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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?

Failure modes due to mechanical shock include shattered MEMS, cracks in structures, packaging fractures, wafer breakage, die adhesion loss, and particle movement into critical locations. Proper design of the MEMS device is key to withstanding shock levels that may be experienced in the subsequent manufacturing environments, shipping, and user environment. Damping MEMS with stoppers, allowance of over-travel (for high shock resistance), and simulations of shock pulses are all important in the design phase. In manufacturing, choosing materials that can withstand the required shock levels and not be prone to crack initiation defect sites are areas to focus on. The final product containing the MEMS must also be designed knowing MEMS often need additional damping and protection from shock or drop events. With understanding, compactly packaged consumer electronics can pass high shock and drop testing.

Figure 6: Simulation results of a dropped cell phone.

The key to characterisation of mechanical shock exposure and identifying where failure occurs, once parts are available, is performing experiments to evaluate sensitivity to shock levels of various pulses and amplitudes. Experimentation can be performed with various testing set ups. Very high g testing can be achieved with a modified split Hopkinson pressure bar. A drop tower is used for moderate levels, and for lower g levels, pneumatic testers. Appropriate fixtures for MEMS and/or the consumer electronic part (with MEMS inside) with a reference accelerometer are important to proper monitoring and delivery of the shock profile.

Tips for systems developers
So what do these failure modes mean to the system developers who integrate MEMS in their products? Some, like stiction and contamination, are things the device manufacturer should have already addressed in their design and can discuss with system developers. Others, like ESD, need system designer attention. For instance, datasheets will typically have ESD ratings identified. Treating the MEMS device like an IC with the ratings is critical so to not to have yield issues in manufacturing and/or introduce weakened parts into the field. ESD mats, ground straps, ESD packaging in shipping, and system design are very similar to the IC industrythese methodologies are well known and should be replicated for an ESD sensitive MEMS device.

MEMS datasheets will also include temperature and relative humidity limits. System developers should think carefully before locating a MEMS device with a lower temperature limit near a heat-generating component in their design. Excess heat can reduce device lifetime via some thermally-driven failure mechanisms. Again, this situation can be simulated to determine how much heat would reach the MEMS device from an adjacent power dissipating device. As for humidity, devices with open packaging, such as MEMS microphones, could need to have their operation restricted to the relative humidity range of the datasheet. (Many MEMS microphones have solved the moisture problem but some other devices could still have sensitivity to humidity.)

The best bet for developers, then, is to always work with the MEMS manufacturer when designing a MEMS device into their system. The cost to redesign a system can be astronomical, and the timeframe for redesign could cause the project to miss the market window.

Summary
In closing, development and manufacturing of MEMS can successfully be performed with reliability in mind throughout the entire product development phase and into production. Upon product launch, reliability test data will both have proven the part reliable and identified the weak links in the design and manufacturing with time for completed cycles of learning. It is important to:
???Use learnings from previous MEMS products to accelerate the time to market by incorporating design and manufacturing fixes for applicable well-known failure mechanisms early in the program.
???Simulate these fixes with various design iterations to narrow down choices in a design for reliability.
???Focus on a fast and effective yield ramp, and elimination of the few surprise failure mechanisms that that were not even predicted can reduce the chaos and put the product into the market on time.
???Have system developers work with the MEMS manufacturers to determine important operational parameters and have the manufacturer suggest solutions to assure the MEMS device works reliably in the final system.

About the author
Allyson Hartzell is a Managing Engineer at Veryst Engineering with more than three decades of professional experience in emerging technologies and a broad background in semiconductor and MEMS fabrication, yield enhancement, emerging technology manufacturing and reliability, packaging materials and processing, and cleanroom scienceincluding particulate and molecular contamination. She is an internationally recognised expert in MEMS reliability, and has expertise in surface chemistry and analytical techniques for failure analysis. Prior to joining Veryst Engineering, Ms. Hartzell was Director of Engineering for Reliability, Failure Analysis, and Yield at Pixtronix, a wholly owned subsidiary of Qualcomm. She was a Senior Staff Scientist in Reliability and Yield at Analog Devices Micromachined Products Division, and has worked at IBM and Digital Equipment Corporation. Allyson has an M.S. in Applied Physics from Harvard University and a B.S. in Materials Engineering from Brown University.


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