Kevin H.-L. Chau

An integrated force-balanced capacitive accelerometer for low-g applications

Abstract A low-cost monolithic accelerometer which incorporates a surface-micromachined polysilicon sensor with bipolar/MOS interface circuitry on a single chip has been developed for a measurement range of ± 5g. The accelerometer has a noise floor of 0.6 mg (Hz) 1 2 and a shock survival rating of 1000g. A sensitivity temperature coefficient ± 50 ppm °C−1 and an offset temperature coefficient of ±5 mg °C−1 have been achieved over the −40 to +105 °C automotive temperature range. A wide bandwidth from d.c. to 4 kHz is adjustable using a single external capacitor. The accelerometer operates on a single 5 V power supply and consumes 40 mW of power. It provides an analog output of 200 mV g−1 and a digitally activated self-test output of −5g.

Technology for the high-volume manufacturing of integrated surface-micromachined accelerometer products

Since the inception of silicon surface micromachining more than fifteen years ago, the technology has successfully emerged from academic research and has found practical applications in many high-volume sensor products. The integration of on-chip electronics, in particular, represents the highest level of technological accomplishment to date in surface micromachining technology. The integrated approach offers tremendous performance enhancements while presenting numerous challenges to actual production implementation. This paper uses the Analog Devices accelerometer product series as an example to review the integrated surface micromachining technology and the practical solutions to overcome many of its challenges for high-volume manufacturing.

A versatile polysilicon diaphragm pressure sensor chip

A multidiaphragm piezoresistive pressure sensor with built-in front side over-pressure protection has been developed which is capable of simultaneous measurement of absolute and differential pressures over a wide range. The sensor chip has six polysilicon pressure sensing diaphragms, three of which measure differential pressures of -1 to 1, -5 to 5, and -30 to 30 psi: the remaining diaphragms measure absolute pressure against vacuum-sealed microcavities and have linear pressure ranges of 0 to 100, 0 to 500, and 0 to 2000 psi. Front side over-pressure protection is at least 6000 psi for every range. First-order temperature correction is achieved by using pressure-insensitive matching resistors. An additional temperature sensor provides on-chip temperature measurement and data for further digital compensation of the sensor array. The sensor chip has been characterized with full-scale output span ranges from 5mV/V for the 1 psi device to 62 mV/V for the 500 psi device. Hysteresis and nonrepeatability are typically within 0.1% of span. The chip thus offers pressure measuring capability over a wide range and the highest built-in over-pressure protection ever reported for a silicon pressure sensor.<<ETX>>

Single-chip 1×84 MEMS mirror array for optical telecommunication applications

The inherent ability of silicon micromachining to provide a multitude of precision aligned optical components on a single die naturally facilitates optical communication trends towards installing larger cross-connects and transmitting many channels on individual fibers. A micromachined mirror array has been designed and fabricated using the Analog Devices, Inc. (ADI) Optical iMEMS (R) process. The single-chip mirror die consists of 84 mirrors arranged in a linear array with an average pitch of 95 μm. Each mirror is equipped with a pair of polysilicon actuation electrodes located beneath the mirror. These two electrodes allow each mirror to be independently rotated around the axis parallel to the long dimension of the array using off-chip voltage commands. An operating mirror tilt of +/- 2 degrees is achieved with less than 130 volts of actuation. The design objectives including high mirror fill factor, optimal air damping, low mirror-to-mirror cross talk, acceptable voltage levels, and robustness posed significant challenges. This paper will describe how these challenges were overcome using an interdigitated mirror layout. The mirrors were successfully fabricated with good yield and characterized through both customer and ADI testing.

Over-range behavior of sealed-cavity polysilicon pressure sensors

Abstract This paper reports the over-range behavior of sealed-cavity polysilicon pressure sensors with full-scale pressures ranging from 0.24 to 2.0 MPa (35 to 290 psi). Because of a built-in over-pressure stop, a fracture pressure between 24 and 41 MPa (3500 to 6000 psi) is observed, which is relatively insensitive to the particular size or range of the sensors investigated. Stress simulations using the finite-element method confirm this and provide a theoretical framework for designing over-range capabilities. Below the fracture range, no degradation in sensor performance is apparent after repetitive cycles of over-pressure to 21 MPa (3000 psi) and the zero repeatability is better than 0.1% of span. These results suggest outstanding over-range capabilities of sealed-cavity polysilicon sensors and exceptional mechanical strength and reproducibility of polysilicon diaphragms for pressure sensing.

High-stress and overrange behavior of sealed-cavity polysilicon pressure sensors

The overrange behavior of sealed-cavity polysilicon pressure sensors with full-scale pressures ranging from 35 to 300 psi (0.24 to 2.0 MPa) is investigated. Because of a built-in overpressure stop, rupture pressure between 3500 to 6000 psi (24 to 41 MPa) was observed. After repetitive cycles of overpressure to 3000 psi, no degradation in sensor performance was apparent, and the zero repeatability was better than 0.1% of span. These results demonstrate the outstanding overrange capabilities of sealed-cavity polysilicon sensors and the exceptional mechanical strength and reproducibility of polysilicon diaphragms for pressure sensing. The excellent built-in overrange protection represents a significant advantage of the sealed-cavity polysilicon sensors for industrial applications where high overrange pressure is common.<<ETX>>

MEMS pressure sensors for high-temperature high-pressure downhole applications

This paper outlines the approach to realize a MEMS pressure sensor suitable for meeting the stringent requirements of sensing in a downhole environment reaching 175-oC temperature and 200-MPa pressure while maintaining an accuracy of better than 0.02% for an extended measurement period of several weeks during oil and gas exploration. A few sensing structures are proposed and strategies for their optimization are discussed for this unique application.