microelectromechanical systems

microelectromechanical systems
Name	microelectromechanical systems
Caption	A Texas Instruments Digital Light Processing chip, a prominent commercial application.

microelectromechanical systems integrate mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. While the electronics are fabricated using integrated circuit process sequences, the micromechanical components are created using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers. This field, which has roots in the 1960s and 1970s with developments at institutions like Bell Labs and the University of California, Berkeley, enables the creation of highly miniaturized systems with applications ranging from automotive safety to consumer electronics and biomedical devices.

Overview

The fundamental concept involves creating miniature mechanical and electromechanical devices with dimensions ranging from a few micrometers to millimeters. Key early milestones include the development of the resonant-gate transistor at Westinghouse Electric Corporation and pioneering surface micromachining work at the University of California, Berkeley under Roger T. Howe. The field gained significant commercial traction in the 1990s with the high-volume production of Analog Devices accelerometers for airbag deployment systems in automobiles. These systems often interface with the physical world through transducers, converting mechanical motion into electrical signals or vice versa, and are a foundational technology for the Internet of Things.

Fabrication

Fabrication primarily utilizes techniques derived from the semiconductor device fabrication industry. Bulk micromachining, a core process, involves the selective removal of the silicon substrate using wet etchants like potassium hydroxide or dry processes such as deep reactive-ion etching. Surface micromachining, pioneered at University of California, Berkeley and later advanced by the DARPA-funded MEMS Exchange, builds structures by depositing and patterning thin films, typically silicon nitride or polycrystalline silicon, on the surface. More specialized techniques include the SCREAM process developed at Cornell University and LIGA, a high-aspect-ratio process using X-ray lithography and electroplating originally developed at the Karlsruhe Institute of Technology.

Materials

While single-crystal silicon is the most common structural material due to its excellent mechanical properties and compatibility with CMOS processing, many other materials are employed. Polycrystalline silicon is widely used in surface micromachining, and silicon germanium is explored for integrated processes. For optical applications, materials like silicon dioxide and silicon nitride are key. Metals such as aluminum, gold, and tungsten are used for electrical interconnects. Polymers like SU-8 photoresist and PDMS are common in bio-MEMS, while advanced materials like diamond-like carbon and piezoelectric materials such as lead zirconate titanate are used for specialized sensors and actuators.

Applications

Applications are vast and cross-disciplinary. In automotive systems, they are ubiquitous in yaw rate sensors for electronic stability control and manifold absolute pressure sensors. Consumer electronics heavily utilize them in the form of microphones in Apple Inc. iPhones, gyroscopes in the Nintendo Wii Remote, and digital micromirror devices in Texas Instruments projectors. In telecommunications, optical switches and tunable lasers rely on this technology. Biomedical uses include lab-on-a-chip devices for diagnostics and Boston Scientific's implantable pressure sensors. Industrial and aerospace applications include Micro-Pirani gauges and systems for James Webb Space Telescope.

Design and modeling

Design requires specialized software tools that combine electrical, mechanical, and fluidic domains. Leading commercial simulation environments include COMSOL Multiphysics and ANSYS. The design process must account for complex multi-physics phenomena such as electrostatic actuation, squeeze-film damping, and piezoresistive effects. Foundries like MEMSCAP and the Sandia National Laboratories SUMMiT process provide standardized design rules and process design kits. Key design considerations include achieving desired resonance frequencies, managing stiction during release, and ensuring compatibility with CMOS integration, often pursued through approaches developed at institutions like the Fraunhofer Society.

Reliability and packaging

Packaging is often the most costly step and is critical for reliability, as devices must be protected while allowing interaction with the environment. Hermetic packaging in ceramic or metal enclosures is common for sensitive components like resonators. Challenges include managing thermal stress from mismatched coefficient of thermal expansion between materials and preventing contamination. Failure modes can include fatigue in cyclic structures, charging in dielectric layers, and wear in contacting surfaces. Standards for testing and qualification have been developed by organizations like the Joint Electron Device Engineering Council and the International Electrotechnical Commission to ensure robustness in applications from Medtronic implants to Bosch automotive sensors.

Category:Microtechnology Category:Semiconductor devices Category:Electromechanical engineering