A case history is presented for the failure analysis of a 0.5u CMOS A/D converter in which high fallout occurred after autoclave stressing. The observed failure mode of degraded signal-to-noise distortion (SINAD) ratio (measured in dB) was found to affect devices within a specific bandwidth centered around 5MHz. From a circuit designer’s viewpoint, an explanation for this unique failure mode did not readily present itself. Yet, using straightforward failure analysis techniques, involving laser ablation of photoresist and selective etching of passivation, the specific failing circuit block was isolated. Crosssectional analysis, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) found evidence of residual photoresist at topography related voids in the nitride passivation layer. The photoresist reacts with moisture in the autoclave, resulting in increased capacitance at minimum-spaced top layer metal lines. This failure mechanism correlates with the observed maximum SINAD degradation around 5MHz: at this frequency the signals along the affected metal lines are at their maximum voltage swing. This failure mechanism is potentially an issue for any similar high-speed, high-resolution designs.

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