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MEMs I Distribution

100 (i.e. length/100 and width/100 ) , the rectangle area shrinks by a factor of (1/100)2 = 10,000. Therefore, the area scales as S2. Likewise, volume scales as S3 – so it follows that at increasingly smaller scales, surface (area) effects have greater impact than volume effects. Carefully considering the scale factor of

different forces can reveal which physical phenomena are most relevant at a given

Subsystem modelling Models are especially needed for MEMS designs due to the non-intuitive nature of sub-millimeter devices. Generally, an entire microelectromechanical system is too complex to model analytically as a whole, so it is usually necessary to break the model into subsystems.

One way of doing this is to categorise parts by function, such as sensors,

Again, as with electronics, the system can be modelled even more abstractly using block diagrams. At this level it becomes convenient to put aside the physicality of each element and instead describe the system in terms of transfer functions. This results in a MEMS model that is much more conducive to control theory techniques – an important set of tools for most high performance designs.

Design integration Whereas standard IC designs are often implemented as a series of discrete steps, MEMS design is much different; the design, layout, materials, and packaging of MEMS are inherently intertwined. Because of this, MEMS design can be more complex than IC design – often requiring the simultaneous development of every design "phase".

Figure 2: Simplified MEMs accelerator

scale. Force due to surface tension scales as S1, pressure and electrostatics related forces as S2, magnetic forces as S3, and gravity scales as S4. This helps explain how water striders (or "water bugs") can walk on water, and why a pair of ball bearings do not behave like a binary star system. Scale factors can also guide our understanding of how to design MEMS- sized devices, although developing a full mathematical model is eventually necessary in any design.

actuators, microelectronics, mechanical structures, etc. Lumped element modelling uses this approach, representing physical parts of the system as discrete elements with idealised characteristics. Electronic circuits are modelled the same way, using idealisations of resistors, capacitors, diodes, etc. of various complexities. We understand that, when possible, electrical engineers will use the simplified Kirchhoff's circuit laws rather than Maxwell’s equations to model a circuit.

MEMS packaging is the process that diverges perhaps most widely from the CMOS design. MEMS packaging is meant primarily to protect the device from environmental damage while also providing an interface and mitigating unwanted external stress. MEMS sensors often use stress as a means of measurement; excessive stress can impair functionality by deforming the device and inducing sensor drift. The packaging requirements for a given MEMS design are often unique and the package must be designed specifically for that device. In the industry it is well known that packaging can account for a huge portion of the total product cost – in some cases exceeding 50%. There is no single standard for MEMS packaging, and only recently have any sort

of packaging technologies emerged, among these being MEMS wafer-level packaging (WLP) and through-silicon via (TSV) techniques.

Fabrication Taking from microelectronics, the strength of MEMS fabrication is the batch process. Mass production adds economy of scale to MEMS devices just like it does any other product. As with IC fabrication, photolithography methods are often the most cost-efficient and certainly the most common technique. However, other processes, both additive and subtractive, are indeed used as well, including chemical/physical vapour deposition (CVD/PVD), epitaxy, and dry etching. Materials used in MEMS devices are often chosen more for their mechanical properties than electrical. Although much depends on the given application, desirable mechanical properties can include: high stiffness, high fracture strength and fracture toughness, chemical inertness, and high temperature stability. Micro-optical- electromechanical systems (MOEMS) may require a substrate that is transparent, while many sensors and actuators must use some amount of piezoelectric or piezoresistive materials.

MEMS technology will continue to grow and will shrink the size of how technology is implemented.

Mouser |

David Askew is a technical content specialist for Mouser Electronics, specialising in embedded systems and software

Components in Electronics

December 2013/January 2014 15

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