Experimental Study on Electric-Current Induced Damage Evolution at the Crack Tip in Thin Film Conductors

Abstract

The time dependent temperature distribution induced by electric current heating in a double edge cracked, unpassivated thin aluminum or gold film interconnect lines is monitored using a high resolution infrared imaging system. A pure aluminum or gold film, with a thickness of 0.2 μm, is deposited by high vacuum evaporation coating and patterned into test structures of varying widths. The operative mechanisms of mass transport are assessed in view of the monitored temperature profile. The pre-cracked aluminum film shows fine crack growth towards the positive electrode, which originates from the initial crack tips. The crack-tip temperature is close to melting, during propagation. After the initial crack propagation, a hot spot is formed between the two elongated cracks, and leads to failure. The crack growth generates a backward mass flow towards the negative electrode. The gold film shows a different pattern, in which the original cracks propagate towards each other with a slight tilt towards the negative electrode. The tip temperature is lower than the melting temperature. These time dependent failure mechanisms are rationalize using a proposed critical current intensity factor and a normalized current intensity rate, similar to the fracture toughness KIC for brittle fracture.