The Effect of Insulation Coatings on Forward Beta Degradation in Bipolar Transistors

Uncoated, SiO2-coated, and polyimide-coated NPN discrete bipolar transistors were subjected to forward life stressing at temperatures between 100°C and 200°C, and at stress current densities in metal lines between 0.2 and 0.8 × 106 A/cm2. The changes of current gain (beta) in these transistors were monitored as the function of stress time up to 1000 hours. Beta degradation in uncoated transistors started immediately. Later, however, when the mechanical stress generated by electromigration reached the yield point of CrAlCu metallurgy in the emitter area, the beta showed a strong recovery to values about 10% higher than the beta at time zero. This explanation was corroborated by the observation of extrusions in the scanning electron microscope. The overlayer of 2.4 ¿m of sputtered SiO2 strongly retarded the beginning of beta degradation but did not prevent it toward the end of life stressing. The overlayer of 2.7 ¿m of spun-on polyimide initially had a negligible effect on the beginning of beta degradation. Later, however, when the yield point of metallurgy was achieved, the polyimide seemed to partially block the extrusion of hillocks and prevented the recovery of beta. This was confirmed by SEM photography. The experiment verified the basic premise that the forward beta degradation in bipolar transistors can be described in terms of the mechanical stress generated by electromigration in the emitter region.