Aluminum double‐layer metallization is a very important technology for VLSI circuits. Aluminum corrosion and electrolytic leakage current of double‐layer metallization have been studied using mainly an unsaturated pressure cooker test (USPCT). Major results were that (i) a vertical electrolytic leakage current through the inter‐layer between top and lower metals is a main factor, (ii) the plasma‐deposited silicon nitride is an excellent dielectric layer for moisture prevention, and (iii) the combination of passivation and inter‐layer is a strong structure to mechanical stress.