Metal film crawling in interconnect structures caused by cyclic temperatures

Abstract

Thermal cycling is widely used as a qualification test in the microelectronic industry. This paper investigates an intriguing failure mode observed in such a test. Near the corners of a silicon die, shear stresses arise due to thermal expansion mismatch between the silicon and the packaging substrate. These shear stresses may have a small magnitude, being transmitted through packaging polymers, but sometimes motivate metallic interconnect films to crawl toward the center of the die during thermal cycling, even when the temperatures are low and the metal creeps negligibly. The phenomenon has been observed for two decades, but no mechanistic explanation has been given so far. This paper shows that the metal films can crawl by ratcheting plastic deformation. When the temperature cycles, the thermal expansion mismatch between the silicon and the metal causes the metal films to yield. Directed by the small shear stresses, the films shear plastically by a small amount in each cycle, and accumulate a large deformation after many cycles. We develop an idealized model to demonstrate this mechanism, and to study the effects of temperature-dependent yield strength and strain hardening. Several analytical solutions are obtained. Implications for the qualification test and interconnect design are discussed. The study clearly shows the need for basic research on large plastic deformation at small length scales.