Georg M. Reuther

Acoustic detection of micro-cracks in small electronic devices

Abstract We demonstrate the feasibility of in-situ acoustic detection of micro-cracks in small electronic devices. Applying precisely controlled damage to test vehicles using a nanoindenter, we record brittle fracture of thin layers by means of an ultra-sound piezo sensor, which is able to detect micro-cracks in the moment they emerge. This robustness test does not require further preparation effort that may induce additional stress to a sample or modify it physically, inhibiting unambiguous failure analysis. With regard to its applicability and limitations, we put acoustic emission into context with standard ex-situ experimental procedures for crack characterization in micro-electronic structures.

Brittle fracture and damage in bond pad stacks — A study of parameter influences in coupled XFEM and delamination simulation of nanoindentation

Wire bonding as well as wafer probing can lead to oxide layer cracking in Backend of Line (BEOL) Bond Pad Stacks. This, along with metal migration into formed cracks, may lead to electrical failures. Mechanical loading conditions comparable to those during the wafer test and the wire bonding process can be achieved using a nanoindenter. This work addresses the finite element (FE) simulation of an indentation process during which a spherical tip imprints into a silicon nitride film. Based on experimental results we have established a FE-model using ABAQUS Standard™ that reproduces the experimentally observed load-displacement behavior. Introducing of the extended finite element method (XFEM) as well as a cohesive contact approach allow describing different failure modes observed in our experiments. The simulation results show dependencies on the used crack initiation criteria and fracture toughness properties. Resulting patterns of damage initiation and propagation and also fracture patterns will be discussed on the basis of 2D axisymmetric and 3D quarter symmetric models. The latter includes circumferential cracks on top of the silicon nitride film with directional crack initiation as well as interface delamination reproduced by means of the energy-based Cohesive Zone approach. The results of these investigations enable predicting and avoiding failures, such as oxide layer cracking during wire bonding or wafer testing.

Brittle fracture and damage in bond pad stacks: Novel approaches for simulation-based risk assessment

We have presented an advanced FEM simulation model that is able to map the scenario of indentation of a diamond tip into a thin brittle SiNx:H film deposited on a Si substrate. Our 2D axisymmetric and 3D quarter and half models are able to reproduce all failure modes observed in preceding indentation experiments. On the one hand, brittle fracture in both film and surface were successfully modelled via the XFEM approach implemented in ABAQUS™ Standard, using the VCCT method. This includes circumferential cracks on top of the silicon nitride film with directional crack initiation. On the other hand, we have replicated interface delamination by means of an energy-based Cohesive Zone approach. Simplified 2D models already enable appreciable insight into the nature of underlying failure mechanisms, i.e., over-critical mechanical stress. A complete 3D model comprising all relevant failure mechanisms and modes will be content of a future work. The mutual dependence of fracture onset, delamination, and elastic-plastic deformation will require further refinement and even higher exactitude of the embedded material models. All in all, the volume of information gathered in the scope of this work will be of paramount importance as to sophisticated FE models. In particular, top metallisation layers will add complexity in models describing the indentation process into BEOL bond pad stacks.