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Sensors/MEMS??

Preventing common MEMS failure mechanisms

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

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

To ensure that a device can overcome stiction as needed, the design needs to properly address all these forces. Increasing a structure's stiffness (spring contact) will increase Frelease, for instance, but there are serious trade-offs. Stiffness affects the pull-in voltage needed to move an element.

Another approach to ensuring that stiction can be overcome would be the reduction of surface work of adhesion. Such reduction traditionally been performed with both design and manufacturing methods to reduce Fcontact. A popular design method for reducing Fcontact involves reducing surface area of contact through the inclusion of stoppers or bumpers. Manufacturing methods include reducing surface area through surface roughening techniques, also called 'nano-texturing'. In figures 3a and 3b, diagrams represent surface texture through, for example, oxidation of polysilicon. Lower surface area of contact is the goal.

Figure 3a: Representation of smooth surface.

Figure 3b: Rougher surface due to oxidation.

Another popular and long-used manufacturing method for reducing contact forces involves the use of anti-stiction coatings that reduce the surface work of adhesion. Early coatings such as OTS (octadecyltrichlorosilane) reduced work of adhesion to a polysilicon surface by 3 to 4 orders of magnitude. Due to growth in MEMS designs and applications, new surface coatings are always in development.

Electrostatic discharge
Along with the mechanical problems of stiction, MEMS are susceptible to electrical problems, such as ESD (electrostatic discharge). The generation of static charge and its transfer between two objects (discharge) will frequently occur in normal use and ESD generation is so well-known to cause havoc in semiconductor devices that entire careers have been spent in designing ESD protect circuits.

ESD can also cause failure for some MEMS. If your MEMS device is electrostatically actuated, for instance, then ESD is a likely failure mechanism for your part. ESD could cause the actuator to move beyond its intended range, possibly resulting in contact and stiction. You should test your part to the proper standards both to quantify the effect and to determine if the device will fail in the field.

Electromechanical failure due to ESD can be reduced by eliminating the electric potential differences between the MEMS element and any potential landing location. Again, this is a design decision and landing features can be designed with ESD effects in mind. Yet in severe cases where design cannot prevent the failure, protect circuitry is recommended for MEMS ESD prevention.

For high resonant-frequency devices, the structure is so stiff that ESD related motion is less likely. The timescale of the ESD pulses are on the order of nanoseconds, too short to result in significant motion. In these cases, the movement in the MEMS is primarily that of resonance and damping.

An ESD event can yield a combination of electrical and mechanical failures in MEMS devices. Joule heating effects, for instance, can cause melting of MEMS structures such as a MEMS comb finger. It can end up 'welded' to the ground plane as the result of ESD. (The welding mechanism in this case can appear like a stiction event.) Of course ESD damage can also be purely electrical, such as an insulator breakdown or metal lead fusing and melting.

Methods for elimination of ESD in MEMS are similar to those for semiconductor devices. ESD protective handling procedures and packing materials, for instance, can be critical to protecting an ESD-sensitive MEMS device. Design changes a developer might consider to reduce sensitivity to ESD in MEMS include wider spacing between leads and wider leads to carry higher current densities.

Testing for ESD failure is a case where industry standards for semiconductors can be applied to MEMS devices. HBM (human body model), MM (machine model), and CDM (charged device model) are the typical standard tests (table). The human body model simulates when a person touches a device. The machine model has a faster pulse and a more severe discharge from a charged machine. The charged device model is common in semiconductors. Recently, many standard organisations have obsoleted the Machine Model standards. JEP172A explains the discontinuation yet it is recommended to test MEMS to this model and use scientific methods to determine applicability of results to the

Table: Three ESD Models and associated specifications.

Micro-contamination
In the manufacture of MEMS devices, micro-contamination is another potential failure source. Micro-contamination can be split into two categories: molecular and particulate. Molecular contamination is gaseous in phase. This contamination is in the form of a molecular cluster that has not reached the critical cluster size to grow into the condensed form of a particle. Such molecular contamination can deleteriously affect the MEMS surface.

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