MEMS Reliability

 

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MEMS Reliability:

An important challenge in achieving successful commercial MEMS products is associated with MEMS reliability.  Reliability and qualification can be much more complex than for Integrated Circuits.  Many of the MEMS failure mechanisms are not well understood. This lack of understanding presents a challenge in developing practical qualification techniques for MEMS products. 

For the world of integrated circuits, there are industry standard tools and techniques for understanding and quantifying reliability.  For the world of MEMS, this knowledge base is much more limited.  In many cases, companies that do have a firm grip on techniques for quantifying reliability view that knowledge as a competitive advantage, and are hesitant to share it.

In order to develop reliable MEMS devices, reliability must be considered at the earliest stages of product development. Decisions made in the design stage can result in devices that will never be reliable.  Reliability must be understood at a fundamental physical and statistical level.

There is often a perspective that by there very nature MEMS will be unreliable because they have moving parts.  The truth is that it is not moving parts that kill reliability, but rubbing surfaces.  MEMS can be designed with moving surfaces, but no rubbing parts, and can be very reliable.

Avoiding rubbing surfaces is one of the key elements in achieving reliable MEMS devices.  A second primary issue affecting reliability is the issue of packaging.  Again, it is useful to consider the case of integrated circuits.  Integrated circuits are known for their reliability.  It should be noted, that integrated circuits are packaged in such a way as to protect the sensitive transistors on the surface of the chip from the environment.  The chips are typically packaged in a hermetic environment, or are potted to protect the devices. For the case of MEMS, some devices by their very nature require them to be exposed to the environment, creating a reliability challenge.

The types of MEMS devices that are the most reliable, and the easiest to qualify are devices that can be packaged in such a way as to protect them from the environment.  An excellent example of this is the case of optical MEMS devices.  These devices can be packaged in a traditional ceramic package, with a glass lid.  The glass lid, hermetically attached to the ceramic package creates a "safe" environment for the sensitive MEMS chip, while still allowing photons to interact with the MEMS device.

The data presented below demonstrates the high level of reliability that can be achieved when MEMS are designed with no rubbing surfaces, and are packaged properly.  The

This chart represents 300 MEMS mirrors under test at 70 degrees C.  It can be seen that there have been no failures with the total accumulated device cycles in excess of a half trillion.

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paul.mcwhorter@memx.com

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