Abstract

Analog Devices General Trias (ADGT) received a challenging customer return unit (CRU) for failure analysis (FA). The CRU is a frequency synthesizer failing to lock at the customer’s application frequency at cold temperature condition. The failure has been simulated during bench testing at the ADGT FA laboratory at ambient temperature with VDD supply voltage lowered to 2.38V from nominal value of 3.0V. Series of fault isolation analyses using Light Emission Microscopy (LEM) and Optical Beam Induced Resistance Change (OBIRCH) localized the failure to a bipolar transistor (NPN) in the feedback divider block of the frequency synthesizer. Circuit level spice simulation replicated the functionality failure when a 2.6K-ohms resistance is connected to the emitter of the transistor. Physical failure and elemental analyses found traces of native oxide in the Nitrogen (N2) rich Titanium Nitride (TiN) layer between the Tungsten (W) plug and Titanium (Ti) liner in the emitter plug of the failing transistor. The oxide interface acts as a barrier to the flow of charge carriers in the emitter leading to the temperature dependency failure; failing at cold temperature and passing at ambient temperature. Corrective actions have been put in-place as a result of the successful identification of the defect.

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