Factors contributing to the corrosion of the aluminum metal on semiconductor devices packaged in plastics

Abstract

Using a test structure with an evaporated aluminum metallization pattern on the surface of a phosphorus “doped” oxide layer of a silicon semiconductor chip, the rate of corrosion of the aluminum metallization in a d.c. electric field at 85°C and 85% relative humidity was studied. The test structure was attached to a metal frame and molded in a variety of epoxy and silicone plastics. The relative contribution of the nature of the plastic encapsulant used, the presence of phosphorus in the silicon oxide layer and the strength of the electric field on the corrosion of the aluminum metallization was investigated. Under the above conditions it was established that in the presence of phosphoric acid extracted from the phosphorus-doped oxide layer corrosion occurs exclusively at the cathode. The rate is dependent upon the applied d.c. voltage. In the absence of phosphorus in the oxide layer and other ionic contaminants in the moisture on the surface of the device (pH 6–8) the rate of aluminum corrosion is low and occurs principally at the anode. The type of plastic encapsulant used influences the rate of moisture penetration to the device surface but once there the rate of corrosion under high temperature and bias is similar for all plastics if the acidity of the surface layer is high. It is shown that moisture penetration is diffusion-controlled and does not correlate with adhesion of the plastic to the external leads. On the basis of observations of the conditions favoring cathodic or anodic corrosion of the aluminum on semiconductor devices together with aluminum electrodes in various electrolytes, the electrochemical and chemical reactions occurring at the electrodes between the aluminum and the electrolytes are postulated.