Fabrication of 2d Material Based Resonators

Improvements in manufacturing processes inspired by the semiconductor integrated circuit industry have seen a sharp reduction in dimensions of microelectromechanical systems (MEMS), leading to the emergence of its submicron counterpart – nanoelectromechanical systems (NEMS). NEMS resonators, nano-scale vibrating structures, have proven to be extremely sensitive sensors of physical phenomena which affect its resonance behaviour. The arrival of graphene and other 2D materials with ultrahigh surface to volume ratio, ultralow mass and a diverse range of superior electrical and mechanical properties marked an important moment in NEMS resonators’ history as the reality of having extremely sensitive NEMS resonator based sensors seems imminent. Although a fair amount of research has demonstrated the potential of 2D materials for NEMS resonators, the employed fabrication methods either are not scalable or involve too many fabrication steps. An efficient and mass-reproducible fabrication process for 2D material based NEMS resonators has been successfully demonstrated. This fabrication method is predicted to be compatible with any 2D materials although graphene was chosen for availability and cost purposes. Metal electrodes for electrostatic actuation and detection are lithographically patterned on a silicon wafer with wet thermal oxide which was then diced into small substrates to host graphene. Graphene was transferred using a wet-transfer method on these electrode chips before being optimally patterned with photolithography and oxygen plasma etch. Freestanding graphene beams were obtained by wet etching the underlying oxide and critical point drying. Compared to the existing large-scale fabrication methods of 2D material based NEMS resonators, suspended graphene is clamped on top of metal electrodes fabricated on the wafer level, resulting in a smaller number of fabrication steps and minimal exposure of graphene to sacrificial layers. The fabricated devices were electrically characterised and the graphene’s sheet resistances were derived and benchmarked against those found in literature. The characteristic bipolar field effect transistor (FET) behaviour of graphene was confirmed. The generic fabrication approach demonstrated in this thesis holds great promise for 2D material based resonators being fabricated with high reproducibility and scalability.

Fabrication of 2D Material Based NEMS Resonators

Tom Larsen, An Tran

2D materials such as graphene, MoS2 and phosphorene are promising candidates for resonating mechanical sensors due to excellent tunable electrical/- mechanical properties, low masses, and maximal surface to volume ratios. Mechanical resonators based on supsended graphene have been realized. - CVD graphene wet-transferred onto 10x10 mm Si/SiO2 chips with Au electrodes - Graphene patterned in O2 plasma with a resist mask - Graphene is released by etching SiO2 in BHF and critical point drying

2017

Nano-electromechanical systems based on ultra-thin semiconductors

Sajedeh Manzeli

Nowadays, the interest in 2D materials has gone far beyond graphene. Specially, monolayers of transition metal dichalcogenides (TMDs) offer a broad spectrum of electronic and optical properties, and show the potential to revolutionize the electronics industry. The promising electronic properties of 2D semiconductors combined with their mechanical strength and flexibility, makes them ideal candidates for nanoelectromechanical systems (NEMS). This thesis focuses on realizing NEMS based on graphene and MoS2, which is one of the most appealing TMDs. First, we present the realization of graphene NEMS by fabricating single and bilayer graphene transistors featuring a doubly clamped suspended channel. The electromechanical response of monolayer graphene nanoribbons show a strain-induced increase in their electrical resistance, making it possible to estimate an upper limit for the piezoresistive gauge factor. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. Our numerical simulations indicate that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of the two graphene layers with respect to each other. Our results report on the rare observation of room temperature electronic interference in bilayer graphene. Next, we investigate the static electromechanical response of atomically thin MoS2. MoS2 exhibits high youngâs modulus and fracture strength. Furthermore, the bandgap of MoS2 is highly strain-tunable. This coupling between electrical and mechanical properties makes MoS2 a promising material for NEMS. Here we incorporate monolayer, bilayer and trilayer MoS2 in a suspended membrane configuration with the electrodes acting as mechanical clamps. Strain-induced bandgap tuning is detected via electrical conductivity measurements and the emergence of the piezoresistive effect in MoS2 is demonstrated. We observe a reversible bandgap modulation in atomically thin MoS2 membranes with a thickness dependent modulation rate. Finite element method (FEM) simulations are used to obtain the spatially varying bandgap profile on the membrane and to quantify the rate of bandgap change. The piezoresistive gauge factor is calculated for single layer, bilayer and trilayer MoS2. Our results reveal that monolayer and bilayer MoS2 show a piezoresistive effect which is comparable to the state-of-the-art silicon strain sensors and two orders of magnitude higher than in graphene. Finally, we present the investigation of MoS2 NEMS resonators working in the VHF range and featuring piezoresistive transduction. The atomic thickness of monolayer MoS2 places it as a promising candidate for miniaturization of electromechanical devices to the limits of vertical scaling. While the small mass of MoS2 leads to an increased resonant frequency and a higher mass sensitivity, the presence of piezoresistivity offers a transduction mechanism in addition to the traditional capacitive transduction. Operating in the tension-dominated regime, monolayer MoS2 NEMS resonators not only allow tunability of the resonant frequency using an external voltage, but also show the strain-induced enhancement of their dynamic range. Furthermore, the resonators are driven into the nonlinear regime allowing the study of nonlinear effects. This work sheds light on the potential of TMD based NEMS as ultra-low power switches, sensors and resonators for applications in RF range.

EPFL2018

Carbon Nanotube Electromechanical Devices for Radio Frequency Applications

Anupama Arun

Carbon nanotubes (CNT) have been investigated extensively in the recent years for application in nano-electromechanical systems (NEMS). CNTs have extraordinary electrical and mechanical properties. They are stiff, with low density, high electrical conductivity and hence are the ideal candidates for NEMS. Beyond miniaturization, NEMS can also offer better performance than its predecessor micro-electro mechanical systems (MEMS). NEMS find application in ultra-sensitive mass and force detection, RF signal processing, low power operated switches with fast switching speeds, to name a few. Despite tremendous promise the main challenge in CNT research has been the fabrication technology to realize CNT based devices with precise control of location and properties of CNT. CNTs are one-dimensional structures with high aspect ratio. To realize devices for practical application with high signal-to-noise-ratio, it becomes necessary to operate many of them in parallel. To fabricate an array of CNTs in parallel posse's additional challenge to the fabrication technology but the resulting structures could provide benefits compared to both NEMS fabricated with one dimensional structures and MEMS. During the course of this thesis, different fabrication technologies were investigated to realize CNT array based devices. A self assembly method, dielectrophoresis and CVD growth were optimized to realize CNT based devices. A novel technique using a modified dielectrophoresis method was setup to be able to trap a single CNT. A CNT crossed junction was fabricated using the same. The electrical properties of the junction were studied. A Suspended Gate CNT array Field Effect Transistor (SG-CNT-FET) based resonator was demonstrated for the first time. SG-CNT-FET combines the excellent mechanical properties of CNTs with the gain of the solid state field effect transistor. The use of an array configuration ensures better signal-to-noise-ratio compared to device implemented with a single CNT. A novel process flow combining the fabrication of a FET and deposition of CNTs by dielectrophoresis was developed. Resonance frequency up to 150 MHz with a quality factor of approximately 50 was obtained. The resonance frequency is tuneable by modulating the tension in the CNT array by applying a DC bias. Dependence of the resonance frequency on the applied bias has been studied under the frame work of a continuum mechanics model. The small signal circuit parameters have been extracted, enabling future circuit design. Vertically grown CNT array were grown on metal lines to realize a tuneable MEM capacitor. The CNT array forms a membrane like structure which can be electro-statically actuated. Morphological and electrical characterizations were performed on the fabricated devices. DC characterization revealed the pull-in behavior like conventional MEMS. The Young's modulus of the grown vertical CNT array was extracted from the measured pull-in voltage. An effective Young's modulus of 30-100 MPa was obtained. An equivalent electrical model was developed to extract the capacitance from the S-parameters measurements, measured up to 6 GHz. The model was validated by ADS circuit simulations. A voltage controlled oscillator at 2.45 GHz has been implemented using the thus realized tuneable MEM capacitor.