Nicol E. McGruer

Micromechanical switches fabricated using nickel surface micromachining

Micromechanical switches have been fabricated in electroplated nickel using a four-level surface micromachining process. The simplest devices are configured with three terminals, a source, a drain, and a gate and are 30 /spl mu/m wide, 1 /spl mu/m thick, and 65 /spl mu/m long. A voltage applied between the gate and source closes the switch, connecting the source to the drain. Devices switch more than 10/sup 9/ cycles before failure and exhibit long-lifetime hot switching currents up to 5 mA. The initial contact resistance is less than 50 m/spl Omega/. The breakdown (stand-off) voltage between the source and the drain is greater than 100 V and the off-current is less than 20 fA at 100 V.

Substituent Effects in Pentacenes: Gaining Control over HOMO−LUMO Gaps and Photooxidative Resistances

A combined experimental and computational study of a series of substituted pentacenes including halogenated, phenylated, silylethynylated and thiolated derivatives is presented. Experimental studies include the synthesis and characterization of six new and six known pentacene derivatives and a kinetic study of each derivative under identical photooxidative conditions. Structures, HOMO-LUMO energies and associated gaps were calculated at the B3LYP/6-311+G**//PM3 level while optical and electrochemical HOMO-LUMO gaps were measured experimentally. The combined results provide for the first time a quantitative assessment of HOMO-LUMO gaps and photooxidative resistances for a large series of pentacene derivatives as a function of substituents. The persistence of each pentacene derivative is impacted by a combination of steric resistance and electronic effects as well as the positional location of each substituent. Silylethynyl-substituted pentacenes like TIPS-pentacene possess small HOMO-LUMO gaps but are not the longest lived species under photooxidative conditions, contrary to popular perception. A pentacene derivative with both chlorine substituents in the 2,3,9,10 positions and o-alkylphenyl substituents in the 6,13 positions is longer lived than TIPS-pentacene. Of all the derivatives studied, alkylthio- and arylthio-substituted pentacenes are most resistant to photooxidation, possess relatively small HOMO-LUMO gaps and are highly soluble in a variety of organic solvents. These results have broad implications for the field of organic molecular electronics where OFET, OLED, and other applications can benefit from highly persistent, solution processable pentacene derivatives.

Study of contacts in an electrostatically actuated microswitch

Surface micromachined, electrostatically actuated microswitches have been developed at Northeastern University. Microswitches have an initial contact resistance of 0.5-1 /spl Omega/, and current handling capability of about 20 mA. Typically, contact resistance degrades progressively when the switches are cycled beyond approximately 10/sup 6/ cycles. In this work, the microswitch contact resistance is studied on the basis of a simple, clean metal contact resistance model. Comparison of measured contact resistance (measured as a function of contact force) with the characteristics predicted by the model shows the measured resistance to be higher than the prediction, approximately by an order of magnitude, suggesting that insulating films at the contact interface need to be taken into account. Microswitches with a large number of parallel contacts have also been developed, and measurement data is presented showing that these devices have a current handling capability greater than 150 mA.

A dynamic model, including contact bounce, of an electrostatically actuated microswitch

Microelectromechanical devices are increasingly being integrated into electronic circuitry. One of these types of devices is the microswitch, which acts much like a three-terminal field-effect transistor (FET). While various microswitches are currently being developed, their dynamic behavior is not well understood. Upon closing, switches bounce several times before making permanent contact with the drain. In this paper, a time-transient finite difference analysis is used to model the dynamic behavior of two different electrostatically actuated microswitch configurations. The model uses dynamic Euler-Bernoulli beam theory for cantilevered beams, includes the electrostatic force from the gate, takes into account the squeeze-film damping between the switch and substrate, and includes a simple spring model of the contact tips. The model and simulation can be used as design tools to improve switch performance and reduce switch bounce in future designs.

Contact resistance study of noble metals and alloy films using a scanning probe microscope test station

The proper selection of electrical contact materials is one of the critical steps in designing a metal contact microelectromechanical system (MEMS) switch. Ideally, the contact should have both very low contact resistance and high wear resistance. Unfortunately this combination cannot be easily achieved with the contact materials currently used in macroswitches because the available contact force in microswitches is generally insufficient (less than 1mN) to break through nonconductive surface layers. As a step in the materials selection process, three noble metals, platinum (Pt), rhodium (Rh), ruthenium (Ru), and their alloys with gold (Au) were deposited as thin films on silicon (Si) substrates. The contact resistances of these materials and their evolution with cycling were measured using a specially developed scanning probe microscope test station. These results were then compared to measurements of material hardness and resistivity. The initial contact resistances of the noble metals alloyed with Au a...

Modeling, simulation and measurement of the dynamic performance of an ohmic contact, electrostatically actuated RF MEMS switch

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 (Va), actuation time (ta), holding voltage (Vh) and turn-on time (ton), 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.

Oxidation-sharpened gated field emitter array process

Structural and electrical characteristics of silicon field emitter arrays are reported. The authors present a process using anisotropic etching of silicon, and silicon oxidation, to form self-aligned gated field emitter structures. The process uses plasma etching and oxidation to form the field emission tips, and allows control of the aspect ratio of the devices. Processing limits and process latitude are discussed. The authors observed average currents of 0.3 mu A/emitter in 1300-emitter arrays, and the emission is stable at 5*10/sup -8/ torr. The arrays exhibit a soft failure behavior, where individual emission tips fail as the gate voltage is increased, but the array as a whole continues to operate. >

Integrated self-biased hexaferrite microstrip circulators for millimeter-wavelength applications

Planar microstrip Y-junction circulators have been fabricated from metallized 130-/spl mu/m-thick self-biased strontium hexaferrite ceramic die, and then bonded onto silicon die to yield integrated circulator circuits. The impedance matching networks needed to transform the low-impedance circulator outputs were deployed on low-loss alumina or glass dielectrics to minimize circuit losses. These magnetically self-biased circulators show a normalized isolation and insertion loss of 33 and 2.8 dB, respectively, and a 1% bandwidth for an isolation of 20 dB. Application of small (H<1.5 kOe) magnetic bias fields improved the isolation and insertion loss values to 50 and 1.6 dB, respectively. This design may form the basis for future monolithic millimeter-wave integrated circulator circuits that do not require magnets.

Plasma immersion ion implantation doping using a microwave multipolar bucket plasma

Using plasma immersion ion implantation, silicon has been doped with boron in a high-voltage pulsed microwave multipolar bucket plasma system. Diborane gas (1%) diluted in helium is used as an ion source. A sheet resistance of 57 Omega / Square Operator and an implanted dose of 1.9*10/sup 15//cm/sup 2/ are obtained in 10 min. when the target potential is pulsed to -10 kV with a 1% duty cycle. The boron profile in the silicon substrate is different from that predicted for a conventional 10-keV ion implantation. Silicon p-n junctions fabricated by this technique are of good quality. >

Mechanical, Thermal, and Material Influences on Ohmic-Contact-Type MEMS Switch Operation

Microswitch performance and reliability are affected by the coupled influences of actuator properties, material and process properties, and device thermal properties. Different contact materials show large differences in immunity to contamination. Contact shape and the adherence force between contact surfaces both evolve as the contact pair is cycled, with the adherence force often reaching a maximum between 105and 107cycles for gold. A typical switch actuation with a step function voltage results in an contact impact force 5 times greater than the static force, increasing the adherence force and the rate of change of the shape of the contact. Heating of the conductors in the switch results in intermodulation products, which are small at a transmitted power of 1 W, but increase with increasing power levels.