MEMS sensors
Making sense of MEMS
MEMS technologies are the rising star in the sensors market.However, there are a number of misconceptions surrounding their capabilities,and conventional sensors continue to meet a much wider range of applications. Jesse Bonfeld of Sherborne Sensors examines the evolution of MEMS fabrication, microsystems,and MEMS devices, and their impact on the sensors market.
M
icro Electro Mechanical Systems (MEMS) describes both a type of device or sensor, and a manufacturing process. MEMS sensors incorporate tiny devices with miniaturised mechanical structures typically ranging from 1-100 µm (about the thickness of a human hair), whilst MEMS manufacturing processes provide an alternative to conventional macro-scale machining and assembly techniques.
Also known as ‘microsystems’ in Europe, and ‘micromachines’ in Japan, MEMS devices have come to the fore in recent years with the wide-scale adoption of MEMS sensors by the automotive industry, and the growing use of accelerometers and gyroscopes in consumer electronics. Perhaps the most well known consumer electronics incorporating MEMS motion sensors include a number of the leading smart phones, and gaming consoles/controllers.
Rise of the micromachines
MEMS development stems from the microelectronics industry, and combines and extends the conventional techniques developed for integrated circuit (IC) processing with MEMS-specific processes, to produce small mechanical structures measuring in the micrometer scale (one millionth of a meter).
As with IC fabrication, the majority of MEMS sensors are manufactured using a Silicon (Si) wafer, whereby thin layers of materials are deposited onto a Si base, and then selectively etched away to leave microscopic 3D structures such as beams, diaphragms, gears, levers, or springs. This process, known as ‘bulk micromachining’, was commercialised during the late 1970s and early 1980s, but a number of other etching and micromachining concepts and techniques have since been developed.
Advances in IC technology and MEMS fabrication processes have enabled commercial MEMS devices that integrate microsensors, microactuators and microelectronic ICs, to deliver perception and control of the physical environment. These devices, also known as ‘microsystems’ or ‘smart sensors’, are able to gather information
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from the environment by measuring mechanical, thermal, biological, chemical, optical, or magnetic phenomena. The IC then processes this information and directs the actuator(s) to respond by moving, positioning, regulating, pumping, or filtering. Any device or system can be deemed a MEMS device if it incorporates some form of MEMS-manufactured component.
Demand for MEMS devices was initially driven by the government and military/defence sectors. More recently, a maturing of the semiconductor manufacturing processes associated with the microchips used within personal computers, and the intersection with the huge requirement in the automotive and consumer electronics sectors, has propelled MEMS sensors into the mainstream. The key MEMS sensors today are accelerometers, gyroscopes, and pressure sensors.
Innovation & limitation
All too often, MEMS technologies are perceived as being all- encompassing solutions made using standardized processes, when in actual fact, they remain a largely one product, one process business. A number of companies develop and produce MEMS devices themselves, and are defined as ‘IDMs’ (integrated device manufacturers), whereas some outsource production (fabless), and others operate both models. Much of the confusion in the market can be attributed to this diversity, and the way in which the various verticals subsequently interface make the MEMS market notoriously difficult to define.
At the point of fabrication, there are very few companies operating in the sensors market that offer MEMS together with another technology because of the high cost of market entry and the cost of packaging MEMS devices. Likewise, once a company has committed to manufacturing MEMS devices, it is difficult for that company to change focus, due to low margins, higher development costs, and greater complexity. That said, MEMS does enable high-volume production, due to the batch fabrication techniques employed, typically resulting in very low costs for each single device.
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