Phenomenological Observations on Electromigration

Abstract

Direct dynamic observations were made of aluminum electromigration by an in situ scanning electron microscope technique. Time lapse 16 mm moving pictures were taken of electromigration which occurred in a commercially available MC 14050 hex buffer. The circuit was biased above rated specifications to obtain a current density of 7×105 Amps/cm2. The movie film clearly demonstrated a number of new phenomena not previously noted by after the fact examination. Some voids were very mobile and moved up the metallization stripe by a voiding-refill process. Another voiding process was observed in which the aluminum disappeared from the top of a mesa at a point where the current density was essentially zero. The formation of a hillock was observed in which the aluminum growth stretched the aluminum silicate glass which covers the stripes, first by forming a dome and then by rapid vertical and lateral growth. Voids were observed forming within several microns of a growing hillock literally emptying aluminum by a river-like mechanism into the hillock structure that was not constrained by grain boundaries or other defects. Changes in growth patterns (hillocks and voids) were noticed when the circuits were exposed to air ambients for short periods of time. It was observed that the presence of a surface oxide on the walls of a void greatly retard the void growth indicating that a primary mechanism for aluminum transport is along the oxide free wall structure. Additional evidence supporting this migration mechanism is given in the text by high resolution electron micrographs.