RF MEMS

| Reconfigurable antennas, with the ability to radiate more than one pattern at different frequencies and polarizations, are necessary in modern telecommunication systems. The requirements for increased functionality (e.g., direction finding, beam steering, radar, control, and command) within a confined volume place a greater burden on today's transmitting and receiving systems. Reconfigurable antennas are a solution to this problem. This paper discusses the different reconfigurable components that can be used in an antenna to modify its structure and function. These reconfiguration techniques are either based on the integration of radio-frequency microelectromechanical systems (RF-MEMS), PIN diodes, varactors, photoconductive elements, or on the physical alteration of the antenna radiating structure, or on the use of smart materials such as ferrites and liquid crystals. Various activation mechanisms that can be used in each different reconfigurable implementation to achieve optimum performance are presented and discussed. Several examples of reconfigurable antennas for both terrestrial and space applications are highlighted, such as cognitive radio, multiple-input-multipleoutput (MIMO) systems, and satellite communication.

We study the pull-in instability in microelectromechanical (MEMS) resonators and find that characteristics of the pull-in phenomenon in the presence of AC loads differ from those under purely DC loads. We analyze this phenomenon, dubbed dynamic pull-in, and formulate safety criteria for the design of MEMS resonant sensors and filters excited near one of their natural frequencies. We also utilize this phenomenon to design a low-voltage MEMS RF switch actuated with a combined DC and AC loading. The new switch uses a voltage much lower than the traditionally used DC voltage. Either the frequency or the amplitude of the AC loading can be adjusted to reduce the driving voltage and switching time. The new actuation method has the potential of solving the problem of high driving voltages of RF MEMS switches.

This paper reports on the design, fabrication, and testing of a low-actuation voltage Microelectromechanical systems (MEMS) switch for high-frequency applications. The mechanical design of low spring-constant folded-suspension beams is presented first, and switches using these beams are demonstrated with measured actuation voltages of as low as 6 V. Furthermore, common nonidealities such as residual in-plane and gradient stress, as well as down-state stiction problems are addressed, and possible solutions are discussed. Finally, both experimental and theoretical data for the dynamic behavior of these devices are presented. The results of this paper clearly underline the need of an integrated design approach for the development of ultra low-voltage RF MEMS switches.

Low-loss microwave microelectromechanical systems (MEMS) shunt switches are reported that utilize highly compliant serpentine spring folded suspensions together with large area capacitive actuators to achieve low actuation voltages while maintaining sufficient off-state isolation. The RF MEMS switches were fabricated via a surface micromachining process using P12545 polyimide as the sacrificial layer. The switch structure was composed of electroplated nickel and the serpentine folded suspensions had a varying number of meanders from 1 to 5. DC measurements indicate actuation voltages as low as 9 V with an on-to-off capacitance ratio of 48. Power handling measurement results showed no “self-biasing” or failure of the MEMS switches for power levels up to 6.6 W. RF measurements demonstrate an isolation of -26 dB at 40 GHz

We show that the effects of photoinduced screening of the bias field as a function of optical spot shape for ultrafast excitation of photoconductors to generate terahertz radiation using spatially resolve Monte Carlo simulations coupled with a self-consistent Poisson solver. The results of the simulations demonstrate that given the same photoexcited carrier density, a line focus parallel to the direction of the bias field suppresses saturation at high optical fluence due to space-charge screening and enhances the peak terahertz power in the time domain.

A review of radio frequency microelectromechanical systems (RF MEMS) technology, from the perspective of its enabling technologies (e.g. fabrication, RF micromachined components and actuation mechanisms) is presented. A unique roadmap is given that shows how enabling technologies, RF MEMS components, RF MEMS circuits and RF microsystems packaging are linked together; leading towards enhanced integrated subsystems. An overview of the associated fabrication technologies is given, in order to distinguish between the two distinct classes of RF microsystems' component technologies; non-MEMS micromachined and true MEMS. An extensive literature survey has been undertaken and key papers have been cited; from these, the motivations behind different RF MEMS technologies are highlighted. The importance of understanding the limitations for realising new and innovative ideas in RF MEMS is discussed. Finally, conclusions are drawn as to where future RF MEMS technology may lead. It is likely that the switch will continue to be the most important RF MEMS component, with future work investigating its enhanced functionality, subsystem integration and volume production. The focus of RF MEMS circuits will shift from the digital phase shifter to high-Q tuneable filters.

A new kind of double-and single-arm cantilever type DC-contact RF MEMS actuators has been monolithically integrated with an antenna architecture to develop a frequency reconfigurable antenna. The design, microfabrication, and characterization of this "reconfigurable antenna (RA) annular slot" which was built on a microwave laminate TMM10i ( = 9.8, tan = 0.002), are presented in this paper. By activating/deactivating the RF MEMS actuators, which are strategically located within the antenna geometry and microstrip feed line, the operating frequency band is changed. The RA annular slot has two reconfigurable frequencies of operation with center frequencies low =2.4 GHz and high = 5.2 GHz, compatible with IEEE 802.11 WLAN standards. The radiation and impedance characteristics of the antenna along with the RF performance of individual actuators are presented and discussed.

This paper explores contact heating in microelectromechanical systems (MEMS) switches with contact spot sizes less than 100 nm in diameter. Experiments are conducted to demonstrate that contact heating causes a drop in contact resistance. However, existing theory is shown to over-predict heating for MEMS switch contacts because it does not consider ballistic transport of electrons in the contact. Therefore, we extend the theory and develop a predictive model that shows excellent agreement with the experimental results. It is also observed that mechanical cycling causes an increase in contact resistance. We identify this effect as related to the build-up of an insulating film and demonstrate operational conditions to prevent an increase in contact resistance. The improved understanding of contact behavior gained through our modeling and experiments allows switch performance to be improved.

An RF MEMs microelectromechanical system variable capacitor has been demonstrated with a 22:1 tuning range, tuning from 1.5 to 33.2 pF of capacitance. This capacitor was constructed using bistable MEMs membrane capacitors with individual tuning ranges of 70:1 to 100:1, control voltages in the 30-55 V range, switching speeds less than 10 S, and operating frequencies as high as 40 GHz. These devices may eventually provide a viable alternative to electronic varactors with improved tuning range and lower loss.

This paper reports on the design, fabrication and testing of novel one and two port piezoelectric higher order contour-mode MEMS resonators that can be employed in RF wireless communications as frequency reference elements or arranged in arrays to form banks of multi-frequency filters. The paper offers a comparison of one and two port resonant devices exhibiting frequencies approximately ranging from 200 to 800 MHz, quality factor of few thousands (1000-2500) and motional resistances ranging from 25 to 1000 Ω. Fundamental advantages and limitations of each solution are discussed. The reported experimental results focus on the response of a higher order one port resonator under different environmental conditions and a new class of two port contour resonators for narrow band filtering applications. Furthermore, an overview of novel frequency synthesis schemes that can be enabled by these contour-mode resonators is briefly presented.

A novel process for monolithic integration of RF MEMS switches with three-dimensional (3-D) antenna elements on a microwave laminate printed circuit board (PCB) is presented. This process calls for a low temperature (90-170 o C) high-density inductively coupled plasma chemical vapor deposition (HDICP CVD) technique that allows the choice of any PCB substrate, such as RO4003-FR4 (ε r =3.38, tanδ=0.002), with the desired electrical properties for antenna applications. A twoelement diversity antenna system monolithically integrated with RF MEMS switches is designed and demonstrated. Index Terms-RF MEMS switches, monolithic integration, low temperature process, printed circuit board.

This paper investigates both theoretically and experimentally the dielectric charging effects of capacitive RF microelectromechanical system switches with silicon nitride as dielectric layer. Dielectric charging caused by charge injection under voltage stress was observed. The amphoteric nature of traps and its effect on the switch operation were confirmed under both positive and negative control voltages. It has been confirmed that charging is a complicated process, which can be better described through the stretched exponential relaxation. This mechanism is thermally activated with an activation energy being calculated from the temperature dependence of the capacitance transient response. The charging mechanism, which is responsible for the pull-out voltage and the device failure, is also responsible for the temperature-induced shift of the capacitance minimum bias.

This paper reports on the development of a broadband, low insertion loss, ohmic-contact MEMs switch for microwave and millimeter-wave applications. The switch is surface-micromachined and electrostatically-actuated with electromechanical performance simulated with analytical and numerical models that compare well to experiment. The switch has an actuation voltage of 30 V, a response time of 20 s, and mechanical strength to withstand 10 9 actuations. A 3D electromagnetics scattering model developed iteratively with the electromechanical modeling predicts RF performance demonstrated to be greater than 50 dB of isolation below 2 GHz and less than 0.2 dB of insertion loss from DC through 40 GHz.

Planar array antennas are attractive for use in future automotive radar systems due to their flexibility in design and control of radar beams. The complexity and cost of a radar front-end phased array can be decreased by applying a beam-steering/switching concept, which reduces the number of parallel RF and baseband signal paths. RF-microelectromechanical systems (MEMS) subsystems are employed because of their excellent RF properties and potential low-cost manufacturability.

The conduction mechanisms through a lead zirconate titanate (PZT) thin film grown by pulsed laser deposition with a La 0.67 Sr 0.33 MnO 3 (LSMO) buffer layer on epitaxial Pt (111) were assessed in the 230-330 K temperature range. X-Ray diffraction and transmission electron microscopy evidenced a columnar growth of (001)-and (011)-oriented PZT grains. The leakage current through the Pt/PZT/LSMO/Pt structure was then systematically measured. From current vs. time curves, a threshold voltage was found below which stable and reproducible current values are obtained, thus avoiding resistance degradation. The conduction mechanism changes from interface controlled at low temperatures to bulk controlled around room temperature. The hopping-type conductivity evidenced above 270 K is consistent with the extended defects and columnar microstructure of the PZT film.

We designed, fabricated, and characterized metamaterial-based RF-microelectromechanical system (RF-MEMS) strain sensors that incorporate multiple split ring resonators (SRRs) in a compact nested architecture to measure strain telemetrically. We also showed biocompatibility of these strain sensors in an animal model. With these devices, our bioimplantable wireless metamaterial sensors are intended, to enable clinicians, to quantitatively evaluate the progression of long-bone fracture healing by monitoring the strain on the implantable fracture fixation hardware in real time. In operation, the transmission spectrum of the metamaterial sensor attached to the implantable fixture is changed when an external load is applied to the fixture, and from this change, the strain is recorded remotely. By employing telemetric characterizations, we reduced the operating frequency and enhanced the sensitivity of our novel nested SRR architecture compared to the conventional SRR structure. The nested SRR structure exhibited a higher sensitivity of 1.09 kHz/kgf operating at lower frequency compared to the classical SRR that demonstrated a sensitivity of 0.72 kHz/kgf. Using soft tissue medium, we achieved the best sensitivity level of 4.00 kHz/kgf with our nested SRR sensor. Ultimately, the laboratory characterization and in vivo biocompatibility studies support further development and characterization of a fracture healing system based on implantable nested SRR.

This paper presents three coating methods of photoresist on large three-dimensional (3-D) topography surfaces. Two special methods, spray and electrodeposition (ED) are introduced and investigated for the fabrication of 3-D microstructures and RF-MEMS devices. Characteristics of each method as well as its advantage and disadvantages are outlined. A comparison is made to point out the most suitable coating method in terms of complexity, performance and type of application. The potential of these coating methods is demonstrated through several applications such as fabrication of multilevel micromachined structures and RF MEMS devices.

Entangled networks of carbon nanofibers are characterized both mechanically and electrically. Results for both tensile and compressive loadings of the entangled networks are presented for various densities. Mechanically, the nanofiber ensembles follow the micromechanical model originally proposed by van Wyk nearly 70 years ago. Interpretations are given on the mechanisms occurring during loading and unloading of the carbon nanofiber components.

Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass-Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to applied contact pressures were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating and contact forces. This modeling framework was used to understand how contact resistance evolves due to changes in the real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on integrated modeling and experimental measurements, which can be used in the design of reliable RF MEMS devices with extended life cycles.

This paper gives a new insight into the problem of RF MEMS irreversible stiction due to dielectric charging. Previous reported works describing the phenomenon only account for a drift of the actuation characteristics as a whole as they consider uniform charge densities. We demonstrate how the spatial charge distribution in the dielectric layer can result in the failure of the devices. We emphasize the role of the variance of the distribution, a parameter neglected in the literature. Our model can account for a shift of the C-V actuation characteristics but also for a change in its profile. In particular, the pull-out window can be narrowed and even made to disappear as a result of the non-zero variance of the charge distribution. We identify the processing, the contact conditions and the distributed charge depths as variance-and thus failure-enhancer parameters.

Three-dimensional multiphysics Finite Element Analysis (FEA) was performed to investigate the reliability of RF MEMS switches at various operational temperatures. The investigated MEMS capacitive switch consists of a freestanding metal membrane actuated by a bottom electrode coated by a dielectric film. Coupled-field simulations between thermal, structural and electrostatic domains were performed. The simulations show that temperature significantly changes both the membrane stress state and out-plane-geometry geometry. In particular, the membrane buckles when a temperature increase, from room temperature, takes place. The buckling temperature, i.e., the upper bound to the operational temperature is a function of manufacturing residual stress state, membrane initial out-of-plane profile, and mismatch in materials coefficient of thermal expansion. The analysis also shows that a temperature reduction, from room temperature to minus 40 o C, causes an increase in pull-in voltage to values that could compromise the switch reliability as a result of charge build-up in the dielectric layer. Our analyses illustrate that by proper design of the membrane out-of-plane profile, it is possible to keep the pull-in voltage, at all operational temperatures, within allowable values. This design feature of RF MEMS switches offers an effective way to achieve reliable pull-in voltages in applications where large temperature variations are expected such as in satellites and airplane condition monitoring based on wireless communication.

In this paper we present a 3D nonlinear dynamic model which describes the transient mechanical analysis of an ohmic contact RF MEMS switch, using finite element analysis in combination with the finite difference method. The model includes real switch geometry, electrostatic actuation, the two-dimensional non-uniform squeeze-film damping effect, the adherence force, and a nonlinear spring to model the interaction between the contact tip and the drain. The ambient gas in the package is assumed to act as an ideal and isothermal fluid which is modeled using the Reynolds squeeze-film equation which includes compressibility and slip flow. A nonlinear contact model has been used for modeling contact between the microswitch tip and the drain electrode during loading. The Johnson-Kendall-Roberts (JKR) contact model is utilized to calculate the adherence force during unloading. The developed model has been used to simulate the overall dynamic behavior of the MEMS switches including the switching speed, impact force and contact bounce as influenced by actuation voltage, damping, materials properties and geometry. Meanwhile, based on a simple undamped spring-mass system, a dual voltage-pulse actuation scheme, consisting of actuation voltage (V a ), actuation time (t a ), holding voltage (V h ) and turn-on time (t on ), has been developed to improve the dynamic response of the microswitch. It is shown that the bouncing of the switch after initial contact can be eliminated and the impact force during contact can be minimized while maintaining a fast close time by using this open-loop control approach. It is also found that the dynamics of the switch are sensitive to the variations of the shape of the dual pulse scheme. This result suggests that this method may not be as effective as expected if the switch parameters such as threshold voltage, fundamental frequencies, etc. deviate too much from the design parameters. However, it is shown that the dynamic performance may be improved by increasing the damping force. The simulation results obtained from this dynamic model are confirmed by experimental measurement of the RF MEMS switches which were developed at the Northeastern University. It is anticipated that the simulation method can serve as a design tool for dynamic optimization of the microswitch. In addition, the approach of tailoring actuation voltage and the utilization of squeeze-film damping may provide further improvements in the operation of RF MEMS switches.

A micromechanical 13.1 MHz bulk acoustic mode (BAW) silicon resonator is demonstrated. The vibration mode can be characterized as a 2-D plate expansion that preserves the original square shape. The prototype resonator is fabricated of single-crystal silicon by reactive ion etching a silicon-on-insulator (SOI) wafer. The measured high quality factor (Q = 130 000) and current output (i MAX ≈ 160 µA) make the resonator suitable for reference oscillator applications. An electrical equivalent circuit based on physical device parameters is derived and experimentally verified.

As the need for low-loss phase shifters increases, so does the interest in radio frequency (RF) MEMS as a solution to provide them. In this paper, progress in building low loss Ka-band phase shifters using RF MEMS capacitive switches is demonstrated. Using a switched transmission line 4-bit resonant phase shifter, an average insertion loss of 2.25 dB was obtained with better than 15-dB return loss. A similar 3-bit phase shifter produced an average insertion loss of 1.7 dB with better than 13-dB return loss. Both devices had a phase error of less than 13 in the fundamental states. To our knowledge, these devices represent the lowest loss Ka-band phase shifters reported to date.

This paper presents a novel electronically scanning phased-array antenna with 128 switches monolithically implemented using RF microelectromechanical systems (MEMS) technology. The structure, which is designed at 15 GHz, consists of four linearly placed microstrip patch antennas, 3-bit distributed RF MEMS low-loss phase shifters, and a corporate feed network. MEMS switches and high-metal-air-metal capacitors are employed as loading elements in the phase shifter. The system is fabricated monolithically using an in-house surface micromachining process on a glass substrate and occupies an area of 6 cm 5 cm. The measurement results show that the phase shifter can provide nearly 20 /50 /95 phase shifts and their combinations at the expense of 1.

This paper reports on a comparison of gold and gold-nickel alloys as contact materials for microelectromechanical systems (MEMS) switches. Pure gold is commonly used as the contact material in low-force metal-contact MEMS switches. The top two failure mechanisms of these switches are wear and stiction, which may be related to the material softness and the relatively high surface adhesion, respectively. Alloying gold with another metal introduces new processing options to strengthen the material against wear and reduce surface adhesion. In this paper, the properties of Au-Ni alloys were investigated as the lower contact electrode was controlled by adjusting the nickel content and thermal processing conditions. A unique and efficient switching degradation test was conducted on the alloy samples, using pure gold upper microcontacts. Solid-solution Au-Ni samples showed reduced wear rate but increased contact resistance, while two-phase Au-Ni (20 at.% Ni) showed a substantial improvement of switching reliability with only a small increase of contact resistance. Discussion of the effects of phase separation, surface topography, hardness, and electrical resistivity on contact resistance and switch degradation is also included.

RF MEMS capacitive switches show great promise for use in wireless communication devices such as mobile phones, but the successful application of these switches is hindered by reliability concerns: charge injection in the dielectric layer (SiN) can cause irreversible stiction of the moving part of the switch.

In this paper, we study the effect of stress voltage and temperature on the dielectric charging and discharging processes of silicon nitride thin films used in RF-MEMS capacitive switches. The investigation has been performed on PECVD-SiN x dielectric materials deposited under different deposition conditions. The leakage current was found to obey the Poole-Frenkel law. The charging current decay was found to be affected by the presence of defects which are generated by electron injection at high electric fields. At high temperatures power law decay was monitored. Finally, the temperature dependence of leakage current revealed the presence of thermally activated mechanisms with similar activation energies in all materials.

In this article materials issues that arise during the processing and operation of micro-electro mechanical systems devices (MEMS), and their impact on the functionality and reliability, are discussed. The article starts with the example of an RF-MEMS switch process flow, indicating how and why certain materials are chosen. Then two specific MEMS materials issues, e.g. stiction and stress, are covered. Finally, the so-called Ôzero-levelÕ packaging of MEMS as well as the effect packaging can have on the device and its operation will be reviewed.

A summary of the current state of the art in MEM switches using high T C superconducting transmission line elements is presented. The most significant advantages of the MEM technology are the low mass and volume of the implemented devices. On the other hand one of the principal drawbacks of the room temperature MEM rf-switches are their losses. These are primarily resistive losses in the waveguide structure and contacts and are large due to the dimensions required (on the order of 100 µm wide by 3 µm thick). This places serious limitations on their use in noise/loss sensitive applications. The use of superconductors for the waveguide elements in MEM switches can reduce these losses very significantly (several orders of magnitude are not unusual in devices with identical dimensions). Adding a high dielectric constant insulator to the switch can also help shrink the necessary dimensions or extend the low frequency response of the switch. INTRODUCTION In this work, we summarize the results of our efforts to date on transitioning a rf micro-electro-mechanical (MEM) switch from normal metals to su-perconductors. The motivation of this was to reduce losses to an absolute minimum. This would enable the use of MEM switches in low noise, frequency agile , applications where low system intermodulation products and as high a third-order intermodulation intercept point (IP3) as possible are required. In order for the reader to place the work in proper perspective and benefit as much as possible from the historical body of work on these devices we have included a general background survey of the literature along with the most recent relevant results from our group on the superconducting rf MEM switch. This background section is followed by an overview of the various processing and fabrication techniques that we have used in our work to date and a discussion of the reasons for the choices made. The section on results presents the most recent results which we have obtained to date and these are discussed in terms of their impact on fabrication, yield and device performance. The final segment of this paper presents an outline of where we see this work heading and what its potential may be. BACKGROUND

The paper presents a systematic investigation of the dielectric charging and discharging process in silicon nitride thin films for RF-MEMS capacitive switches. The SiN films were deposited with high frequency (HF) and low frequency (LF) PECVD method and with different thicknesses. Metal-Insulator-Metal capacitors have been chosen as test structures while the Charge/Discharge Current Transient method has been used to monitor the current transients. The investigation reveals that in LF material the stored charge increases with the film thickness while in HF one it is not affected by the film thickness. The dependence of stored charge on electric field intensity was found to follow a Poole-Frenkel like law. Finally, both the relaxation time and the stored charge were found to increase with the electric field intensity.

A novel fabrication process for directly constructing RF MEMS capacitive switches has been developed on microwave laminate printed circuit boards (PCBs). The integrated process uses metal wet etching to form the metal lines for coplanar waveguides, compressive molding planarization (COMP) to planarize uneven surface heights for switch membrane formation, and high density inductively coupled plasma chemical vapor deposition (HDICP-CVD) for low temperature silicon nitride deposition. This technology will also allow further monolithic integration with antennas on the same PCBs to form reconfigurable antennas without the concerns of mismatching among components.

In this paper we report recent advances in pulsed-laser-deposited AIN thin films for high-temperature capping of SiC, passivation of SiC-based devices, and fabrication of a piezoelectric MEMS/NEMS resonator on Pt-metallized SiO2/Si. The AlN films grown using the reactive laser ablation technique were found to be highly stoichiometric, dense with an optical band gap of 6.2 eV, and with a surface smoothness of less than 1 nm. A low-temperature buffer-layer approach was used to reduce the lattice and thermal mismatch strains. The dependence of the quality of AlN thin films and its characteristics as a function of processing parameters are discussed. Due to high crystallinity, near-perfect stoichiometry, and high packing density, pulsed-laser-deposited AlN thin films show a tendency to withstand high temperatures up to 1600°C, and which enables it to be used as an anneal capping layer for SiC wafers for removing ion-implantation damage and dopant activation. The laser-deposited AlN thin films show conformal coverage on SiC-based devices and exhibit an electrical break-down strength of 1.66 MV/cm up to 350°C when used as an insulator in Ni/AlN/SiC metal-insulator-semiconductor (MIS) devices. Pulsed laser deposition (PLD) AlN films grown on Pt/SiO2/Si (100) substrates for radio-frequency microelectrical and mechanical systems and nanoelectrical and mechanical systems (MEMS and NEMS) demonstrated resonators having high Q values ranging from 8,000 to 17,000 in the frequency range of 2.5–0.45 MHz. AlN thin films were characterized by x-ray diffraction, Rutherford backscattering spectrometry (in normal and oxygen resonance mode), atomic force microscopy, ultraviolet (UV)-visible spectroscopy, and scanning electron microscopy. Applications exploiting characteristics of high bandgap, high bond strength, excellent piezoelectric characteristics, extremely high chemical inertness, high electrical resistivity, high breakdown strength, and high thermal stability of the pulsed-laser-deposited thin films have been discussed in the context of emerging developments of SiC power devices, for high-temperature electronics, and for radio frequency (RF) MEMS.

The dependence of the electrical properties of silicon nitride, which is a commonly used dielectric in nanoand micro-electromechanical systems (NEMS and MEMS), on the deposition conditions used to prepare it and, consequently, on material stoichiometry has not been fully understood. In this paper, the influence of plasma-enhanced chemical vapor deposition conditions on the dielectric charging of SiN x films is investigated. The work targets mainly the dielectriccharging phenomenon which constitutes a major failure mechanism in electrostatically driven NEMS/MEMS devices and particularly in capacitive MEMS switches. The charging/ discharging processes are studied using two nanoscale characterization techniques: Kelvin probe force microscopy (KPFM) and, for the first time, force-distance curve (FDC) measurements. KPFM is used to investigate dielectric charging at the level of a single asperity, while FDC is employed to measure the multiphysics coupling between the charging phenomenon and tribological issues, mainly meniscus force. The electrical properties of the SiN x films obtained from both techniques show a very good correlation. X-ray photoelectron spectroscopy and Fourier transform infrared Manuscript

This paper presents fast switching RF MEMS varactors with measured switching times between 150 and 400 ns. By introducing bended sides on a planar microbeam, it is shown experimentally that the resonance frequency of aluminum bridges is increased by a factor 25 compared to standard RF MEMS components. In addition, this original shape can be implemented easily in post-processing of CMOS circuits. Several designs are presented with measured mechanical resonance frequencies between 1 and 3 MHz and measured switching times under 400 ns. Using this original approach, sub-microsecond RF MEMS switched varactors have been designed and fabricated on quartz substrate. Their resulting RF performance is presented with a measured capacitance ratio of 2.3. Reliability tests have also been performed and have demonstrated no significant mechanical behavior variation over 14 billion cycles and a moderate sensitivity to temperature variation.

We present a method for the fabrication of vertical inductors for radio-frequency and microwave applications. This process uses ®ve levels of lithography and electroplating, with no substrate removal, high temperatures or serial process steps. Rotation of the inductors perpendicular to the substrate is achieved by surface tension driven self-assembly, giving separation from the substrate and therefore increasing substantially the Q and self-resonant frequencies. Meander and spiral air-bridged inductors of up to 5.5 nH are demonstrated. The Q of 2 nH inductors shows an increase from 4 to 20 after having undergone the self-assembly process. Mechanical sensitivity is evaluated through ®nite element modelling, and the maximum displacements indicated are below 10 nm for 1 g of loading. Mechanical resonance frequencies are found within the 6±15 kHz range. #

In this paper, we present CMOS compatible fabrication of monocrystalline silicon micromirror arrays using membrane transfer bonding. To fabricate the micromirrors, a thin monocrystalline silicon device layer is transferred from a standard silicon-on-insulator (SOI) wafer to a target wafer (e.g., a CMOS wafer) using low-temperature adhesive wafer bonding. In this way, very flat, uniform and low-stress micromirror membranes made of monocrystalline silicon can be directly fabricated on top of CMOS circuits. The mirror fabrication does not contain any bond alignment between the wafers, thus, the mirror dimensions and alignment accuracies are only limited by the photolithographic steps. Micromirror arrays with 4 4 pixels and a pitch size of 16 m 16 m have been fabricated. The monocrystalline silicon micromirrors are 0.34 m thick and have feature sizes as small as 0.6 m. The distance between the addressing electrodes and the mirror membranes is 0.8 m. Torsional micromirror arrays are used as spatial light modulators, and have potential applications in projection display systems, pattern generators for maskless lithography systems, optical spectroscopy, and optical communication systems. In principle, the membrane transfer bonding technique can be applied for integration of CMOS circuits with any type of transducer that consists of membranes and that benefits from the use of high temperature annealed or monocrystalline materials. These types of devices include thermal infrared detectors, RF-MEMS devices, tuneable vertical cavity surface emitting lasers (VCSEL) and other optical transducers.

The paper investigates the dependence of charging process on the dielectric charging of radiation induced defects in Si 3 N 4 and SiO 2 dielectric films, which are used in RF-MEMS switches. The radiation has been performed with 5 MeV alpha particles. The assessment has been carried out in Metal-Insulator-Metal capacitors with the thermally stimulated depolarization currents and discharge current transient methods. This allowed monitoring the defects introduction as a function of radiation fluence. The defects electrical characteristics that are the activation energy and corresponding depolarization time constant were determined from the evolution of the thermally stimulated current spectra and the transient response of discharge currents at different temperatures.

The dielectric charging is one of the major failures reducing the reliability of capacitive switches with electrostatic actuation. Then the control of the charging/discharging processes is a key factor to allow a fast recovering of the dielectric after charging. From transient current measurements on MIM capacitors it is possible to select the best material for RF-MEMS. We have studied different PECVD silicon nitride obtained under low (380 KHz), high (13.56 MHz) or mixed (380kHz/13.56MHz) frequency power supply. The conduction mechanism into the dielectrics has been deduced from current measurements on MIM capacitors. Then the film properties have been studied by infrared measurement in order to identify the chemical bond into the dielectric which can explain the charging behaviour. It was observed that low hydrogen content in the films is in good correlation with electrical quality and kinetic of the charging/discharging processes.

It is well-recognized that MEMS switches, compared to their more traditional solid state counterparts, have several important advantages for wireless communications. These include superior linearity, low insertion loss and high isolation. Indeed, many potential applications have been investigated such as Tx/Rx antenna switching, frequency band selection, tunable matching networks for PA and antenna, tunable filters, and antenna reconfiguration. However, none of these applications have been materialized in high volume products to a large extent because of reliability concerns, particularly those related to the metal contacts. The subject of the metal contact in a switch was studied extensively in the history of developing miniaturized switches, such as the reed switches for telecommunication applications. While such studies are highly relevant, they do not address the issues encountered in the sub 100µN, low contact force regime in which most MEMS switches operate. At such low forces, the contact resistance is extremely sensitive to even a trace amount of contamination on the contact surfaces. Significant work was done to develop wafer cleaning processes and storage techniques for maintaining the cleanliness. To preserve contact cleanliness over the switch service lifetime, several hermetic packaging technologies were developed and their effectiveness in protecting the contacts from contamination was examined. The contact reliability is also very much influenced by the contact metal selection. When pure Au, a relatively soft metal, was used as the contact material, significant stiction problems occurred when clean switches were cycled in an N 2 environment. In addition, various mechanical damages occurred after extended switching cycling tests. Harder metals, while more resistant to deformation and stiction, are more sensitive to chemical reactions, particularly oxidation. They also lead to higher contact resistance because of their lower electrical conductivity and smaller real contact areas at a given contact force. Contact reliability issues could also be tackled by improving mechanical designs. A novel collapsing switch capable of generating large contact forces (>300µN) was shown to be less vulnerable to contamination and stiction.

Air-suspension of transmission-line structures using microelectromechanical systems (MEMS) technology provides the effective means to suppress substrate losses for radio-frequency (RF) signals. However, heating of these lines augmented by skin effects can be a major concern for RF MEMS reliability. To understand this phenomenon, a thermal energy transport model is developed in a simple analytical form. The model accounts for skin effects that cause Joule heating to be localized near the surface of the RF transmission line. Here, the model is validated through experimental data by measuring the temperature rise in an air-suspended MEMS coplanar waveguide (CPW). For this measurement, a new experimental methodology is also developed allowing direct current (dc) electrical resistance thermometry to be adopted in an RF setup. The modeling and experimental work presented in this paper allow us to provide design rules for preventing thermal and structural failures unique to the RF operation of suspended MEMS transmission-line components. For example, increasing the thickness from 1 to 3 m for a typical transmission line design enhances power handling from 5 to 125 W at 20 GHz, 3.3 to 80 W at 50 GHz, and 2.3 to 56 W at 100 GHz (a 25-fold increase in RF power handling).

A major issue in the reliability of RF MEMS capacitive switches is charge injection in the dielectric. In this study we try to establish the time and voltage dependence of dielectric charging in RF MEMS with silicon nitride as a dielectric. It is shown that the voltage shift of the CV-curve due to injected charge shows a √ t dependence over a large time range.

Atomic layer deposition (ALD) was used to deposit an alternative dielectric barrier layer for use in radio frequency microelectromechanical systems (rf MEMS). The layer is an alloy mixture of Al 2 O 3 and ZnO and is proposed for use as charge dissipative layers in which the dielectric constant is significant enough to provide a large down-state capacitance while the resistivity is sufficiently low to promote the dissipation of trapped charges. This paper investigates Al 2 O 3 /ZnO ALD alloys deposited at 100 and 177 • C and compares their material properties. Auger electron spectroscopy was used to determine the Zn concentrations in the alloy films, which was lower than expected. Atomic force microscopy images revealed an average surface roughness of 0.27 nm that was independent of deposition temperature and film composition. The dielectric constants of the Al 2 O 3 /ZnO ALD alloys films were calculated to be similar to pure Al 2 O 3 ALD, being ∼7. Indentation was used to ascertain the modulus and hardness of the ALD films. Both the modulus and hardness were found to increase for the greater deposition temperature. ALD-coated rf MEMS switches showed a low insertion loss, ∼0.35 dB, and a high isolation, 55 dB at 14 GHz. Mechanical actuation of the ALD-coated devices showed lifetimes of over 1 billion cycles.

Surface micromachined metal armatures are commonly used for MEMS applications of which RF-MEMS is the most well known. In most cases metals with a high conductivity, such as aluminum or gold, are used. These metals often have a low melting point and therefore have a low thermal stability and show plastic deformation of the structures at relatively low temperatures (<200 • C). High melting point metals, such as platinum, are expected to show plastic deformation only at higher temperatures which makes them interesting for use as a structural layer in RF-MEMS devices. In this paper, we present a technology to realize suspended platinum structures by means of surface micromachining. An improved lift-off process allows patterning 1 μm Pt films on a polyimide sacrificial layer. A comparison of the characteristics and armature resonance frequencies between RF-MEMS switches with Pt armatures and AlCu 0.5% alloy armatures reveals an increased thermal stability for the former up to at least 250 • C. This enables zero-level packaging of switches at relative high temperatures without affecting their performances. The lower conductivity of Pt compared to AlCu 0.5% does not lead to a significant increase in RF losses. Implementing AlN as a dielectric material, the Pt-based capacitive shunt switches reported in this paper showed lifetimes in excess of 5×10 7 cycles under standard testing conditions.

This paper presents a novel temperature-compensated two-state microelectromechanical (MEM) capacitor. The principle to minimize temperature dependence is based on geometrical compensation and can be extended to other devices such as MEM varactors. The compensation structure eliminates the effect of intrinsic and thermal stress on device operation. This leads to a temperature-stable device without compromising the quality factor (Q) or the voltage behavior. The compensation structure increases the robustness of the devices, but does not require any modifications to the process. Measurement results verify that the OFF and ON capacitance change is less than 6% and the pull-in voltage is less than 5% when the temperature is varied from 30 to +70 C.

S uccessful deployment of wireless sensor and actuator networks (WSNs) in numbers large enough to provide true ambient intelligence requires the confluence of progress in several disciplines, including distributed computing, networking, wireless communications, and, most importantly, ultra-low-power design. The latter is an essential cornerstone to the ultimate success of this innovative and paradigm-shifting concept for a number of reasons. Foremost, physical access to nodes in a deployed network may be difficult, and the sheer number of nodes makes changing batteries or manual refueling unrealistic. Hence, nodes should be energy self-contained for the lifetime of the application. In addition, node size is dominated by the energy storage and generation modules. Improved energy scavenging and lower power consumption result in smaller nodes, opening the door for a wider range of applications. Finally, energy storage and generation is responsible for a sizable fraction of the cost of a node. Ubiquitous deployment of large networks requires node costs substantially below what is available today.

This paper presents the robust design optimization of an RF-MEMS direct contact cantilever switch for minimum actuation voltage and opening time, and maximum power handling capability. The design variables are the length and thickness of the entire cantilever, the widths of the sections of the cantilever, and the dimple size. The actuation voltage is obtained using a 3-D structural-electrostatic finite-element method (FEM) model, and the opening time is obtained using the same FEM model and the experimental model of adhesion at the contact surfaces developed in our previous work. The model accounts for an unpredictable variance in the contact resistance resulting from the micromachining process for the estimation of the power handling. This is achieved by taking the ratio of the root mean square power of the RF current ("signal") passing through the switch to the contact temperature ("noise") resulting from the possible range of the contact resistance. The resulting robust optimization problem is solved using a Strength Pareto Evolutionary Algorithm, to obtain design alternatives exhibiting different tradeoffs among the three objectives. The results show that there exists substantial room for improved designs of RF-MEMS direct-contact switches. It also provides a better understanding of the key factors contributing to the performances of RF-MEMS switches. Most importantly, it provides guidance for further improvements of RF-MEMS switches that exploit complex multiphysics phenomena.