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MEMS Accelerometer Autoclave Failure Mechanisms and Design Improvement
Keywords: MEMS, Accelerometer, Autoclave
Autoclave test is often a prescribed quality test for devices used in a harsh environment. Until recently, automotive safety industry has started to impose such requirement to validate MEMS accelerometers used for airbag sensors. The sensing structure of MEMS capacitive accelerometer is typically encapsulated in a hermetic environment to prevent moisture intrusion. However, the properties of packaging materials can be changed by the over-pressure and over-humidity condition, which causes failure of accelerometer offset shift (output deviation at non-excited state). This article discusses the autoclave failure mechanisms, the failure analysis techniques to identify the root cause and the design technique to prevent autoclave failure. In particular, there are three possible autoclave failure mechanisms: package stress induced offset change, parasitic capacitance change and ohmic leakage. First, the hygroscopic swelling of epoxy mold compound material could alter the stress state of the package and transducer to cause offset shift. FEA co-simulation of the package and MEMS was performed to study such effect. The results were further confirmed by the offset measurement results when removing the mold compound by laser etching. Also, change in dielectric properties of epoxy materials due to moisture intake could result in parasitic capacitance change or ohmic leakage between the electrical interconnects. Such changes (<1fF, >1Gohm) are difficult to be measured directly. Instead a modulator frequency sweep measurement was performed. It is shown that the offset of failed devices will vary with the modulator clock frequency while the good devices keep the same offset. This indicates the offset failure is related to the leakage as the amount of charge to be integrated could vary with integration time. A silicon nitride passivation layer is therefore proposed as the design fix to eliminate such leakage paths.
David Y. Lin, MEMS Design Engineer
Freescale Semiconductor
Tempe, AZ

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