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We have developed a single-wafer vacuum encapsulation for microelectromechanical systems (MEMS), using a thick 120-[micro]m) polysilicon encapsulation to package micromechanical resonators in a pressure 600 cycles of -50 to 80[degrees]C, and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated temperature. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400[degrees]C have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400[degrees]C without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high-temperature environment during the sealing process. Index Terms--Encapsulation, hermetic, microelectromechanical systems (MEMS), micromechanical resonator, packaging, pressure, wafer-level.

Thermoelastic dissipation (TED) is analyzed for complex geometries of micromechanical resonators, demonstrating the impact of resonator design (i.e., slots machined into flexural beams) on TED-limited quality factor. Zener first described TED for simple beams in 1937. This work extends beyond simple beams into arbitrary geometries, verifying simulations that completely capture the coupled physics that occur. Novel geometries of slots engineered at specific locations within the flexural resonator beams are utilized. These slots drastically affect the thermal-mechanical coupling and have an impact on the quality factor, providing resonators with quality factors higher than those predicted by simple Zener theory. The ideal location for maximum impact of slots is determined to be in regions of high strain. We have demonstrated the ability to predict and control the quality factor of micromechanical resonators limited by thermoelastic dissipation. This enables tuning of the quality factor by structure design without the need to scale its size, thus allowing for enhanced design optimization. Index Terms--Damping, energy dissipation, microelectromechanical systems (MEMS), micromechanical resonator, quality factor, thermoelastic dissipation.

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions ( Index Terms--Accelerometer, piezoresistive, wafer level packaging, ultraminiature sensors.

The use of advanced embedded system technologies such as microelectromechanical system (MEMS) sensors and wireless communications hold great promise for measuring the response of civil structures to ambient and external disturbances. In this paper, the design of a high-performance, planar piezoresistive MEMS accelerometer is discussed in detail. The piezoresistive accelerometer possesses superior performance characteristics including low noise densities when measuring local structural responses characterized by high-frequency content. A low-cost wireless sensing unit, designed for automated structural monitoring, is utilized to record and wirelessly transmit measurements obtained by the piezoresistive accelerometer. To validate the performance of the wireless monitoring system including the interfaced piezoresistive accelerometer, a five degrees-of-freedom laboratory test structure is utilized. CE Database subject headings: Monitoring; Sensors; Structural response; Acoustic measurement.

Electrical through-wafer interconnects (ETWI) which connect devices between both sides of a substrate are critical components for microelectromechanical systems (MEMS) and integrated circuits (IC), as they enable three-dimensional (3-D) structures and permit new packaging and integration geometries. Previously demonstrated ETWI are very difficult to integrate with standard semiconductor fabrication processes, not compatible with released sensors, do not permit extensive processing on both sides of the wafer, and are in general very application specific. This work describes the design, fabrication, and characterization of an ETWI technology for silicon substrates that can be broadly integrated with MEMS and IC processes. This interconnect is a passively isolated electrical through-wafer polysilicon plug, with a 20 [micro]m diameter, 10-14 [OMEGA] resistance, and less than 1 pF capacitance. Plasma etching from both sides of the wafer is used to achieve a high-aspect ratio via (20:1 through 400 [micro]m). The process is compatible with standard lithography, standard wafer handling, subsequent high-temperature processing, and released sensors integration. N-type and p-type versions are demonstrated, and isolated ground planes are added to provide shielding against substrate noise. Electrical properties of these ETWI are measured and analytically modeled. These ETWI are appropriate for integration with devices with impedances much greater than the ETWI, such as piezoresistive and capacitive sensor arrays. [850] Index Terms--Electrical through-wafer interconnects (ETWI), MEMS and IC integration, through-wafer via deep silicon plasma etch.

A new high-sensitivity bulk-silicon-micromachined tunneling accelerometer with micro-g resolution has been successfully fabricated and tested at Stanford University. This accelerometer is a prototype intended for underwater acoustics applications and is required to feature micro-g resolution at frequencies between 5 Hz and 1 kHz and can be packaged with circuitry in an 8-[cm.sup.3] volume with a total mass of 8 g. This paper briefly describes the mechanical design of this tunneling accelerometer and focuses on the experiments carried out in our laboratory to test the tunneling transducer as well as on the experimental determination of accelerometer resolution. The exponential relationship between tunneling gap and tunneling current is verified and results in an effective tunneling barrier height of about 0.2 eV. The goal of this paper is to outline the measurements which are necessary to verify that the sensor is actually tunneling and to confirm that the accelerometer performance is consistent with what should be expected from a tunneling accelerometer.

The micromachined piezoresistive accelerometer is now 20 years old. Design variations have been investigated, but commercial devices have generally maintained a consistent topology with incremental improvements. In this paper, a new approach is introduced to the design and construction of this device that offers functional and manufacturing advantages. Piezoresistive accelerometers are described that combine deep reactive ion etching and oblique ion implantation to form self-caging proof masses and flexures with vertical sidewalls and sidewall piezoresistive strain sensors. These devices deflect in-plane rather than out-of-plane, which allows one to form multiaxis accelerometers on one substrate. Performance is comparable to inexpensive commercial capacitive accelerometers and is limited by 1/f noise. The design, fabrication, and experimental characterization is presented. This new topology provides the foundation for a new category of piezoresistive accelerometers. [438] Index Terms--Deep reactive ion etching, fusion wafer bonding, lateral implantation, micromachines, oblique implantation, piezoresistive accelerometers, piezoresistors.

This paper presents a dual-microelectromechanical system (MEMS) resonator-based temperature sensor. In this sensor, the readout circuit estimates the temperature by measuring the frequency ratio of the two clocks generated by separate resonators with different temperature coefficients. The circuit is realized in a 0.18- \mu \textm CMOS process and achieves a resolution of 20 \mu \textK over a bandwidth of 100 Hz while consuming 19 mW of power, leading to a resolution FOM of 0.04 pJK2. It enables us to implement a MEMS-based programmable oscillator with an Allan deviation of <1 e^-10 over 1 s averaging time, and a frequency stability of <±0.1 parts per million in the temperature range from −45 °C to 105 °C. Such oscillators are key building blocks in telecom, datacom, and precision timekeeping applications. [ABSTRACT FROM AUTHOR]

MEMS-based oscillators offer a silicon-based alternative to quartz-based frequency references. Here, a MEMS-based programmable oscillator is presented which achieves better than \pm\ 0.5-ppm frequency stability from -\40 ^\circ \C to 85 ^\circ \C and less than 1-ps (rms) integrated phase noise (12 kHz to 20 MHz). A key component of this system is a thermistor-based temperature-to-digital converter (TDC) which enables accurate and low noise compensation of temperature-induced variation of the MEMS resonant frequency. The TDC utilizes several circuit techniques including a high-resolution tunable reference resistor based on a switched-capacitor network and fractional-N frequency division, a switched resistor measurement approach which allows a pulsed bias technique for reduced noise, and a VCO-based quantizer for digitization of the temperature signal. The TDC achieves 0.1-mK (rms) resolution within a 5-Hz bandwidth while consuming only 3.97 mA for all analog and digital circuits at 3.3-V supply in 180-nm CMOS. [ABSTRACT FROM AUTHOR]

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (

Packaging of micro-electro-mechanical systems (MEMS) devices has proven to be costly and complex, and it has been a significant barrier to the commercialization of MEMS. We present a packaging solution applicable to several common MEMS devices, such as inertial sensors and micromechanical resonators. It involves deposition of a 20 μm layer of epi-polysilicon over unreleased devices to act as a sealing cap, release of the encapsulated parts via an HF vapor release process, and a final seal of the parts in 7 mbar (700 Pa) vacuum. Two types of accelerometers, piezoresistive and capacitive sensing, were fabricated. Piezoresistive accelerometers with a footprint smaller than 3 mm² had a resolution of 10 μg/√Hz at 250 Hz. Capacitive accelerometers with a 1 mm² footprint had a resolution of 1 mg √Hz over its 5 kHz bandwidth. Resonators with a quality factor as high as 14,000 and resonant frequency from 50 kHz to 10 MHz have also been built. More than 100 capacitive accelerometers and 100 resonators were tested, and greater than 90% of the resonators and accelerometers were functional. [ABSTRACT FROM AUTHOR]