Degradation of Evaporated Aluminium Contacts on Silicon Planar Transistors

At current densities of about 3 × 106 A/cm2 evaporated aluminium contacts on silicon become degraded in 1 to 3 hours at about 250°C. At low magnifications the degraded contacts appear to have blackened, but further examination shows their surfaces to have become roughened, with the appearance of discrete regions of a silicon-rich phase in the aluminium. This degradation is accompanied by irreversible increases in the room temperature sheet resistance of the contacts.