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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?

Demand for micro electro-mechanical systems (MEMS) technology is ever growing. To service that demand with reliable products, both developers and users of MEMS devices need to know about likely failure mechanisms and how to avoid them. Stiction, electrostatic discharge (ESD), micro-contamination, and mechanical shock are key reliability failure mechanisms to understand.

An important driver for the demand of MEMS is, and will continue to be, the Internet of Things (IoT). MEMS and sensors are being used increasingly in healthcare, consumer electronics, low power applications, security, asset tracking, automotive technologies, and smart homes, to name a few, with MEMS marketing and industry analysts predicting tens of billions of shipments within the next decade. With so many interacting MEMS and sensors in the field, up-time is important and reliability is critical.

The first step in ensuring MEMS reliability is to avoid common pitfalls during the design and process development phase to assure a stronger and more reliable part-upon-marketplace introduction. And the time to market for MEMS is fast. Upon product launch, the part must meet all its datasheet specifications as well as storage, shipping, and operational environment reliability tolerances. One of the first things to be prepared to deal with is stiction.

Stiction
The term stiction comes from "static friction" and it has been a factor for years in a wide variety of technologies, including suspension linkages for cars, polished glass, hard disc drives, and precision gage blocks. It occurs when two objects are initially brought into contact (figure 1). In a MEMS device, objects that could come in contact include elements such as actuators, proof masses, and sensing fingers. Such contact may occur as part of the device's normal operation, or may unintentionally occur as the result of an external force such as a mechanical shock. Either way, however, once contact has occurred the device needs a reliable way to ensure that it can separate the surfaces again in order to keep functioning properly.

Figure 1: Surfaces are brought into contact (lateral stiction)

The primary forces that come into play upon bringing two surfaces very close together are electrostatic attraction and surface work of adhesion. The force of electrostatic attraction is proportional to 1/d2, and surface work of adhesion is proportional to 1/d3. Surface work of adhesion in MEMS is primarily due to van der Waals and hydrogen bond forces.

The two surfaces must be very close to be drawn into contact for stiction. The electrostatic force attraction distance is a function of the potential difference between the surfaces, and is typically in the micron range. Once the two surfaces are in single digit Ångstrom range, van der Waals and hydrogen bonding forces come into play. Although the latter forces are classically defined as weak interactions, they are additive and become significant in stiction.

Figure 2: Surfaces are in contact (lateral stiction).

Release from stiction is only possible if the release forces, also called restoring forces, exceed the forces that allow the surfaces to stay in contact: Frelease > Fcontact. Release forces in a MEMS device include the mechanical properties of the MEMS design (spring constant) and, when packaged in a gas or fluid, squeeze film damping.

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