Simultaneous Electrical and Thermal Modeling of a Contact-Type RF MEMS Switch

Improving the power handling capability of direct contact RF MEMS switches requires a knowledge of conditions at the contact. This paper models the temperature rise in a direct contact RF MEMS switch, including the effects of electrical and thermal contact resistance. The maximum temperature in the beam is found to depend strongly on the power dissipation at the contact, with almost no contribution from dissipation due to currents in the rest of the switch. Moreover, the maximum temperature is found to exceed the limit for metal softening for a significant range of values of thermal and electrical contact resistance. Since local contact asperity temperature can be hundreds of degrees higher than the bulk material temperature modeled here, these results underscore the importance of understanding and controlling thermal and electrical contact resistance in the switch.

Formation of Hexagonal Monolayers by Flow of Bead Suspensions Into Flat Microchambers

To realize a highly parallel optical detection in bead-based bioanalytical assays, we investigate the hydrodynamic aggregation of bead suspensions in a hexagonally periodical monolayer by a pressure-driven flow through a microfluidic structure. This device consists of one inlet channel connected to a shallow chamber with a depth that only slightly exceeds the diameter of the beads. To enforce the aggregation of the beads, the flow leaves the chamber via outlet channels possessing a depth smaller than a bead diameter. This way the outlets act as barriers to the beads and force them to accumulate in the chamber. Benchmarking different chamber and outlet designs we found an optimum filing behavior for a rhombus-like aggregation chamber connected to a single outlet channel at the same width as the chamber. Here, the aperture angle of 60° fosters hexagonal aggregation patterns which leads to the highest packaging density. Reproducible filling ratios of more than 94% have been achieved. The rhombus-like chamber also shows the shows the smallest increase of the hydrodynamic resistance during filling and the best rinsing behavior which allows to minimize the volume of washing detergents used for a bioassay. Zones of accumulated beads redistribute the hydrodynamic flow through the device during the filling process. CFD-simulations, embedded in an iterative master-routine, are carried out to describe the complete process of filling and to assist the process of design optimization.

Micro Ultrasonic Machining of Ceramic MEMS With Micro Metallic Dies

Micro ultrasonic Machining (MUSM) is useful for producing micro parts in brittle materials, especially ceramics. By use of suitable micro metallic dies, the efficiency of fabrication can be significantly enhanced. In this study, the LIGA process was used to generate micro nickel dies, which also served as microelectrodes in Die-sinking Electrical Discharge Machining (EDM) to produce micro tungsten dies for MUSM. With these micro metallic dies, micro ceramic components were fabricated.

Heat Transfer and Electrokinetic Flow Analysis in Poly(Dimethylsiloxane) Microfluidic Systems

Electrokinetic pumping is commonly used as a mechanism for species transport in microfluidic systems. Joule heating, caused by current flow through the buffer solution during electroosmotic flow, can lead to significant increases in the system temperature which can be detrimental to electrophoretic separations and temperature sensitive chemical reactions. In this paper, a combined experimental and numerical approach was used to examine Joule heating and heat transfer at a T intersection for PDMS/PDMS and PDMS/Glass hybrid microfluidic systems. In general it was found the PDMS/Glass chips maintained a more uniform and lower buffer temperature than the PDMS/PDMS systems, since the internally generated heat could be transferred more efficiently (due to the higher thermal conductivity of the glass component) from the channel network to the room temperature reservoir. This increase in temperature was shown to significantly increase the current load and the volume flow rate through the PDMS/PDMS system.

Design and Fabrication of a High Efficiency Piezoelectric Vibration-Induced Micro Power Generator

To fulfill the increasing self-power demanding of the embedded and remote microsystems, theoretical and experimental study of a piezoelectric vibration-induced micro power generator that can convert mechanical vibration energy into electrical energy is presented. A complete energy conversion model regarding the piezoelectric transducer is discussed first. To verify the theoretical analysis, two clusters of transducer structures are fabricated. The piezoelectric lead zirconate titanate (PZT) material that has better energy conversion efficiency among the piezoelectric materials is chosen to make of the energy conversion transducer. The desired shape of the piezoelectric generator with its resonance frequency in accordance with the ambient vibration source is designed by finite element analysis (FEA) approach. Conducting wires and load resistor are soldered on the electrodes to output and measure the vibration induced electrical power. Experimental results shows that the maximum output voltages are generated at the first mode resonance frequencies of the structure. It is also found from the experimental results that the induced voltage is irrelevant to the width of the structure but is inverse proportion to the length of the structure. It takes 7 minutes to charge a 10,000 μF capacitors array to a 7 V level. The total amount of electricity and energy stored in the capacitors are 0.7 Coulomb and 0.245 J, respectively. The experimental results are coincidence with the theoretical analysis.

Characterization of a Radiantly Driven Multistage Knudsen Compressor

Knudsen Compressors are meso/micro scale gas compressors/pumps based on thermal transpiration or thermal creep. The design of radiantly driven Knudsen Compressors is discussed, along with a model that was developed to understand their performance. Experimental pumping performances for Knudsen Compressors with one, two, five, and fifteen stage, radiantly driven cascades are also discussed. Temperature measurements across the transpiration membranes, for various pressures of Nitrogen, were obtained and compared to those predicted by the performance model. The results agree with the model to within 15% consistently under predicting the measured hot side temperature of the transpiration membrane. The pump-down curves, steady-state maximum pressure differences, and maximum flow rates produced by a single stage Knudsen Compressor were obtained. A variety of configurations were studied at pressures from 500 mTorr to atmospheric pressure. The experimental results agreed with the performance model’s predictions to within 20%.

Design and Fabrication of a Temperature Sensor Based on Thermopile in CMOS Technology

This paper describes a new geometry for integrated micromachined thermopile structures. Different arrangements for the thermocouples in proximity to the heating element are examined, to optimize the accuracy of the temperature measurement. Several design parameters including thermopile lengths, and the number of thermocouples, are examined. The test chip was designed and fabricated in CMOS technology, including the appropriate opening for post-processing micromachining. The thermopile used was fabricated with polysilicon/aluminum contacts on a silicon oxide/nitride layer provided by the CMOS process. Different microbeam and bridge membrane support structures were designed for the thermopile, in order to investigate the optimal geometry for mechanical stability and to avoid structure buckling.

Static Chaos Microfluid Mixers Using Microblock-Induced Alternating Whirl and Microchannel-Induced Lamination

We characterize two types of noble static chaos microfluid mixers for the applications to Micro Total Analysis Systems (μTAS): an AW-type microfluid mixer, having a series of microblocks along a flow channel for generating alternating direction whirl (AW) flows, and an AWL-type microfluid mixer, coupling the AW-type microfluid mixers with divided microchannels for generating lamination flows between the alternating whirl flows. For generating whirling flow in microchannels, we design rotating block geometry in microchannels. For chaos mixing, we suggest alternating-directional whirling flows in microchannels. AW, AWL-type microfluid mixers are made of PDMS (Polydimethylsiloxane). We quantify mixing state using phenolphthalein visualization experiments and measure pressure drop through microfluid mixers.

Microfabricated PDMS Check Valves

This paper presents two types of novel micro check valves that are based on PDMS. The valves consist of a thin flap and a flow restriction block inside a microchannel. The flap is perpendicular to the flow and lies near the block, which forms a restricted fluid path inside the channel. The valves are fabricated entirely from PDMS through replica molding techniques and can be readily integrated in PDMS-based complex microfluidic systems. Testing results show that for a reverse flow, the 2D valve has a saturated leakage rate at higher pressures, leading to an interesting “fluid diode” phenomenon, while the 3D valve has zero leakage at higher pressures.

Temperature-Dependence of Water Bridge Formation in Atomic Force Microscopy

When an Atomic Force Microscope (AFM) is operated in air, capillary condensation induces meniscus formation between the AFM tip and substrate. At present, no models account for the temperature-dependence of meniscus formation. This paper describes experiments measuring capillary forces between an AFM tip and mica at various temperatures and times. At low humidity, the capillary force decreases with increasing surface temperature in a manner unaccounted for by merely the dependence of water surface energy on temperature. We propose that this is due to water evaporation off the heated surface. The adhesion is also shown to decrease significantly with time until stabilizing after approximately an hour of experiments. Localized heating of the surface by the AFM laser is proposed as the cause of adhesion decrease. The decrease in force occurring at high surface temperatures implies a reduction in meniscus size that may potentially improve the resolution of AFM-based nanolithography techniques.