Numerical simulation and experiments of fatigue crack growth in multi-layer structures of MEMS and microelectronic devices

Bo Wang, Purdue University

Abstract

The study is concerned with Fatigue Crack Growth (FCG) in elastic-plastic solids under consideration of constraint and size effects. Numerical and experimental studies on FCG in thin film structures relevant to integrated circuit and MEMS devices are reported. It was demonstrated that numerical simulations of FCG can successfully be conducted within a framework where material separation is described by a Cohesive Zone Model (CZM) with an irreversible constitutive relationship. The traction-separation law does not follow a predefined path, but depends on the evolution of the damage dependent cohesive zone properties. The approach enables analysis of fatigue failure in cases where the Paris Law is no longer applicable. The influences of geometric constraint (thin film confinement, presence of interfaces), mechanical constraint (T-stress), and structure size on FCG are studied. Numerical simulations predict that increased constraint will accelerate fatigue failure due to changes in plastic deformation and changes in crack closure. For cracks growing perpendicular to a bi-material interface, crack growth rate acceleration, deceleration or arrest, as well as crack bifurcation at the interface are predicted depending on the plastic property mismatch and interface properties. It was demonstrated that as structural size is reduced, fatigue failure of cracked structures no longer occurs by crack propagation but by transitions to uniform debonding. Then, an evolution equation for the internal cohesive length scale under cyclic loading is proposed. Measurements of fracture toughness and FCG rates for a material system where aluminum films (thickness 0.05 to 2.0 μm) are confined between two elastic substrates are reported. Improved test protocols for the commonly used 4-point bend delamination test are developed such that cracks can be propagated along both sides of the specimen. Experimental results indicate that the fracture toughness is nearly independent on the metal film thickness considered but show FCG rates to be dependent on film thickness. The dependence of the failure behavior on film thickness arises primarily due to enhanced crack path deflection and crack bridging for decreasing film thickness. The appearance of extrinsic mechanisms of crack deflection and crack bridging is dependent on the constraint and size effects.

Degree

Ph.D.

Advisors

Siegmund, Purdue University.

Subject Area

Mechanical engineering

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