Static and dynamic aspects of an air gap capacitor

This paper deals with the theory of an air-gap capacitor used as a micromechanical resonator. Both static and dynamic aspects are discussed. A single-element approach for the electrostatic excitation and capacitive detection of the vibrational motion of the resonators is described. The non-linear character of the electrostatic force is accounted for in the static analysis. The behaviour of the air-gap capacitor is modelled as a lumped spring-mass system and its limitations are discussed. Also an equivalent electrical one-port network is derived, which can be used in a circuit simulation to account for the mechanical behaviour of the resonator. The results obtained from the spring-mass system are compared with the results obtained from a more elaborate numerical analysis of the air-gap capacitor. The lumped spring-mass system is adequate for modelling the air-gap capacitor

Quasi-monolithic planar load cells using built-in resonant strain gauges

Two load cell designs are presented using resonant strain gauges providing a frequency output. One design is based on a four-point beam deflection jig. It offers high sensitivity, but suffers from robustness and impractical geometries for a broad force range. A modified planar design (typical dimensions 1-10 mm) removes these drawbacks and in addition features built-in force reduction, overhead protection and compensation of common mode effects. Load ranges vary from high (1 500 N) to very low (0.03 N), with theoretically achievable resolutions as high a 1 part in 107

Nonlinearity and hysteresis of resonant strain gauges

Nonlinearity and hysteresis effects of electrostatically activated, voltage driven resonant microbridges have been studied theoretically and experimentally. It is shown, that, in order to avoid vibration instability and hysteresis to occur, the choices of the ax. and d.c. driving voltages and of the quality factor of a resonator, with a given geometry and choice of materials, are limited by a hysteresis criterion. The limiting conditions are also formulated as hysteresis-free design rules. An expression for the maximum attainable figure of merit is also given. Experimental results, as obtained from electrostatically driven vacuum-encapsulated polysilicon microbridges, are presented and show good agreement with the theory

Electrostatically driven vacuum-encapsulated polysilicon resonators part I. design and fabrication

Basic design issues and a fabrication process based on surface-micromachining techniques for electrostatically driven vacuum-encapsulated polysilicon resonators are presented. A novel freeze-drying method that does not require vacuum equipment is presented. Reactive sealing with LPCVD silicon nitride is used to create the evacuated cavity, resulting in cavity pressures close to the deposition pressure. Design issues regarding choice of materials, technology and layout are discussed. First experimental results, including an admittance plot of the one-port resonator and a plot indicating the dependence of the Q-factor on the resonator geometry and ambient pressure, are presented

A differential resonator design using a bossed structure for applications in mechanical sensors

Theory and experimental results are presented of a differential resonator design employing a bossed structure for applications in mechanical sensors. The effects of residual strain, temperature and mechanical load on the resonance frequency are investigated. Mismatches in the resonators are accounted for in the analysis, resulting in a predicted temperature dependence of the offset and of the sensitivity. Experimental data obtained from a macroscopic brass model, mounted on a steel bar and applied as a force sensor, are given. Compared to a design employing a single resonator, the measurements indicate a doubling in force sensitivity and a reduction of both the intrinsic temperature dependence and of the differential thermal expansion effects. The results of this research are directly applicable to micromachined structures in silicon

Stiction of surface micromachined structures after rinsing and drying: model and investigation of adhesion mechanisms

The mechanisms causing stiction of polysilicon structures fabricated by surface micromachining techniques have been investigated. It is found that during drying from rinse liquids attractive dynamic capillary forces are responsible for bringing micromechanical structures into contact with the underlying substrate. Measured adhesion energies of sticking microbridges indicate that van der Waals forces are responsible for the stiction of hydrophobic surfaces and that hydrogen bridging is an additional adhesion mechanism for hydrophilic surfaces. Methods to reduce the stiction problem are indicated

Resonant diaphragm pressure measurement system with ZnO on Si excitation

The principle of measuring pressure by means of a resonant diaphragm has been studied. An oscillator consisting of an integrated amplifier with a piezoelectrically driven diaphragm in its feedback loop has been built. The oscillator frequency is accurately proportional to the square of the pressure in the range of 60 to 130 Torr.\ud The frequency range is 1324 to 1336 Hz (this range being limited by a spurious mode which could be suppressed by better processing) for a 25 mm diameter diaphragm made of a silicon wafer and with PZT ceramics as driver and receptor. We have made an integrated version (1 × 1 mm2) of a square resonant diaphragm pressure guage by selective etching of (1 0 0) planes with ethylenediamine. The piezoelectric driving materials was sputtered zinc oxide. A driver was deposited midway between the bending point and the point of greatest curvature.\ud A receptor was located at a symmetrical position to give a optimum transfer condition.\ud The integrated current amplifier had a low impedance differential input stage, two gain cells and a high impedance output stage. These electrical conditions ensured maximum elastic freedom of the diaphragm. A digital circuit in I2L technology has been designed and made with eight-bit parallel read out of the frequency. This circuit may be directly connected to a microprocessor. The whole system contains the sensor chip, the analog amplifier chip and the digital chip, all in compatible technology.\ud \u

Fibbing in action: On-demand load-balancing for better video delivery

Video streaming, in conjunction with social networks, have given birth to a new traffic pattern over the Internet: transient, localized traffic surges, known as flash crowds. Traditional traffic-engineering methods can hardly cope with these surges, as they are unpredictable by nature. Consequently, networks either have to be overprovisioned, which is expensive and wastes resources, or risk to periodically incur congestion, which infuriates customers. This demonstration shows how Fibbing can improve network performance and preserve users' quality of experience when accessing video streams, by implementing a fine-grained load-balancing service. This service leverages two unique features of Fibbing: programming per destination load-balancing and implementing uneven splitting ratios

Wafer-level packaged RF-MEMS switches fabricated in a CMOS fab

Reports on wafer-level packaged RF-MEMS switches fabricated in a commercial CMOS fab. Switch fabrication is based on a metal surface micromachining process. A novel wafer-level packaging scheme is developed, whereby the switches are housed in on-chip sealed cavities using benzocyclobutene (BCB) as the bonding and sealing material. Measurements show that the influence of the wafer-level package on the RF performance can be made very small.\ud \u