Simulations of Deformation and Stress During Copper Wirebond on ULK Chips

In this paper, we develop a multi-level modeling procedure for copper wirebonding that provides insights into (a) deformation and stress in wire, pad, and die (b) an assessment of the risk of ULK fracture during impact stage and ultrasonic vibration steps. First, we construct a nonlinear, dynamic finite element model (global) to study the mechanical responses of wire, pad, and the underlying ULK stacks during the impact stage and the last cycle of ultrasonic vibration in copper wirebonding. Specifically, these process steps are modeled through prescribing touch down and in-plane oscillatory motions on capillary, which result in dissimilar critical states of stress locally in the ULK stacks. Next, we develop a isogeometric model (local) for a generic configuration of ULK stacks with eight levels of metallization by composing the geometric primitives representing ILD layers, copper lines/vias, as well as the material interfaces following the Hierarchical Partition of Unity Field Composition technique. The description for material moduli in the entire ULK stacks is further enriched with a bi-linear damage law. The critical states of stress obtained in the global wirebond model are then converted into boundary conditions for the local ILD model under plane strain condition to simulate the crack initiation in the ULK stacks. We observe, from the simulation results, potential crack initiation sites along vertical /horizontal interfaces in the ULK stacks due to local compressive/tensile loading during impact/vibration step, respectively.