Principles of Corrosion

The principles of corrosion have been reviewed from two aspects: Thermodynamics and kinetics. Thermodynamic considerations yield the relative corrosion tendencies ofthe elements. Three types of corrosion cells have also been mentioned: Galvanic, concentration and simple anodic. Kinetics attempts to explain and predict the actual corrosion behavior of materials. The relative corrosion tendencies, the metallurgical aspects and the environmental conditions must all 'be considered. The environmental conditions of interest are normally temperature, relative humidity, applied electrical bias and ionic contamination. Examples of practical corrosion problems have been discussed. Corrosion is a difficult subject because each case is highly specific. Corrosion of IC devices is even more difficult because of the small sizes of components and because the local corrosion conditions within the IC packages are unknown Several examples of metallization corrosion have been briefly described, such as Mo-Au (galvanic), Al and NiCr (simple anodic), and Al in the presence of phosphorus and chlorine(ionic contamination). The function of failure analysis laboratories is to identify the corrosion mechanism and the Mfaterial causing corrosion. Then that source of corrosion must be related back to the device processing and fabrication stages in order to prevent future problems. There are two approaches to preventing corrosion failures: (1) Protect corrosion susceptible materials from the corrosive environment, as in applying oxide and nitride passivation coatinqs and in packaging finished devices in plas tic or hermetically sealed containers; (2) utilize corrosion resistant systems, such-as the (thus far suecesssful) .Ti/W/Au system.