Optimization of the passivation scheme for a 0.35 μm TLM process is presented. The passivation layer is required to provide mechanical and chemical protection during the assembly and packaging process and long term environmental protection. The passivation scheme was optimized by testing a product-like test vehicle at accelerated environmental stress conditions of high humidity and high temperature. The initial passivation scheme showed a unique electrical test signature - a voltage dependent failure with open circuit at low voltage/pass at high voltage. Advanced wafer-level failure analysis techniques and equipment were used to isolate and identify the failure site. Further cross-section analysis revealed that the failure site was a fracture across a via and adjoining IMO. This cracking was attributed to stress corrosion fracture of the IMO based on root cause analysis. This hypothesis required a path for moisture to the stressed IMO-2 location to cause the fracture. The moisture path was identified by further analysis and process changes to eliminate this were implemented. All subsequent parts passed the accelerated environmental tests confirming the stress corrosion fracture hypothesis.

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