Polycrystalline silicon carbide NEMS for high-temperature logic

Microwave switches are extensively used in many wireless and satellite communication systems. They are mainly used for signal routings and reconfigurations. Recently, due to the development of micro-mechanical systems (MEMS), RF MEMS switches are being widely investigated. Although RF MEMS switches have been intensively studied, limited work was done on switches that are suitable for integration in large switching networks such as switch matrices and multi-bit phase shifters [1, 2]. These switching networks require very small switches each with independent controls. Their major existing problems are the high actuation voltage and the low switch yield. High actuation voltage is undesired as a more complicated circuitry is required to provide suitable control signal. Low yield is another drawback as it could result in malfunctioning of some of the switches and thus the entire integrated system. One of the major contributions in resulting of low yield in RF MEMS switches are their dependency to the residual stress of the fabrication process both across one wafer and across batches. This problem is much more pronounced on switches with long length as they deform to their curled shape and prevent switching. In this paper, a novel type of switch is proposed that could reduce the stress sensitivity of the switch as well as providing a low actuation voltage. The overall switch structure is design with size miniaturization.

This paper presents the design, analysis, modeling and simulation of a novel RF MEMS series switches with low actuation voltage. A mechanical modeling is presented to describe the behavior of the series switch. The switch is designed with special mechanical structures. The novel mechanical and mathematical modeling of the switch leads to calculation of the accurate actuation voltage. The spring constant has been calculated in relation to the presence of the residual stress in the beam. The calculated spring constant for this beam is used to determine the accurate actuation voltage. The size of the switch is 60 ? 110 µm2. The designed RF MEMS series switch was simulated using Intellisuite MEMS tool. He calculated actuation voltage is 4.05 V and simulated one is 4.2 V for 0.6 µm beam thickness. The calculated result is also very close with simulated one. The proposed switch compared with other electrostatic switches has low actuation voltage and small size. The RF characteristics were simulated using HFSS software and the switch has good RF performance. The insertion loss of 0.067 dB, return loss of 26 dB and isolation of 16 dB were achieved at 40 GHz.

In today’s world one of the major concerns for the electronics engineers and scientists is how to reduce the power consumption of the electronics devices. To overcome this problem we come across different technologies. Radio Frequency (RF) Microelectromechanical (MEMS) switches are one of the leading technologies for past two decades. But the main concern with RF MEMS switches is its high actuation voltage and high switching time. This paper generally concentrates on the simulation of RF MEMS contact switch having Y-shaped cantilever beam to obtain low actuation voltage with less switching time. The simulation and results are obtained by the Intellisuite 8.7v software. The actuation voltage depends on the air gap, Young’s modulus, thickness of the beam etc. The “pull-in” voltage is found to be 1.2 V and the switching time is about 52.3 µs.

Graphite and silicon carbide (SiC) are important materials of fuel elements in High Temperature Reactor-Pebble-bed Modules (HTR-PM) and it is essential to analyze the source term about the radioactive products adsorbed on graphite and SiC surface in HTR-PM. In this article, the adsorption behaviors of activation product Cobalt (Co) on graphite and SiC surface have been studied with the first-principle calculation, including the adsorption energy, charge density difference, density of states, and adsorption ratios. It shows that the adsorption behaviors of Co on graphite and SiC both belong to chemisorption, with an adsorption energy 2.971 eV located at the Hollow site and 6.677 eV located at the hcp-Hollow site, respectively. Combining the charge density difference and density of states, it indicates that the interaction of Co-SiC system is stronger than Co-graphite system. Furthermore, the variation of adsorption ratios of Co on different substrate is obtained by a model of grand canonical ensemble, and it is found that when the temperature is close to 650 K and 1700 K for graphite surface and SiC surface, respectively, the Co adatom on the substrate will desorb dramatically. These results show that SiC layer in fuel element could obstruct the diffusion of Co effectively in normal and accidental operation conditions, but the graphite may become a carrier of Co radioactivity nuclide in the primary circuit of HTR-PM.

Graphene, as a monolayer of graphite, has an ultra-high Young’s modulus of ~1 TPa, which makes it a promising candidate for Nano-Electromechanical (NEM) devices. The graphene NEM switches is expected to show minimized electrical leakage, sharp switching response, low actuation voltage, and high on/off ratio. A simple bottom-up procedure using a polymer sacrificial spacer is utilized to fabricate graphene electromechanical contact switch devices. Low pull-in voltage of below 5 V is achieved with good consistency [1], which is compatible with the conventional complementary metal-oxide-semiconductor circuit requirements. In addition, the formation of carbon-gold bonds at the contact position is proposed as another important mechanism for the irreversible switch other than the well-known irreversible static friction. In order to overcome the stiction issue and realize repetitive switching operations, we exploited a natural chromium oxide layer formed on the chromium electrode. We fabricated a doubly-clamped beam and cantilever graphene nanoelectromechanical switches with a local top actuation electrode. The low pull-in voltage below 5 V was realized in both of the switches. The naturally formed chromium oxide at the graphene-electrode contact interface prevents the formation of chemical bonds between graphene and metal, resulting in reversible switching operation successfully. Thanks to highly-stable SP2 bonds in a 2D honeycomb lattice, graphene exhibits an extremely suppressed electrical noise, making it possible to realize extremly-sensitive sensor. For the physisorbed CO2 molecules - graphene vdW complexes, we found it is possible to tune the vdW interaction with external electric fields. The field-dependent charge transfer in the CO2−graphene complex was unveiled along with small variations in the equilibrium CO2−graphene distance and the O−C−O bonding angle. By reversing the substrate bias polarity, the charge transfer direction also switched, signifying the role of physisorbed CO2molecules can be altered electrically between donor and acceptor. The range of such electrical tunability was found inherent in individual molecular species [2]. In recent years, direct milling of graphene by Helium ion beam (HIB) has been actively explored. The main advantages of HIB milling over typical gallium focused ion beam (FIB) milling are: high resolution and less damage to the graphene. This HIB milling can be actively explored to carve the suspended graphene into a very narrow beam, which will help to realize narrower suspended graphene beams, which can’t be realized with the standard electron beam lithography technique. Towards this direction, gas field ionization source (GFIS) system with Helium and Nitrogen beams is introduced. In order to use HIB milling, a suspended monolayer graphene beam device was first fabricated without a top gate. These devices were used for the further HIB milling. We successfully patterned a 60-nm-long and 15-nm wide suspended graphene beam. As the next step to utilize our unique graphene nano-electro-mechanical systems (NEMS) to realize a well challenging application, the NEMS sensor devices were explored to realize individual CO2 molecule detection. Detection of a single molecule’s adsorption and desorption, which determines the ultimate resolution of gas sensing, has rarely been achieved with conventional solid-state sensor devices. A doubly clamped bilayer graphene beam was first fabricated and then by applying a voltage to the lower gold electrode the graphene beam was electrostatically pulled down. It was found that despite the negligible charge transfer from a single very poor doping CO2 physisorbed molecule, it strongly affects the electronic transport in suspended graphene beam by inducing charged impurity, which can shut down part of conduction of the graphene channel with Coulomb impurity scattering. We detected each individual physisorption of CO2molecule as a step-wise resistance change with a quantized interval for the graphene channel [3].

Silicon carbide (SiC) is a promising material for power electronic devices; however, the fabrication of low-cost and high-quality single-crystal SiC wafers with a smooth surface remains difficult, due to its high mechanical hardness and chemical stability. We report an abrasive-free electrochemical mechanical polishing (AF-ECMP) for a SiC surface using Ce film deposited on a polishing pad. The SiC surface is oxidized by electrolysis between the SiC (anode) and the polishing plate (cathode), the resulting oxide is then removed by interaction with the Ce film. The excess oxide produces a rough surface during continuous electrolysis on the SiC surface. The periodic electrolysis improves the surface quality, and electrolysis conditions such as duty ratio, duty cycle, and applied voltage are optimized. The abrasive ECMP, using CeO2 particles, broadens the initial scratches on SiC, whereas no scratch broadening is observed by the AF-ECMP. Despite a lower material removal rate (MRR) than the abrasive ECMP, the surface roughness was improved by the AF-ECMP with less polishing time. AF-ECMP produces an atomically smooth SiC surface with a roughness of ∼0.5 nm Ra, which is promising for low-cost and environment-friendly finishing process for SiC.

The wetting of silicon carbide surfaces

Aluminum nitrate (Al(NO3)3) and sodium polystyrene sulfonate (PSS) were used to improve the dispersion of fine silicon carbide (SiC) powders. Effects of modification parameters on the viscosity of modified SiC slurry were studied by orthogonal experiments. Modified SiC slurry with the solid loading of 50 vol% reached the lowest viscosity of 34 MPa s. The adsorption processes of PSS on the as‐received and Al(NO3)3 premodified SiC surface were investigated. The results indicated that the adsorption between PSS and premodified SiC surfaces was a high affinity type and was mainly controlled by active sites on an SiC surface. The Langmuir model and the pseudo‐second‐order model could better fit the adsorption isotherm and kinetics data, respectively. The contact angle decreased from 32.8 to 15.2° and the wettability was improved by modification. The isoelectric point of modified SiC powder shifted to the acidic region and the maximum zeta potential was obtained at pH 11. Sedimentation results also showed that a stable dispersed suspension of modified SiC was achieved at pH 11. Density–pressure curves demonstrated that the flowability and formability of SiC powder were improved by modification. The dispersion effect of PSS on SiC and Al2O3 composite powder was verified by viscosity and sedimentation results.

As a new post-implantation activation annealing of Silicon Carbide (SiC), we propose the Si-vapor ambient anneal using Tantalum Carbide / metal Tantalum composite materials (TaC/Ta). In this technique, semi-closed TaC/Ta container which can supply Si-vapor ambient is used, and Si vapor compensates thermal desorption Si atoms from the SiC surface above 1500°C and can maintain the original surface morphology by controlling a process temperature and Ar back pressure. Therefore the Si-vapor ambient anneal is able to simplify the process of conventional activation anneal methods using refractory cap-layers for protecting SiC surface from thermal damage of Si-atom desorption. Experiments were performed under Ar 1.3kPa at 1600/1700°C for 5min optimized conditions in a 6inch TaC/Ta container, and the Al+ ion-implanted 4H-SiC properties after annealing were characterized by atomic force microscopy (AFM), Rutherford Back-scattering Spectrometry (RBS) channeling method, and four-point probe method. According to evaluation, there was no roughening of SiC surface from AFM topographic images and recovery of crystallinity at the ion-implanted layer was equivalent to by the conventional cap-layer method from RBS channeling measurement. The sheet resistance of 12kΩ/ at 1700°C equal to the typical Al+ ion implanted p-type SiC is confirmed by four-point probe method.

RF-MEMS switches may be divided into capacitive and metal-to-metal types in terms of type of connection. In capacitive switches in the OFF-state the beam does not attach to the transmission line due to the presence of a thin dielectric layer and it gets released easily. Capacitive switches are better compared with the metal-to-metal types due to their ability to transmit signals of higher frequency and power. In this paper, a novel RF MEMS shunt capacitive switch with a unique spring design is proposed. The proposed design benefits from high isolation, low loss, low actuation voltage, small size and weight compared to similar switches and is designed for application in the Ka-band. Step structure is used in the mechanical design of the switch to reduce the air gap between the bridge and the transmission line that has resulted in reduction of actuation voltage to 2.2 V. The proposed switch has been placed upon a coplanar waveguide (CPW) line with an impedance of 50 Ohms. $${\text{SiO}}_{2}$$ with a thickness of 1000 A has been used as dielectric to increase isolation and down-state capacitance. The unique design of the spring results in the reduction of stress to a minimum at all points of the switch especially the springs at the time of applying the actuation voltage. This advantage would reduce switch failures as time passes and results in increased life of the switch. Up-state capacitance is 93.3 fF and down-state capacitance is 2.64 pF. Therefore, a capacitive ratio of 28 is obtained. The dynamic behavior of the switch is studied by using the COMSOL Multiphysics software package and the RF characteristics have been obtained by using the HFSS software and show good RF performance. The results show that in the up-state, the designed switch including S11 is less than − 11.5 dB and S21 is more than − 0.85 dB at 1–40 GHz. In the down-state, the switch has excellent isolation in the Ka-band. Maximum isolation is − 71 dB that occurs at the resonance frequency of 30.5 GHz.

Cellulose based electro-active paper(EAPap) is considered as a new smart material which has a potential to be used for biomimetic actuators and sensors. Beside of the natural abundance, cellulose EAPap is fascinating with its biodegradability, lightweight, high mechanical strength and low actuation voltage. When the external stress is applied to EAPap, it can generate the electrical output due to its piezoelectric property. Using piezoelectric behavior of EAPap, we studied the feasibility of EAPap as mechanical strain sensor applications and compared to commercial strain sensor. By measuring the induced output voltage from the thin piezoelectric cellulose EAPap under static and dynamic force, we propose cellulose EAPap film as a potential strain sensor material.

Low Actuation Wideband RF MEMS Shunt Capacitive Switch

Silicon carbide surfaces were heated to 1500 C in a vacuum and analyzed at room temperature with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The basic unit of the surfaces was considered as a plane of a tetrahedron of either SiC4 and CSi4 composition. AES spectra were obtained from 250-1500 C, with an analysis depth of 1 nm revealed the presence of little Si and mostly graphite. XPS analysis depth was 2 nm or less, and Si was found in the second 1 nm. Sliding friction tests with single-crystal silicon carbide in contact with iron in a vacuum were characterized by a stock-slip value. The coefficient of friction increased with increasing temperature up to 400 C, then decreased with increasing temperature from 400-600 C. Reheating surfaces to 800 C after preheating them to that temperature produced no changes in AES readings. It is concluded that the maximum density of silicon and silicon-carbide is at 800 C, and the higher the sliding temperature, the more metal that is transferred.

This paper presents the design a capacitive shunt type RF-MEMS switch with high isolation, high switching speed and low actuation voltage for Ka-band applications. The proposed RF-MEMS switch has chosen different structures to reduce the actuation voltage. This paper investigates various analysis such as Electromechanical and RF characteristics of the proposed switch. The beam material taken as the gold and the dielectric is taken as Si3N4 with ℰr as 7.5 and the pull-in voltage of proposed switch obtained as 3.37 V. The proposed different RF-MEMS switches have been analyzed over the range of 26.5–40 GHz frequency. The clamped-clamped structure type RF MEMS Switch is having high isolation as − 46.37 dB at 40 GHz. The perfection of RF losses makes these switches as a good choice for a high frequency of K-band.

A vertical RF-MEMS (Radio Frequency Micro ElectroMechanical Switch) switch design is proposed in this work that features a shunt capacitive configuration and can operate at a very low actuation voltage. Besides operating on a low switching voltage, the switch behold a small value of insertion loss in Up (ON) state and offers a towering isolation factor in Down (OFF) state. A horizontal structure of bridge membrane is used in the proposed design and the switch function (with ON and OFF operating states) is provided by the vertical movement of this membrane structure that is controlled by the electrostatic actuation technique. The actuation pad is centrally fed by a Coplanar Waveguide structure. The switch performance has been analysed over a wide frequency range from 1 to 40 GHz. For obtaining a 1.5 μm vertical displacement of the bridge membrane, a sheer 3.0 Volts of pull-in voltage (VPI) is required. CoventorWare and HFSSv13 tools are used in this work for designing and analysing the proposed switch. The use of fixed-fixed flexure beam configuration is chosen for switch design. Simulation results showcase excellent RF characteristics with isolation factor of −47 dB (at 32.5 GHz) and insertion loss in range of −0.01 to −1.10 dB (for 1−40 GHz frequency).