FEATURE MEMS SENSORS The pressure is on


Gianluigi Morello from Melexis discusses the reasons whyMEMS implementation is showing itself to be the best way to solve next generation pressure monitoring problems


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n a constantly changing technology landscape, there are a growing number of areas where traditional pressure sensing systems are no longer capable of delivering the performance necessary to be fully effective. As a result there is a great deal of impetus to find suitable alternatives. MEMS technology has been used with increasing frequency across a broad spectrum of industry sectors over the last few years - pressure sensing being one of the more common places where it is now being employed. Industry analyst firm Research & Markets have predicted that the already multi-billion global MEMS pressure sensor business will continue to develop considerably between now and 2018, with a compound annual growth rate of 8.6%. Such sensor devices work on the following


principle of operation. The presence of pressure causes a deformation of an extremely sensitive micro-machined silicon diaphragm structure. The degree to which the structure is deformed will relate directly to the electrical signal that passes through an implanted resistor network. From this a precise measurement of the pressure acting upon the sensor can be determined. The deformation of the diaphragm is both temporary and reversible - this is due to the exceptional elastic properties of silicon. The bulkiness and relatively high unit price tags that characterise electromechanical


16 SPRING 2015 | MICROMATTERS


pressure sensor devices have considerable limitations in space constrained, cost- sensitive application scenarios. Equivalent MEMS devices on the other hand offer compactness and a rugged construction which leads to greater reliability, as well as being comparatively inexpensive. Another major benefit of MEMS is the fact


that it has compatibility with common CMOS process technologies. As a result the sensor element can be brought together with the necessary signal conditioning circuitry on a single semiconductor die to form a monolithic pressure sensing solution. Whereas traditional sensors, which rely on ceramic or metal substrates, can’t integrate these functions together - making them a lot more susceptible to electro-magnetic interference and seriously curbing their overall performance. Finally, MEMS pressure sensors support encapsulation in other media, in order to further heighten the sensing system’s overall robustness. This kind of design is of particular


importance when the sensor has to measure the pressure of a liquid. The application media is isolated from the MEMS sensing


Figure 1:


MEMs device from Melexis


element by a metallic membrane. The gap between the MEMS device and the metallic membrane is filled by a non-electrically conducting liquid that is used to transmit the pressure from the metallic membrane to the MEMS element. Reacting to the growing need for more advanced pressure sensing, Melexis has recently added two new automotive grade MEMS-based pressure sensor ICs to its portfolio. Fabricated using the company’s own proprietary process technology, these AEC-Q100 qualified devices combine high sensitivity with strong linearity and are targeted at automotive pressure monitoring applications (such as engine/transmission oil pressure and air conditioning system pressure monitoring) plus industrial process control systems, medical instrumentation, household appliances (such a refrigerators) and consumer electronics products. The MLX90815 is optimised for measuring pressures from 0 to 30 bar absolute. This is complemented by the MLX90816, which is capable of determining full scale pressures from 30 bar to 50 bar absolute. The sensor element incorporated into each of these devices consists of a piezo-resistive Wheatstone bridge connected to the tiny pressure membrane etched into the semiconductor die. As pressure is applied to the membrane, a differential voltage change occurs across the outputs of the Wheatstone bridge while a bias voltage is applied to the bridge inputs. These sensors can be deployed directly in conventional non-corrosive/non- aggressive media or incorporated into oil- filled sensor module designs if required. As new opportunities emerge for more


“Industry analyst firm


Research & Markets predict that the already


multi-billion global MEMS pressure sensor business will continue to develop considerably”


widespread use of pressure sensors, the inherent disadvantages of conventional sensor technology are becoming increasingly apparent. The large form factors, poor performance and substantial unit cost restrict their suitability. Conversely, through the miniaturised sensing structures and cost optimised wafer batch fabrication, implementation of MEMS technology seems set to rise substantially. Its capacity to cope with demanding applications environments such as automotive, industrial and healthcare without cost penalties being involved, is now starting to become more appreciated by the electronics engineering community. Facing exacting performance demands, the improved sensitivity and robustness


enabled by MEMS, on top of the higher degrees of integration this provides, all add weight to the argument.


Melexis www.melexis.com +49 361 427 7000


Enter 207 / MICROMATTERS


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