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.

/pdf/micromachined-inertial-sensors-esf6197vdc.pdf

Micromachined inertial sensors

Pull-in instability as an inherently nonlinear and crucial effect continues to become increasingly important for the design of electrostatic MEMS and NEMS devices and ever more interesting scientifically. This review reports not only the overview of the pull-in phenomenon in electrostatically actuated MEMS and NEMS devices, but also the physical principles that have enabled fundamental insights into the pull-in instability as well as pull-in induced failures. Pull-in governing equations and conditions to characterize and predict the static, dynamic and resonant pull-in behaviors are summarized. Specifically, we have described and discussed on various state-of-the-art approaches for extending the travel range, controlling the pull-in instability and further enhancing the performance of MEMS and NEMS devices with electrostatic actuation and sensing. A number of recent activities and achievements methods for control of torsional electrostatic micromirrors are introduced. The on-going development in pull-in applications that are being used to develop a fundamental understanding of pull-in instability from negative to positive influences is included and highlighted. Future research trends and challenges are further outlined.

Electrostatic pull-in instability in MEMS/NEMS: A review

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

http://iopscience.iop.org/article/10.1088/0022-3727/40/20/S19/pdf

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

The emergence of micro sensors and actuators is pushing forward the revolution of intelligent systems technology. Intelligent systems technology allows a dramatic change in the way that machines, tools, and systems serve humans. This paper reviews recent R&D and commercial status of inertial sensors using silicon micromachining technologies and discusses the requirement for commercialization of the research results.

Interference microscopy plays a central role in noncontact strategies for process development and quality control, providing full 3D measurement of surface characteristics that influence the functional behavior of manufactured parts. Here I briefly review the history and principles of this important technique, then concentrate on the details of hardware, software, and applications of interference microscopy using phase-shifting and coherence scanning measurement principles. Recent advances considered here include performance improvements, vibration robustness, full color imaging, accommodation of highly sloped surfaces, correlation to contact methods, transparent film analysis, and international standardization of calibration and specification.

Principles of interference microscopy for the measurement of surface topography

This paper describes a fully automated measurement system designed to evaluate the dynamic characteristics of micromechanical structures (millimeter dimensions). To validate the system, vibration measurements have been carried on two structures-a micromachined silicon cantilever and bridge-and the results are presented. Out-of-plane measurements show that for the cantilever, both the mode shapes and resonant frequencies agree with beam theory predictions. However, for the bridge structure, tension due to boron doping causes a change from beam-like behavior and a more complex model is required. Mode-shapes natural frequencies and modal damping are determined from data obtained by vibrating the structures using a piezoelectric mounting system and deriving the transfer function between the piezodrive voltage and beam vibrational velocity.

A system for the dynamic characterization of microstructures

Cubic SiC cantilever resonators designed for electrothermal actuation are presented. Metal electrodes with both open circuit and short circuit designs have been deposited and patterned on top of the 3C–SiC cantilevers. Pt electrodes on single crystal 3C–SiC cantilevers and NiCr electrodes on poly-crystalline 3C–SiC cantilevers have both been fabricated and tested in order to investigate the material property effect on the performance of the devices. An analytical model has been developed to understand the electrical power distribution in the cantilevers for the different material systems as well as the different metal terminations. Electrothermal actuation of resonance has been successfully achieved in all the fabricated cantilevers. The dynamic performance of the cantilever resonators has been systematically studied including resonance frequencies, amplitude response with voltage and actuation efficiencies. During the discussion of these results, the mechanism of the electrothermal actuation in these devices has been identified which allows actuation frequencies up to 100 MHz to be possible.

SiC cantilever resonators with electrothermal actuation

The principal features of a three-axis ring gyroscope are described. The equations of motion, the response to constant rates of turn, and the relationships between the modes of vibration are derived. Tuned and untuned cases are considered.

Principles of a three-axis vibrating gyroscope

A simple one-step inductively coupled plasma etching technique has been developed for the fabrication of SiC resonant beam structures. Straight cantilever and bridge devices have been made successfully. The structures have been actuated and resonant frequencies ranging from ∼120 kHz to ∼5 MHz have been measured. Comparison of the theoretically simulated and experimentally measured resonant frequencies shows the presence of significant tensile stress in bridge structures while the cantilever beams are free of stress. The degree of the tension in the bridge structures has been found to be independent of the bridge length.

Fabrication of SiC microelectromechanical systems using one-step dry etching

Silicon carbide (SiC) is widely recognized as the leading candidate to replace silicon in micro-electro-mechanical systems devices operating in harsh environments. In this work, cantilevers and bridges in SiC are designed, fabricated and evaluated between room temperature (RT) and 600 °C. The active material is a cubic poly-SiC film deposited on a poly-Si layer which is separated from the Si substrate by a thermal oxide. From surface profiling and optical observations, it is deduced that an average residual strain of +5 × 10?4 is present in the 2.7 µm thick film, with a gradient of 2.5 × 10?4 µm?1. The structures are excited either mechanically or electrostatically. Their resonance frequency is measured by laser Doppler velocimetry and used to derive the Young's modulus and residual stress in the heteroepitaxial layer (330± 45 GPa and 200± 20 MPa, respectively). The temperature coefficient of Young's modulus is found to be ?53± 2 ppm K?1 in the range RT to ~ 300 °C, while an analytical expression is given for the temperature dependence of the Young's modulus between RT and 500 °C. The residual tensile stress is found to depend on temperature in a complex manner.

https://iopscience.iop.org/article/10.1088/0022-3727/40/11/012/pdf

Alun Harris

Papers

A system for the dynamic characterization of microstructures

SiC cantilever resonators with electrothermal actuation

Principles of a three-axis vibrating gyroscope

Fabrication of SiC microelectromechanical systems using one-step dry etching

Mechanical properties of a 3C-SiC film between room temperature and 600 °C