Plastic ratcheting induced cracks in thin film structures

In the microelectronic and photonic industries, temperature cycling has long been used as a reliability test to qualify integrated materials structures of small feature sizes. The test is time consuming, and is a bottleneck for innovation. Tremendous needs exist to understand various failure modes in the integrated structures caused by cyclic temperatures. This paper presents a systematic study of a failure mechanism recently discovered by the authors. In a thin film structure comprising both ductile and brittle materials, the thermal expansion mismatch can cause the ductile material to plastically yield in every temperature cycle. Under certain circumstances, the plastic deformation ratchets, namely, accumulates in the same direction as the temperature cycles. The ratcheting deformation in the ductile material may build up stress in the brittle materials, leading to cracking. The paper 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. Design rules with increasing levels of sophistication are described. Concepts presented here are generic to related phenomena in thin film structures.