The relationship among electromigration, passivation thickness, and common-emitter current gain degradation within shallow junction NPN bipolar transistors

In very large-scale integration, the trend towards smaller horizontal dimensions and shallower doping profiles within NPN bipolar devices aggravates degradation mechanisms affecting common-emitter current gain (beta). It is experimentally shown for shallow junction NPN bipolar transistors operating in the conduction mode that the degradation of beta is electromigration related. Increasing the passivation thickness simultaneously improves metal conductor lifetime and aggravates beta degradation. Both increases are attributed to the electromigration-induced compressive stress. The complete recovery of beta after a 400 °C anneal indicates that the electromigration-induced compressive stress is relieved by creep in an aluminum-based metallization. The experimental results for beta degradation are consistent with an existing theory that takes into account the effects of stress on the effective mass of light and heavy holes.