This paper presents a review of silicon micromachined accelerometers and gyroscopes. Following a brief introduction to their operating principles and specifications, various device structures, fabrication, technologies, device designs, packaging, and interface electronics issues, along with the present status in the commercialization of micromachined inertial sensors, are discussed. Inertial sensors have seen a steady improvement in their performance, and today, microaccelerometers can resolve accelerations in the micro-g range, while the performance of gyroscopes has improved by a factor of 10/spl times/ every two years during the past eight years. This impressive drive to higher performance, lower cost, greater functionality, higher levels of integration, and higher volume will continue as new fabrication, circuit, and packaging techniques are developed to meet the ever increasing demand for inertial sensors.

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Micromachined inertial sensors

High-precision miniature accelerometers based on electron tunneling transducers have been successfully fabricated using bulk-silicon micromachining process. This process has been modified to reduce bi-metal effects and results in greatly-suppressed 1/f noise. A comprehensive system model and a robust controller based on mixed /spl mu/ synthesis have also been developed to close the feedback loop, extend the bandwidth, and withstand accelerometer variations due to micromachining process variations. This accelerometer is a prototype intended for underwater acoustics applications and is designed to be packaged in an 8 cm/sup 3/ sphere volume with a total mass of 8 grams. In this paper, we describe the accelerometer design, the feedback control design, and several performance enhancements of these micromachined tunneling accelerometers, which include a high resolution of 20 nano-g//spl radic/Hz and a 5 Hz-1.5 kHz bandwidth.

A high-precision, wide-bandwidth micromachined tunneling accelerometer

A variety of microelectromechanical system devices requires encapsulation of their crucial fragile parts in a hermetically sealed cavity for reasons of protection Hermeticity of the cavity and controllability of the ambient (gas pressure and gas composition) can be critical to the device performance In order to minimize damage during handling, the cavity is preferably realized at the same time the device is fabricated, ie, at wafer level This paper reports the development of a hermetic packaging technique satisfying all the above The method is referred to as the indent-reflow-sealing (IRS) technique, which relies on a multiple-chip fluxless solder-based joining technique and seal Key process steps are the creation of an indent in the solder, the plasma pretreatment of the bonding surfaces, the pre-bonding (or sticking) of the chips and, the closing of the indent during a low-temperature (220/spl deg/C-350/spl deg/C) solder reflow in a clean controlled ambient using a designated oven As opposed to other methods, the IRS method allows a greater flexibility with respect to the choice of the sealing gas and pressure, thereby offering a very hermetic seal and compatibility with low-cost high-throughput batch fabrication techniques Flip-chip assemblies based on SnPb (67/37) solder and Au as the top surface metallization, have been reflowed in a forming gas ambient and have next been characterized on shear strength, hermeticity, and susceptibility to thermal stresses The method has been successfully implemented in the process flow of an electromagnetic microrelay for the realization of the cavity housing the electrical contacts

The indent reflow sealing (IRS) technique-a method for the fabrication of sealed cavities for MEMS devices

The techniques of micromachining silicon are used for the manufacturing of a thermal accelerometer. This sensor requires no solid proof mass and has a low cost production. A heating resistor creates a symmetrical temperature profile and two temperature detectors are placed on both sides. When an acceleration is applied, the temperature profile becomes asymmetric and the two detectors measure the differential temperature. Platinum resistors deposited by electron beam evaporation on a SiN x membrane are used as heater and temperature sensors. This paper presents measurements of temperature profile and sensitivity according to the distance heater–detector, power supplied and room temperature. It shows that the sensitivity is proportional to the heating power and decreases when the room temperature increases. Experimental results in tiltmeter utilization and in a centrifuge are also presented. A 3 dB bandwidth of 20 Hz is measured applying a sinusoidal acceleration and the equivalent acceleration noise is found to be 0.3 mg RMS.

Micromachined thermal accelerometer

Inertial navigation represents a unique method of navigation, in which there is no dependency on external sources of information. As opposed to other position fixing navigation techniques, inertial navigation performs the navigation in a relative sense with respect to the initial navigation state of the moving platform. Hence, inertial navigation systems are not prone to jamming, or spoofing. Inertial navigation systems have developed vastly, from their occurrence in the 1940s up to date. The accuracy of the inertial sensors has improved over time, making inertial sensors sufficient in terms of size, weight, cost, and accuracy for navigation and guidance applications. Within the past few years, inertial sensors have developed from being purely mechanical into incorporating various technologies and taking advantage of numerous physical phenomena, from which the dynamic forces exerted on a moving body could be computed accurately. Besides, the evolution of inertial navigation scheme involved the evolution from stable-platform inertial navigation system, which were mechanically complicated, to computationally demanding strap-down inertial navigation systems. Optical sensory technologies have provided highly accurate inertial sensors, at smaller sizes. Besides, the vibratory inertial navigation technologies enabled the production of Micro-electro-machined inertial sensors that are extremely low-cost, and offer extremely low size, weight and power consumption, making them suitable for a wide range of day-to-day navigation applications. Recently, advanced inertial sensor technologies have been introduced to the industry such as nuclear magnetic resonance technology, cold-atom technology, and the re-introduction of fluid-based inertial sensors. On another note, inertial sensor errors constitute a huge research aspect in which it is intended for inertial sensors to reach level in which they could operate for substantially long operation times in the absence of updates from aiding sensors, which would be a huge leap. Inertial sensors error modeling techniques have been developing rapidly trying to ensure higher levels of navigation accuracy using lower-cost inertial sensors. In this review, the inertial sensor technologies are covered extensively, along the future trends in the inertial sensors’ technologies. Besides, this review covers a brief overview on the inertial error modeling techniques used to enhance the performance of low-cost sensors.

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Inertial sensors technologies for navigation applications: state of the art and future trends

A micromachined thermal accelerometer that is simple, reliable, and inexpensive to make has been developed at Simon Fraser University. The operating principle of this accelerometer is based on the free-convection heat transfer of a small hot air bubble in a sealed chamber. An experimental device that requires only four masking steps to fabricate has been built. This device has demonstrated a 0.6 milli-g sensitivity that can theoretically be extended to sub-micro-g level: A 2-axis accelerometer based on the same operating principle has also been fabricated and tested.

B. Woods

Papers

Micromachined accelerometer based on convection heat transfer