Manufacture and characterization of graphene membranes with suspended silicon proof masses for MEMS and NEMS applications

Graphene's unparalleled strength, chemical stability, ultimate surface-to-volume ratio and excellent electronic properties make it an ideal candidate as a material for membranes in micro- and nanoelectromechanical systems (MEMS and NEMS). However, the integration of graphene into MEMS or NEMS devices and suspended structures such as proof masses on graphene membranes raises several technological challenges, including collapse and rupture of the graphene. We have developed a robust route for realizing membranes made of double-layer CVD graphene and suspending large silicon proof masses on membranes with high yields. We have demonstrated the manufacture of square graphene membranes with side lengths from 7 mu m to 110 mu m, and suspended proof masses consisting of solid silicon cubes that are from 5 mu mx5 mu mx16.4 mu m to 100 mu mx100 mu mx16.4 mu m in size. Our approach is compatible with wafer-scale MEMS and semiconductor manufacturing technologies, and the manufacturing yields of the graphene membranes with suspended proof masses were >90%, with >70% of the graphene membranes having >90% graphene area without visible defects. The measured resonance frequencies of the realized structures ranged from tens to hundreds of kHz, with quality factors ranging from 63 to 148. The graphene membranes with suspended proof masses were extremely robust, and were able to withstand indentation forces from an atomic force microscope (AFM) tip of up to 7000nN. The proposed approach for the reliable and large-scale manufacture of graphene membranes with suspended proof masses will enable the development and study of innovative NEMS devices with new functionalities and improved performances.

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

Fabrication of 2d Material Based Resonators

Tom Larsen, An Tran

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.

2017

Nano-electromechanical systems based on ultra-thin semiconductors

Sajedeh Manzeli

EPFL2018

Fabrication of 2d Material Based Resonators

Tom Larsen, An Tran

2017