Nanoelectromechanical Research Articles

Performance Analysis of Nanoelectromechanical Relay-Based Field-Programmable Gate Arrays

The energy consumption of field-programmable gate arrays (FPGA) is dominated by leakage currents and dynamic energy associated with programmable interconnect. An FPGA built entirely from nanoelectromechanical (NEM) relays can effectively eliminate leakage energy losses, reduce the interconnect dynamic energy, operate at temperatures >225 °C and tolerate radiation doses in excess of 100 Mrad, while hybrid FPGAs comprising both complementary metal-oxide-semiconductor (CMOS) transistors and NEM relays (NEM-CMOS) have the potential to realize improvements in performance and energy efficiency. Large-scale integration of NEM relays, however, poses a significant engineering challenge due to the presence of moving parts. We discuss the design of FPGAs utilizing NEM relays based on a heterogeneous 3-D integration scheme, and carry out a scaling study to quantify key metrics related to performance and energy efficiency in both NEM-only and NEM-CMOS FPGAs. We show how the integration scheme has a profound effect on these metrics by changing the length of global wires. The scaling regime beyond which net performance and energy benefits is seen in NEM-CMOS over a baseline 90 nm CMOS technology is defined by an effective relay beam length of 0.5 $\mu \text{m}$ , on-resistance of 200 $\text{k}\Omega$ , and a via pitch of 0.4 $\mu \text{m}$ , all achievable with existing process technology. For ultra-low energy applications that are not performance critical, NEM-only FPGAs can provide close to $15\times$ improvement in energy efficiency.

Nano-electromechanical Switch Based on a Physical Unclonable Function for Highly Robust and Stable Performance in Harsh Environments.

A physical unclonable function (PUF) device using a nano-electromechanical (NEM) switch was demonstrated. The most important feature of the NEM-switch-based PUF is its use of stiction. Stiction is one of the chronic problems associated with micro- and nano-electromechanical system (MEMS/NEMS) devices; however, here, it was utilized to intentionally implement a PUF for hardware-based security. The stiction is caused by capillary and van der Waals forces, producing strong adhesion, which can be utilized to design a highly robust and stable PUF. The probability that stiction will occur on either of two gates in the NEM switch is the same, and consequently, the occurrence of the stiction is random and unique, which is critical to its PUF performance. This uniqueness was evaluated by measuring the interchip Hamming distance (interchip HD), which characterizes how different responses are made when the same challenge is applied. Uniformity was also evaluated by the proportion of "1" or "0" in the response bit-string. The reliability of the proposed PUF device was assessed by stress tests under harsh environments such as high temperature, high dose radiation, and microwaves.

Adjusting the Operating Voltage of an Nanoelectromechanical Relay Using Negative Capacitance

The operating voltage of a nanoelectromechanical (NEM) relay can be scaled down significantly using negative capacitance (NC), which is a unique property of ferroelectric materials. It has been observed that a ferroelectric capacitor can amplify the internal voltage of a device (higher than the externally applied voltage) by means of the NC effect. In this simulation work, depending on the structural parameters of a ferroelectric capacitor and an NEM relay, an NC + NEM relay (in which the gate electrode of the NEM relay is connected in series to the ferroelectric capacitor) was investigated. As a result, a sub-0.5-V operating voltage is theoretically estimated.

Three-Dimensional Finite Element Method Simulation of Perforated Graphene Nano-Electro-Mechanical (NEM) Switches.

The miniaturization trend leads to the development of a graphene based nanoelectromechanical (NEM) switch to fulfill the high demand in low power device applications. In this article, we highlight the finite element (FEM) simulation of the graphene-based NEM switches of fixed-fixed ends design with beam structures which are perforated and intact. Pull-in and pull-out characteristics are analyzed by using the FEM approach provided by IntelliSuite software, version 8.8.5.1. The FEM results are consistent with the published experimental data. This analysis shows the possibility of achieving a low pull-in voltage that is below 2 V for a ratio below 15:0.03:0.7 value for the graphene beam length, thickness, and air gap thickness, respectively. The introduction of perforation in the graphene beam-based NEM switch further achieved the pull-in voltage as low as 1.5 V for a 250 nm hole length, 100 nm distance between each hole, and 12-number of hole column. Then, a von Mises stress analysis is conducted to investigate the mechanical stability of the intact and perforated graphene-based NEM switch. This analysis shows that a longer and thinner graphene beam reduced the von Mises stress. The introduction of perforation concept further reduced the von Mises stress at the graphene beam end and the beam center by approximately ~20–35% and ~10–20%, respectively. These theoretical results, performed by FEM simulation, are expected to expedite improvements in the working parameter and dimension for low voltage and better mechanical stability operation of graphene-based NEM switch device fabrication.

Stress analysis of perforated graphene nano-electro-mechanical (NEM) contact switches by 3D finite element simulation

In this article, we report the finite element method (FEM) simulation of the suspended double-clamped graphene beam-based NEM switches with standard and perforated beam structures, to analyze the von Mises stress, the contour plot and the electrical field distribution by using FEM simulator provided by IntelliSuite and COMSOL software. This FEM numerical model is used to observe the scaling characteristics of the graphene beam-based NEM switch. The FEM results confirm that the von Mises stress value reduced by ~2–3% for every 0.1 µm increment of the standard graphene beam length. The introduction of perforation in the case of HL = 150 nm, HW = 100 nm and DL = 100 nm, further reduced the von Mises stress at the graphene beam end and the beam center by approximately ~20–35 and ~10–20%, respectively. These low values of von Mises stress along the graphene beam resulted in a lower probability of the device failure. In addition, the strong electrostatic force at the hole’s edges and beam's edges leads to a larger mechanical deflection at the edges compared to the beam center and it is consistent with the analysis of von Mises stress. The results presented here are expected to accelerate developments in the working dimension and parameter of the graphene NEM switch device fabrication for better mechanical reliability operation.

Multi-Functional CMOS Compatible Nano-Electro-Mechanical Relays for Vapor Phase Explosive Detection

Nano-electro-mechanical (NEM) relays play a very important role in ultra-low standby power and ultra-low power vapor phase gas sensor for Internet-of-Things sensor applications. In this paper, for the first time, we demonstrate a multi-functional fully-CMOS compatible titanium-based NEM relay with 1.8V actuation voltage, ~100Ω R ON , 50ns turn ON delay, and 10 9 ON/OFF current ratio in the ambient conditions. The innovative feature of this paper lies on the novel combination of grayscale electron beam lithography and a bi-layer lift-off process at room temperature (RT) to realize an NEM relay. This novel combination facilitates easy release of structures with an air gap of 25nm and a beam size of 1μm × 1.5μm. As an example, using the proposed self-purging NEM relay, a vapor phase gas sensing application is successfully demonstrated for explosives detection in an ambient air at RT.

Monolithic three-dimensional tunnel FET–nanoelectromechanical hybrid reconfigurable logic circuits

Monolithic three-dimensional (M3D) reconfigurable logic (RL) circuits of tunnel field-effect transistors (TFETs)–nanoelectromechanical (NEM) memory switches have been proposed, simulated and demonstrated in order to overcome the limitations of CMOS-only RL circuits. A TFET is considered as one of the most promising extremely-low-power logic devices thanks to its abrupt on–off transition as well as low off-current (Ioff) and a NEM memory switch is a good solution to signal path routing thanks to its nonvolatile storage of data signal paths and stable rail-to-rail voltage swing. In our proposed RL circuits, NEM memory switch routing parts are integrated over complementary TFET logic circuits by using conventional CMOS backend process.

Designing Low Power and Durable Digital Blocks Using Shadow Nanoelectromechanical Relays

Nanoelectromechanical (NEM) relays are a promising emerging technology that has gained widespread research attention due to its zero leakage current, sharp ON-OFF transitions, and complementary metal–oxide–semiconductor compatibility. As a result, NEM relays have been significantly investigated as highly energy-efficient design solutions. A major shortcoming of NEMs preventing their widespread use is their limited switching endurance. Hence, in order to utilize the low-power advantages of NEM relays, further device, circuit, and architectural techniques are required. In this paper, we introduce the concept of shadow NEM relays, which is a circuit-level technique to leverage the energy efficiency of the NEM relays despite their low switching endurance. This technique creates two virtual ground nodes in a block to allow: 1) a low power mode with functional NEM relays and 2) a normal mode with failed NEM relays. To demonstrate the applicability of this concept, we have applied it to a six-transistor SRAM cell as an illustrative example. We also investigate the applicability of this SRAM cell in field-programmable gate arrays and on-chip caches. Experimental results reveal that shadow NEM relays can reduce the power consumption of SRAM cells by up to 80% while addressing the limited switching endurance of NEM relays.

Study of electrical breakdown and secondary pull-in failure modes for NEM relays

In this work, two common failure modes of nano-electro-mechanical (NEM) relays: (1) electrical breakdown and (2) stiction due to secondary pull-in were analyzed. These effects are dominant when dimensions of the device are scaled to the sub-micrometer scale. Like MEMS devices, design adjustments, such as introduction of dimples, cannot provide a solution. The geometrical parameters and working environment drive directly the occurrence of these failure modes. The beam length is the key parameter in driving the electrical breakdown while the distance of the gate to the drain, the beam thickness, and the actuation gap set the limits for secondary pull-in voltage. The analysis shows that these failure modes could be mitigated and a physical parameters design space could be identified to achieve NEM devices for high speed operation.

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

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].

3D Finite Element Simulation of Graphene Nano-Electro-Mechanical Switches.

In this paper, we report the finite element method (FEM) simulation of double-clamped graphene nanoelectromechanical (NEM) switches. Pull-in and pull-out characteristics are analyzed for graphene NEM switches with different dimensions and these are consistent with the experimental results. This numerical model is used to study the scaling nature of the graphene NEM switches. We show the possibility of achieving a pull-in voltage as low as 2 V for a 1.5-μm-long and 3-nm-thick nanocrystalline graphene beam NEM switch. In order to study the mechanical reliability of the graphene NEM switches, von Mises stress analysis is carried out. This analysis shows that a thinner graphene beam results in a lower von Mises stress. Moreover, a strong electrostatic force at the beam edges leads to a mechanical deflection at the edges larger than that around the center of the beam, which is consistent with the von Mises stress analysis.

On the free vibrations of size-dependent closed micro/nano-spherical shell based on the modified couple stress theory

Since classical theory is incapable to justify size-dependency of small scale systems such as micro-electro-mechanics (MEMs) or nano-electro-mechanics (NEMs) systems, modified couple stress theory (MCST) has been developed in order to capture the size effect in the small size investigation. A novel study on the free vibrations of the micro/nano-scale spherical shell based on First-order Shear Deformation Theory (FSDT) and MCST is done. Fullerene (C60) is an appropriate example of spherical micro-scale structures. The governing equations of the modified couple stress spherical shell are derived by using Hamilton's principle. Obtained equations are solved using Generalized Differential Quadrature (GDQ) method. The influences of changing geometrical parameter and scale parameter on the natural frequency are investigated. It is shown that the scale parameter is extremely effective on the natural frequency of the micro/nano-sphere. This issue is bolder in the thick spherical shell. Finally, proper scale parameters are proposed for different nano-scale spheres by comparing numerical results with experimental results.

Role of Nanoelectromechanical Switching in the Operation of Nanostructured Bi2Se3 Interlayers between Conductive Electrodes.

We demonstrate a simple low-cost method of preparation of layered devices for opto- and thermoelectric applications. The devices consist of a functional Bi2Se3 layer of randomly oriented nanoplates and flexible nanobelts enclosed between two flat indium tin oxide (ITO) electrodes. The number of functional interconnections between the ITO electrodes and correspondingly the efficiency of the device can be increased by gradual nanoelectromechanical (NEM) switching of flexible individual Bi2Se3 nanobelts in the circuit. NEM switching is achieved through applying an external voltage to the device. For the first time, we investigate in situ NEM switching and breakdown parameters of Bi2Se3 nanobelts, visualize the processes occurring in the device under the influence of applied external voltage, and establish the limitations to the possible operational conditions.

Multi-Bit Nano-Electromechanical Nonvolatile Memory Cells (Zigzag T Cells) for the Suppression of Bit-to-Bit Interference.

Multi-bit nano-electromechanical (NEM) nonvolatile memory cells such as T cells were proposed for higher memory density. However, they suffered from bit-to-bit interference (BI). In order to suppress BI without sacrificing cell size, this paper proposes zigzag T cell structures. The BI suppression of the proposed zigzag T cell is verified by finite-element modeling (FEM). Based on the FEM results, the design of zigzag T cells is optimized.

Back-end-of-line compatible Poly-SiGe lateral nanoelectromechanical relays with multi-level interconnect

Nanoelectromechanical (NEM) relays show promise in a wide variety of low power applications. NEM relays have near-zero leakage current, in contrast to the relatively high leakage current of nanoscale CMOS transistors, thus enabling hybrid CMOS-NEM relay systems that are more energy efficient. If NEM relays can be fabricated in the back-end-of-line (BEOL) metallization process, they can be added to a CMOS integrated circuit without adding significantly to the die area. In this paper, we demonstrate a CMOS BEOL-compatible fabrication process of NEM relays with protected, buried interconnects. The NEM relay processing steps are at temperatures below 425 °C and all mechanical and chemical processing steps are designed to avoid damage to underlying CMOS transistors. We demonstrate a lateral relay with buried interconnect that switches for more than 1000 cycles with resistances below 300 kΩ in nitrogen at atmospheric pressure.

Modelling of Piezothermoelastic Beam with Fractional Order Derivative

AbstractThis paper deals with the study of transverse vibrations in piezothermoelastic beam resonators with fractional order derivative. The fractional order theory of thermoelasticity developed by Sherief et al. [1] has been used to study the problem. The expressions for frequency shift and damping factor are derived for a thermo micro-electromechanical (MEM) and thermo nano-electromechanical (NEM) beam resonators clamped on one side and free on another. The effect of fractional order derivative on the derived expressions is observed analytically and shown graphically in the case of Lead Zirconate Titanate (PZT)-5A material. For α = 1, our results agree with those that are obtained by Grover and Sharma [20] and other particular cases of interest are also discussed.

Large-scale nanoelectromechanical switches based on directly deposited nanocrystalline graphene on insulating substrates.

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).

Three-terminal nanoelectromechanical switch based on tungsten nitride—an amorphous metallic material

Nanoelectromechanical (NEM) switches inherently have zero off-state leakage current and nearly ideal sub-threshold swing due to their mechanical nature of operation, in contrast to semiconductor switches. A challenge for NEM switches to be practical for low-power digital logic application is their relatively large operation voltage which can result in higher dynamic power consumption. Herein we report a three-terminal laterally actuated NEM switch fabricated with an amorphous metallic material: tungsten nitride (WNx). As-deposited WNx thin films have high Young’s modulus (300 GPa) and reasonably high hardness (3 GPa), which are advantageous for high wear resistance. The first prototype WNx switches are demonstrated to operate with relatively low control voltage, down to 0.8 V for an air gap thickness of 150 nm.

Process-Variation-Adaptive Charge Pump Circuit using NEM (Nano-Electro-Mechanical) Relays for Low Power Consumption and High Power Efficiency

For some low-frequency applications such as power-related circuits, NEM relays have been known to show better performance than MOSFETs. For example, in a step-down charge pump circuit, the NEM relays showed much smaller layout area and better energy efficiency than MOSFETs. However, severe process variations of NEM relays hinder them from being widely used in various low-frequency applications. To mitigate the process-variation problems of NEM relays, in this paper, a new NEMrelay charge pump circuit with the self-adjustment is proposed. By self-adjusting a pulse amplitude voltage according to process variations, the power consumption can be saved by 4.6%, compared to the conventional scheme without the self-adjustment. This power saving can also be helpful in improving the power efficiency of the proposed scheme. From the circuit simulation of NEM-relay charge pump circuit, the efficiency of the proposed scheme is improved better by 4.1% than the conventional.

Three-Dimensional Integration of Complementary Metal-Oxide-Semiconductor-Nanoelectromechanical Hybrid Reconfigurable Circuits

Complementary-metal-oxide-semiconductor(CMOS) and nanoelectromechanical (NEM) hybrid reconfigurable circuits are implemented for the first time using three-dimensional (3D) integration process. In addition, their operation is confirmed experimentally. For the fabrication of the 3D CMOS-NEM hybrid reconfigurable circuits, only the standard CMOS baseline process has been used except for the hydrofluoric acid vapor etch to release the NEM structures and focused-ion-beam patterning to define small patterns.