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Distribution I MEMs


A staggeringly small world


As advances in microelectromechanical systems (MEMs) accelerate, David Askew considers why we need them and looks at their design and manufacture


M


icroelectromechanical systems (MEMS), also known as microsystems technology in


Europe, or micromachines in Japan, are a class of devices characterised both by their small size and the manner in which they are made. MEMS devices are considered to range in characteristic length from one millimeter down to one micron – many times smaller than the diameter of a human hair.


MEMS will often employ microscopic analogues of common mechanical parts and tools; they can have channels, holes, cantilevers, membranes, cavities, and other structures. However, MEMS parts are not machined. Instead, they are created using micro-fabrication technology similar to batch processing for integrated circuits. Many products exist today that use MEMS technology, such as micro heat exchangers, ink jet printer heads, micro- mirror arrays for high-definition projectors, pressure sensors, infrared detectors, and many more.


Why do we want MEMS? MEMS devices perform many of the same tasks as macroscopic devices while also offering many advantages. The first and most obvious of these is miniaturisation. As previously mentioned, MEMS- scale devices are small enough to be manufactured in a batch fabrication process, similar to ICs, which can significantly reduce the costs of mass production. In addition to the prospect of being cheaper, MEMS devices can also be more applicable than their much larger equivalents. Designing metal ball and spring accelerometers into smartphones, cameras, airbag control units, or similarly small sized devices would be impractical at best; by reducing device size by several orders of magnitude, MEMS can be used in applications where a conventional sensor would be far too large. Ease of integration is yet another advantage of MEMS technology. Because they are


14 December 2013/January 2014


fabricated with similar processes used in fabricating ASICs, MEMS structures can be more readily integrated with the microelectronics. And while integration of MEMS and CMOS structures into a truly monolithic device is challenging, progress is being made.


Many manufacturers have employed a hybrid approach to create commercially successful and cost-effective MEMS products. Texas Instruments’ Digita Micro Mirror Device (DMD) is one such example. DMD is at the core of TI’s DLP technology, which is widely used in projector units ranging from business and classroom models to digital cinema. Each 16µm square mirror is actuated electrostatically by a voltage potential between it and the CMOS memory cell below it. Grayscale images are produced by pulse-width modulating the mirrors between on and off states. Colour is added either by using a three-chip solution (one chip for each primary colour), or with a single chip and a colour wheel or RGB LED light source. Designs using the latter technique synchronise the DLP chip with the rotation of the colour wheel,


displaying each colour in quick succession so that the viewer sees a single full- spectrum image.


Perhaps one of the most interesting


features of MEMS is the designer’s ability to exploit the peculiar physics that emerge from such a small-scale physical domain.


MEMS today Many MEMS products are commercially quite successful, with devices already in widespread use. The automotive industry is among the main drivers behind MEMS technology. MEMS vibrating structure gyroscopes, for example, are new and fairly inexpensive devices being used for automotive anti-skid or electronic stability control systems.


The Murata Electonics Oy SCx-series of MEMS accelerometers, gyroscopes, and inclinometers and various combinations of these functions reside on a single chip to enable specific automotive applications, where precision is required in confined spaces.


MEMS-based airbag sensors have universally replaced mechanical type crash sensors in nearly all cars made since the 1990s. Figure 2 shows a simplified example of a MEMS accelerometer similar to what might be used as a crash sensor. A cantilever beam with proof mass is attached to one or more fixtures that act as springs. When the sensor is accelerated along the beam’s axis, the beam is shifted some distance, which is measured as a change in capacitance between the beam "teeth" and the fixed outer conductors. Many commercial and industrial inkjet printers use MEMS based technology in printer heads to withhold ink drops and precisely deposit them only when needed – a technique called Drop-on-Demand (DoD). An ink drop is placed by applying a voltage potential across an element composed of piezoelectric material, such as lead zirconate titanate, causing it to deform. This increases the pressure within the ink chamber of the printer head, forcing a very small amount of the (relatively incompressible) ink out of the nozzle.


Figure 1: TI’S DMD pixel, exploded view Components in Electronics


Other MEMS technologies are only just beginning to enter the market on a large scale. Micromachined relays (MMRs), such as those developed by Omron, offer a class of relays that are faster, more efficient, and capable of an unprecedented degree of on-chip integration. Omron has also brought their MEMS expertise to bear on temperature sensors with their new D6T


non-contact MEMS thermal sensors. The D6T integrates ASIC and thermopile elements in a MEMS fabrication process, resulting in a miniaturised non-contact thermal sensor measuring only 18 x 14 x 8.8mm (4x4 element type). MEMS technology is, however, not limited to single-sensor devices. Consider the human senses: a single eye gives us colour, motion, and (some) positional information, while two eyes enables binocular vision for improved depth perception. In fact, many of our perceptual experiences require a combination of senses in order to be meaningful at all. The idea is that by combining sensory data we can compensate for the weaknesses and drawbacks of each individual sense organ, and arrive at an understanding of the environment that is in some way superior. In the human context, this is called "multimodal integration"; in electronics it’s called sensor fusion. Sensor fusion, especially as it relates to MEMS, is an important development in sensor technology for mobile devices. Many manufacturers are already offering complete solutions, such as Freescale with their 12-Axis Xtrinsic Sensor Platform for Windows 8. This platform integrates a 3- axis accelerometer, 3-axis magnetometer, pressure sensor, 3-axis gyroscope, and ambient light sensor with a ColdFire+ MCU for a total hardware solution – then couples it with proprietary sensor fusion software.


The MEMS market continues to pick up pace as the advantages of MEMS devices gain recognition. According to Yole Développement’s 2012 MEMS industry report, MEMS "will continue to see steady, sustainable double digit growth for the next six years", becoming a $21 billion global market by 2017.


MEMS design and manufacturing Introductions to the design and manufacture of MEMS often begin with a review of scaling and miniaturisation. If we ask, for example, why one cannot simply take an air compressor or a ceiling fan and shrink it down to the size of a flea, the answer is scaling laws. A flea-sized ceiling fan will not behave in the same way as a normal-sized fan 1000 times larger, because the involved forces change in strength relative to each other. The scale factor, S, can aid in understanding how. Consider the area of a rectangle, equal to the product of its length and its width; if the rectangle is scaled down by a factor of


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