh environmental capabilities. This makes them a promising component for digital computing circuits when high energy efficiency under extreme environmental conditions is important. However, to make NEM-based logic circuits commercially viable, NEM switches must be manufacturable in existing semiconductor foundry platforms to guarantee reliable switch fabrication and very large-scale integration densities, which remains a big challenge. Here, we demonstrate the use of a commercial silicon-on-insulator (SOI) foundry platform (iSiPP50G by IMEC, Belgium) to implement monolithically integrated silicon (Si) NEM switches. Using this SOI foundry platform featuring sub-200 nm lithography technology, we implemented two different types of NEM switches: (1) a volatile 3-terminal (3-T) NEM switch with a low actuation voltage of 5.6 V and (2) a bi-stable 7-terminal (7-T) NEM switch, featuring either volatile or non-volatile switching behavior, depending on the switch contact design. The experimental results presented here show how an established CMOS-compatible SOI foundry process can be utilized to realize highly integrated Si NEM switches, removing a significant barrier towards scalable manufacturing of high performance and high-density NEM-based programmable logic circuits and non-volatile memories.