Ratcheting as a mechanism for thermal cycling induced failures in thin film structures

In the microelectronic and photonic industries, temperature cycling has long been used as a reliability test to qualify integrated material structures of small feature sizes. Tremendous needs exist to understand various failure modes caused by cyclic temperatures. This thesis investigates two intriguing failure modes. 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, and overlaying passivation film to crack during thermal cycling. This work shows that ratcheting is responsible for both failure modes.; This thesis first gives a background by reviewing several general themes underlying my work. To understand the concept of ratcheting, Chapter 2 describes a three-layer structure subject to a dead weight and cyclic temperatures. The layers, with different thermal expansion coefficients, may elongate a definite amount for each temperature cycle. This is known as ratcheting.; Chapter 3 examines metal film crawling. 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. An idealized model is developed to demonstrate this mechanism, and to study the effects of temperature dependent yield strength and strain hardening.; Chapters 4 and 5 study passivation film cracking. The ratcheting deformation in the metal film may build up stress in the overlaying passivation films. It is this evolving stress state that cracks the passivation film after some cycles. Strain hardening can reduce the ratcheting effect. Finite element calculations are used to show the mechanism. Implications for design rules and qualification tests are discussed. Chapter 5 introduces an analogy between ratcheting and viscous flow. An analytical model is developed, which explains the experimental observations, and allows one to design the structure to avert this failure mode. Concepts presented in this thesis are generic to related phenomena in thin film structures.