Graphene membranes act as highly sensitive transducers in nanoelectromechanical devices due to their ultimate thinness. Previously, the piezoresistive effect has been experimentally verified in graphene using uniaxial strain in graphene. Here, we report experimental and theoretical data on the uni- and biaxial piezoresistive properties of suspended graphene membranes applied to piezoresistive pressure sensors. A detailed model that utilizes a linearized Boltzman transport equation describes accurately the charge-carrier density and mobility in strained graphene and, hence, the gauge factor. The gauge factor is found to be practically independent of the doping concentration and crystallographic orientation of the graphene films. These investigations provide deeper insight into the piezoresistive behavior of graphene membranes.

This paper presents for the first time an RF nonlinearity analysis of complex multidevice radio frequency microelectromechanical system (RF MEMS) circuits. The IIP3 of different RF MEMS multidevice tunable-circuit concepts including digital MEMS varactor banks, MEMS switched capacitor banks, distributed MEMS phase shifters, and MEMS tunable filters, is investigated. Closed-form analytical formulas for the IIP3 of MEMS multidevice circuit concepts are derived. A nonlinearity analysis, based on measured device parameters, is presented for exemplary circuits of the different concepts using a multidevice nonlinear electromechanical circuit model implemented in Agilent Advanced Design System. The results of the nonlinear electromechanical model are also compared with the calculated IIP3 using derived equations for the digital MEMS varactor bank and MEMS switched capacitor bank. The degradation of the overall circuit linearity with increasing number of device stages is also investigated, with the conclusion that the overall circuit IIP3 is reduced by half when doubling the number of stages, if proper design precautions are not taken. Design rules are presented so that the mechanical parameters and thus the IIP3 of the individual device stages can be optimized to achieve a higher overall IIP3 for the whole circuit. In addition, the nonlinearity of a novel MEMS tunable capacitor concept introduced by the authors, based on an MEMS actuator with discrete tuning steps, is discussed and the IIP3 is calculated using derived analytical formulas.

Pressure sensors based on suspended graphene membranes have shown extraordinary sensitivity for uniaxial strains, which originates from graphene's unique electrical and mechanical properties and thinness [1]. This work compares through both theory and experiment the effect of cavity shape and size on the sensitivity of piezoresistive pressure sensors based on suspended graphene membranes. Further, the paper analyzes the effect of both biaxial and uniaxial strain on the membranes. Previous studies examined uniaxial strain through the fabrication of long, rectangular cavities. The present work uses circular cavities of varying sizes in order to obtain data from biaxially strained graphene membranes.

This paper reports for the first time on RF nonlinearity analysis of complex multi-device RF MEMS circuits. The nonlinearity analysis is done for the two most commonly-used RF MEMS tuneable-circuit concepts, i.e. digital MEMS varactor banks and MEMS switched capacitor banks. In addition, the nonlinearity of a novel MEMS tuneable capacitor concept by the authors, based on a MEMS actuator with discrete tuning steps, is discussed. This paper presents closed-form analytical formulas for the IIP3 (nonlinearity) of the three MEMS multi-device circuit concepts, and an analysis of the nonlinearity based on measured device parameters (capacitance, gap), of the different concepts. Finally, this paper also investigates the effect of scaling of the circuit complexity, i.e. the degradation of the overall circuit linearity depending on the number of stages/bits of the MEMS-tuning circuit.

Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nanoelectromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nanoelectromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.

This paper reports on two novel concepts of areaefficient, ultra-wideband, MEMS-reconfigurable coupled line directional couplers, whose coupling is tuned by mechanically changing the geometry of 3-D micromachined coupled transmission lines, utilizing integrated MEMS electrostatic actuators. Concept 1 is based on symmetrically changing the geometry of the ground coupling of each signal line, while Concept 2 is simultaneously varying both the ground coupling and the coupling between the two signal lines. This enables uniform and well predictable performance over a very large frequency range, in particular a constant coupling ratio while maintaining an excellent impedance match, along with high isolation and a very high directivity. For an implemented micromachined prototype 3-to-6 dB coupler based on Concept 1, the measured isolation is better than 16 dB, and the return loss and directivity are better than 10 dB over the entire bandwidth from 10 to 18 GHz. Concept 2 presents an even more significant improvement. For an implemented 10-to-20 dB prototype based on Concept 2, the measured isolation is better than 40 dB and the return loss is better than 15 dB over the entire bandwidth from 10 to 18 GHz for both states. The directivities for both states are better than 22 dB and 40 dB, respectively, over the whole frequency range. The measured data fits the simulation very well, except for higher through-port losses of the prototype devices. All devices have been implemented in an SOI RF MEMS fabrication process. Measured actuation voltages of the different actuators are lower than 35 V. Reliability tests were conducted up to 500 million cycles without device degradation.

This paper investigates a novel method of integrating microwave microelectromechanical systems (MEMS) chips into millimeter-wave rectangular waveguides. The fundamental difficulties of merging micromachined with macromachined microwave components, in particular, surface topography, roughness, mechanical stress points and air gaps interrupting the surface currents, are overcome by a double-side adhesive conductive polymer interposer. This interposer provides a uniform electrical contact, stable mechanical connection and a compliant stress distribution interlayer between the MEMS chip and a waveguide frame. The integration method is successfully implemented both for prototype devices of MEMS-tuneable reflective metamaterial surfaces and for MEMS reconfigurable transmissive surfaces. The measured insertion loss of the novel conductive polymer interface is less than 0.4 dB in the E-band (60-90 GHz), as compared to a conventional assembly with an air gap of 2.5 dB loss. Moreover, both dc biasing lines and mechanical feedthroughs to actuators outside the waveguide are demonstrated in this paper, which is achieved by structuring the polymer sheet xurographically. Finite element method simulations were carried out for analyzing the influence of different parameters on the radio frequency performance.

This paper gives an overview of recent achievements in microwave micro‐electromechanical systems (microwave MEMS) at KTH Royal Institute of Technology, Stockholm, Sweden. The first topic is a micromachined W‐band phase shifter based on a micromachined dielectric block which is vertically moved by integrated MEMS actuators to achieve a tuning of the propagation constant of a micromachined transmission line. The second topic is W‐band MEMStuneable microwave high‐impedance metamaterial surfaces conceptualized for local tuning of the electromagnetic resonance properties of surface waves on a high‐impedance surface. The third topic covers 3‐dimensional micromachined coplanar transmission lines with integrated MEMS actuators which move the sidewalls of these transmission lines. Multi‐stable switches, tuneable capacitors, tuneable couplers, and tuneable filters have been implemented and characterized for 1‐40 GHz frequencies. As a forth topic, micromachined waveguide switches are presented. Finally, silicon‐micromachined near‐field and far‐field sensor and antenna interfaces are shown, including a micromachined planar lens antenna and a tapered dielectric rod measurement probe for medical applications.

This paper presents a novel concept of RF microelectromechanical systems (MEMS) tunable capacitors based on the lateral displacement of the sidewalls of a 3-D micromachined coplanar transmission line. The tuning of a single device is achieved in multiple discrete and well-defined tuning steps by integrated multi-stage MEMS electrostatic actuators that are embedded inside the ground layer of the transmission line. Three different design concepts, including devices with up to seven discrete tuning steps up to a tuning range of 58.6 to 144.5 fF, (C-max/C-min = 2.46) have been fabricated and characterized. The highest Q factor, measured by a weakly coupled transmission-line resonator, was determined as 88 at 40 GHz and was achieved for a device concept where the mechanical suspension elements were completely de-coupled from the RF signal path. These devices have demonstrated high self-actuation robustness with self-actuation pull-in occurring at 41.5 and 47.8 dBm for mechanical spring constants of 5.8 and 27.7 N/m, respectively. Nonlinearity measurements revealed that the third-order intermodulation intercept point (IIP3) for all discrete device states is above the measurement-setup limit of 68.5 dBm for our 2.5-GHz IIP3 setup, with a dual-tone separation of 12 MHz. Based on capacitance/gap/spring measurements, the IIP3 was calculated for all states to be between 71-91 dBm. For a mechanical spring design of 5.8 N/m, the actuation and release voltages were characterized as 30.7 and 21.15 V, respectively, and the pull-in time for the actuator bouncing to drop below 8% of the gap was measured to be 140 mu s. The mechanical resonance frequencies were measured to be 5.3 and 17.2 kHz for spring constant designs of 5.8 and 27.7 N/m, respectively. Reliability characterization exceeded 1 billion cycles, even in an uncontrolled atmospheric environment, with no degradation in the pull-in/pull-out hysteresis behavior being observed over these cycling tests.

This paper reports for the first time on RF nonlinearity analysis of complex multi-device RF MEMS circuits. The nonlinearity analysis is done for the two most commonly-used RF MEMS tuneable-circuit concepts, i.e. digital MEMS varactor banks and MEMS switched capacitor banks. In addition, the nonlinearity of a novel MEMS tuneable capacitor concept by the authors, based on a MEMS actuator with discrete tuning steps, is discussed. This paper presents closed-form analytical formulas for the IIP3 (nonlinearity) of the three MEMS multi-device circuit concepts, and an analysis of the nonlinearity based on measured device parameters (capacitance, gap), of the different concepts. Finally, this paper also investigates the effect of scaling of the circuit complexity, i.e. the degradation of the overall circuit linearity depending on the number of stages/bits of the MEMS-tuning circuit.