Melt filaments in n+pn+ lateral bipolar ESD protection devices

Lateral bipolar n/sup +/pn/sup +/ devices, with thick field oxide (TFO) separating the collector and emitter, are often used in snapback mode as protection devices for MOS ESD circuit protection. ESD testing for Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM) waveforms was performed on a family of TFO cells which varied the gate length (bipolar base width) as the experimental parameter. For MM and CDM, the data showed a dependence of withstand voltage on the gate length, indicating that longer gate lengths improve performance; while for HBM, withstand voltage was independent of the gate length. The devices exhibited failure modes that manifested as low level current leakage. Failure analysis identified the current leakage sites as melt filaments primarily localized at the TFO ends. Filament distribution was seen to be a function of gate length for MM. Two possible mechanisms are presented to account for the observed filament distribution and follow up experiments are suggested to test the validity of each.