Metal film crawling in interconnect structures caused by cyclic temperatures

Thermal cycling is widely used as a qualification test in the microelectron ic industry. This paper investigates an intriguing failure mode observed in such a test. Near the corners of a silicon die, shear stresses arise due t o thermal expansion mismatch between the silicon and the packaging substrat e. These shear stresses may have a small magnitude, being transmitted throu gh packaging polymers, but sometimes motivate metallic interconnect films t o crawl toward the center of the die during thermal cycling, even when the temperatures are low and the metal creeps negligibly. The phenomenon has be en observed for two decades, but no mechanistic explanation has been given so far. This paper shows that the metal films can crawl by ratcheting plast ic deformation. When the temperature cycles, the thermal expansion mismatch between the silicon and the metal causes the metal films to yield. Directe d by the small shear stresses, the films shear plastically by a small amoun t in each cycle, and accumulate a large deformation after many cycles. We d evelop an idealized model to demonstrate this mechanism, and to study the e ffects of temperature-dependent yield strength and strain hardening. Severa l analytical solutions are obtained. Implications for the qualification tes t and interconnect design are discussed. The study clearly shows the need f or basic research on large plastic deformation at small length scales. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights res erved.