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Preventing common MEMS failure mechanisms

Posted: 01 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:micro electro-mechanical systems? MEMS? Stiction? electrostatic discharge? ESD?

For example, MEMS designers can eliminate stiction mechanisms by reducing the surface work of adhesion with anti-stiction coatings by orders of magnitude (recall the force balance equation). But if outgassing chemical species from, for example, organic die attach materials are able to adsorb to an anti-stiction surface and inhibit its effectiveness, the force balance equation can be reversed. For instance, PDMS (polydimethylsiloxane) is commonly found in adhesives. Upon outgassing and re-deposition to the MEMS surface, the surface work of adhesion increases to the range of polysilicon.

The semiconductor world has studied and treated AMC (airborne molecular contamination) with activated carbon filtration, and filtration methods from semiconductor manufacturing facilities can be used in MEMS fabrication. Elimination of the source is preferred, however. If stiction is of concern in the MEMS design, knowledge of what airborne molecular contamination can inhibit and deleteriously change your MEMS surface is key to the elimination and/or proper filtration of the AMC source.

Figure 4: AMC molecules adsorbing to anti-stiction coating.

Particles are another form of micro-contamination that can cause failures. In moving MEMS, particles can mechanically obstruct the structure from proper movement, though some MEMS, such as thermal accelerometers, will not experience mechanical obstruction due to particles. Yet in other cases, particles that are conductive in nature can create shorts if they are in a critical location.

The potential for failure due to particle micro-contamination may not always be apparent during manufacturing test. Particles greater than one micron move primarily with gravity and electric field. Thus, a particle in a MEMS package can create no problem initially, but can move into a critical area under the right conditions and create failure in the field.

Particulate contamination is the reason MEMS manufacturing and packaging are performed in cleanroom environments. But cleanrooms are never completely free of particles, so particle shedding materials and particle generating operations must be identified and eliminated if particle contamination is present. In order to elementally identify the particle source and eliminate it, we recommend that developers employ x-ray mapping of the particle using energy dispersive x-ray analysis (EDS) in a scanning electron microscope to separate the particle from background elements.

Figure 5: Representation of particle wedged between MEMS beams.

Manufacturing operations that typically create particles but are not a problem in integrated circuits, such as wafer dicing, require specialised treatment in MEMS fabrication. A common technique is to cap MEMS structures, for instance, so that packaging cleanrooms, typically with more particles per unit air volume than wafer cleanrooms, do not have to be retrofitted into costly uber-clean spaces to eliminate particulate contamination.

The surface forces described in the stiction section apply to small particle adhesion as well, making the removal of small particles difficult. Yet the common semiconductor practice of wet cleans are not realistic in MEMS manufacturing, because capillary stiction can occur upon drying. Critical CO2 cleaning has some limited success in MEMS fabrication but is not implemented widely. Design of the MEMS structure to be less sensitive to particles is the best way to avoid the problems of small particles, using wider spacings, larger distances from the ground plane, and capping structures if possible.

Failure due to mechanical shock
Dropping a product that has MEMS inside, such as consumer cell phones and tablets, can result in thousands of g's of acceleration in the form of a shock profile. The simulation of a cell phone drop in figure 6, for instance, predicts a maximum g level of 5000 g's for a cell phone that has a polymer protective case. Without the case the shock level would be higher. Wearables can also experience high shock levels. Who hasn't broken their watch glass when it is on one's arm? High shocks of wearables with installed MEMS should therefore be expected.

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