The analysis of chemical and metallurgical changes in microcircuit metalization systems

Abstract
Chemical and metallurgical changes in IC metalization systems have been studied during a program of device reliability testing and evaluation. This paper discusses the four reactions responsible for the majority of the observed metalization failures: 1) migration of aluminum under silicon dioxide layers, 2) movement of silicon through both gold and aluminum wires resulting in loss of mechanical strength, 3) structural changes in the molybdenum-gold system occurring during temperature storage testing, and 4) the electrochemical attack of the molybdenum barrier layer resulting from humidity storage testing with applied bias. Using standard optical techniques and a commercially available electron beam microanalyzer/scanning electron microscope, the following results were obtained. 1) At temperatures near the silicon-aluminum eutectic point, aluminum migrated under the thermally grown oxide in the form of triangles oriented with respect to the underlying silicon. Aluminum migration was observed at lower temperatures when bias was applied to the device. This occurred at the interface of the thermal silicon dioxide and a vapor deposited glass. 2) Silicon precipitation at wire grain boundaries was observed in both aluminum and gold lead wires. The mechanisms of solid-state diffusion and high-current density electromigration of silicon resulted in wire fracture or slip at grain boundaries in the wires. 3) Silicon penetration into gold conductor stripes through defects in the molybdenum barrier layer resulted in severe reordering of the gold at temperatures in excess of 300°C. The silicon-gold eutectic was formed at temperatures greater than 370°C causing complete dewetting of the molybdenum layer. At lower temperatures, grain boundary flaking due to loss of gold cohension and adhesion was observed. 4) Whiskers which formed on molybdenum barrier layers during 85-85 humidity storage testing with applied bias contained high concentrations of sodium. Several known compounds of sodium and molybdenum are discussed in light of the X-ray data taken on the electron beam microprobe.

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