Douglas Ray Sparks

Long-term evaluation of hermetically glass frit sealed silicon to Pyrex wafers with feedthroughs

The reliability of bonding silicon to Pyrex wafers using a reflowed glass frit seal is examined in this paper. The Pyrex wafers have metal feedthroughs, which are used to actuate and capacitively sense a single-crystal, silicon resonator. Long-term, high-temperature storage conditions for chips with and without getters are examined. The reflowed glass seal is demonstrated to be hermetic for years at high temperature using both diaphragm deflection and the Q and frequency of resonators as a vacuum indicator. The use of a thin film getter is found to eliminate Q hysteresis due to gas desorption and adsorption, observed in other resonators studies.

Reliable vacuum packaging using NanoGetters and glass frit bonding

A new approach to vacuum packaging micromachined resonant, tunneling, and display devices will be covered in this paper. A multi-layer, thin-film getter, called a NanoGetter, which is particle free and does not increase the chip size of the microsystem has been developed and integrated into conventional wafer-to-wafer bonding processes. Experimental data taken with chip-scale packages using glass frit bonding between the Pyrex and silicon wafers, has resulted in silicon resonators in which Q values in excess of 37,000 have been obtained. Reliability data for vacuum-sealed diaphragms and resonators will be presented. Unlike previous reliability studies without getters, no degradation in Q has been noted with NanoGetter parts after extended high temperature storage. Applications for this technology include gyroscopes, accelerometers, displays, flow sensors, density meters, IR sensors, microvacuum tubes, RF-MEMS, pressure sensors and other vacuum sealed devices.

A MEMS-Based Coriolis Mass Flow Sensor for Industrial Applications

A microfluidic Coriolis mass flow sensor is discussed. The micromachined flow sensors are made using silicon tubes bonded onto a metallized glass substrate. True mass flow rates with an accuracy of better than plusmn0.5% were measured between 1 and 500 g/h. The sensor also provides a temperature and density output. The sensor output was resistant to pressure, temperature, vibration, fluid density, and viscosity. Unlike conventional steel Coriolis mass flowmeters, microelectromechanical-systems-based sensors are immune to external vibration. Applications for these low-flow-rate devices include chemical mixing, additives, biotechnology, chromatography, pharmaceutical development, and other areas where extremely small volumes of liquids are mixed, studied, or metered and where shock and vibration are encountered.

An electroformed CMOS integrated angular rate sensor

A CMOS integrated, surface-micromachined angular rate sensor utilizing an electroformed vibrating metal ring structure on a silicon IC has been developed. Substantial signal-conditioning circuitry is included on the IC with the vibrating structure. Tests of the sensor demonstrate that its performance is equivalent to that required for a variety of angular rate applications. Stiction of electroformed micromachines and other reliability issues will be discussed.

Cyclic fatigue and creep of electroformed micromachines

Resonant micromachines have been used to make accelerometers, angular rate sensors, voltage controlled oscillators, pressure, flow and chemical sensors. Resonant microsystems are in constant motion and so present new challenges in the area of reliability. A variety of materials have been employed to fabricate resonant device including semiconductors, quartz and electroformed metals. Material changes due to fatigue can occur in metallic structures under cyclic load. For many materials, there exists a fatigue limit below which a cyclic load will not lead to failure. Fatigue limits on the microlevel have not been verified. This paper will explore the correlation between macro and micro fatigue limits. Creep is another metallurgical effect, which is understood at the macro level, but has not been studied extensively in microsystems. Sensor output shift can be affected by a variety of factors, including traditional material creep. Electrostatic attraction and gas adsorption/desorption from microcavity surfaces have been found to be sources of parametric drift in this study. Charge build-up at the surface of the CMOS nitride passivation layer has been found to lead to stiction under extended high temperature operational life test conditions. This paper will illustrate how proper circuit and micromachine design, material selection and processing can allow electroformed materials to be used, without creep or cyclic fatigue affecting the performance of the resonant microsystem.

Flexible vacuum-packaging method for resonating micromachines

Abstract The vacuum packaging of resonant micromachined sensors using a solder reflow or brazing technique is disclosed. Various packaging processes, such as die adhesive selection and processing, solder selection and reflow temperature are experimentally treated with respect to the Q -value obtained by both cantilever- and ring-shaped micromachines. The stability of nickel micromachines under low-amplitude resonant conditions is demonstrated using this packaging technique. The advantages of this packaging method for development and low-volume production devices are presented.

The hermeticity of sealed microstructures under low temperature helium and hydrogen exposure

Changes in resonator behavior in the presence of helium were observed at low temperature in sealed microstructures. The effects were seen with microelectromechanical systems (MEMS) resonators employing reflowed glass and/or direct silicon bonded sealing interfaces. Resonator Q, peak gain and frequency all indicate the degradation of vacuum quality due to low temperature ingress of helium into the sealed vacuum chamber. These effects were seen between 23 and 100 °C after and during helium exposure at relatively low pressures of 140 to 380 kPa. Similar effects were not observed for hydrogen, argon and air. The results may suggest diffusion of helium through silicon at low temperatures and have implications for hermeticity testing using MIL-STD 883E, applications for the exposure of a wide variety of MEMS devices to helium and possibly wafer processing using helium.

Automotive applications for micromachining

High volume silicon micromachining has been employed by the automotive industry for 20 years. This paper examines past, current and future applications of MEMS to the automobile. Both sensor and the application of micromachining to other automotive areas are covered. Technologies such as wet and plasma etching, wafer bonding, LIGA, circuit integration and packaging are discussed.

Chemically-accelerated corrosion tests for aluminum metallized ICs

Abstract The study of corrision in microelectronics using a simple, low-cost, chemical-acceleration method is examined. A new corrosion mechanism based on the experimental results, p-type semiconductor enhanced corrosion, is proposed. Wire and passivation lifting owing to corrosion are documented. The corrosion of aluminum and aluminum/gold couples as a function of NaCl concentration is presented. The effects of wafer saw slurry on bond pad corrosion and staining is characterized using this method.

Dislocation and stacking fault interactions in silicon

The influence of dislocations generated by thermal stress during epitaxial processing upon other dislocations and upon epitaxial stacking fault densities and oxidation induced stacking fault densities is examined. High dislocation densities were observed at the intersection of thermally induced slip planes. Like dislocation densities, epitaxial stacking fault densities increased at slip plane intersections. Thermally induced dislocations influenced the arrangement of dislocation loops generated by lattice strain from heavy doping. Oxidation induced stacking faults were found to nucleate at areas of heavy slip, parallel to the slip lines.