The Effect of Ionic Contaminants on Silicon Transistor Stability

The development of the beam-lead sealed-junction (BLSJ) technology (beam-lead contact metals-silicon nitride passivation) included many experiments to study the effects of various ionic contaminants on silicon transistor stability. Stress aging was performed on standard n-p-n silicon transistors (aluminum contacts and silicon dioxide protection) under various conditions of temperature, bias, contamination, and ambient. These experiments showed the following results. 1) Alkali ions and copper in a reducing ambient are detrimental to the devices. 2) A hydrogen ambient accelerates the effect of alkali ions on the transistor degradation. 3) The degradation is approximately a linear function of the reverse bias and the contamination level from 4 to 400 Å of sodium chloride. 4) Anions have only a secondary effect on the migration of alkali ions in the oxide. The BLSJ technology was developed to protect unencapsulated silicon devices from the degradation seen on standard transistors during the preceding experiments. Results have shown that the median time to failure of sodium-contaminated BLSJ transistors aged in air at 300°C is higher than that for standard silicon transistors aged under identical conditions.