Simulation and modeling of materials for microelectronics

One of our major interdisciplinary research activities is the modelling of damage in multi-material composites in microelectronics. First our approach starts from the fundamental question: “What is damage?” ending in the extremely application oriented question: “How can one avoid it?”. Here we follow a material oriented approach, requiring advanced experimental as well as modeling techniques.

Standard fracture mechanical methods are limited in applicability if the crack approaches surfaces and interfaces. Recognizing this fact, Simha at al. and Kolednik et al. in cooperation with the MCL have developed, and constantly evolved, the “Configurational Force” concept. With that method the crack driving force can be computed correctly even if the plastic zone extents to the surface or across interfaces [2,3]. Engineering of new fracture tough components and sophisticated fracture experiments are supported in a founded theoretical manner by this approach.

This allows e.g. the design of thin layered systems with significantly enhanced fracture toughness compared to homogeneous bulk material [4,5]. This is inspired by nature. E.g. nacre has a natural microstructural design, giving it a fracture toughness many orders of magnitude higher than its very brittle constituent material calcium carbonate [6].

Thermal transient simulation, in combination with experimental thermal impedance analysis, and the use of the so-called structure function enables deeper understanding of the thermal behavior of microelectronics components. The structure function is a mapping of the heat path from heat source to heat sink in the form of thermal resistances and capacities. Comparison of simulated and measured structure function constitutes an advanced tool for validation of the simulation and the interpretation of the experiment. By this means improvement of the thermal management of microelectronics by “Virtual Prototyping” is enabled [1].

In this way we e.g. develop special PCB concepts including high thermal conductive but electrical insulating ceramic coatings at critical points within the system. Analyzed compact thermal models in addition can improve the “Condition Monitoring” and “Structural Health Monitoring” of components. This is considered as an innovative and cost effective method for “Predictive Maintenance” e.g. in lighting industry.

The material characterization accompanied by simulation is used in “Non Destructive Testing” using ultrasound and also for material-parameter determination like: eigenstress, elastic constants, flow curves, fracture- and interface properties, thermal conductivity and capacities. In particular numerical modeling is used to go beyond analytical evaluation methods for micro pillar, Raman, XRD, nanoindentation etc. experiments.

Ultrasound simulation is used to support our experimental methods: the “Scanning Acoustic Microscope (SAM)” at MCL and the “Laser Ultrasound Method” in cooperation with the Karl-Franzens-University Graz. The combination of simulation and experiments gives an advantage especially for the interpretation of complex sound wave behavior like propagation and conversion of bulk and surface waves. This enables an advanced interpretation an detection of defects in complex microelectronics components [1,2] and the characterization of elastic parameters e.g. of thin films [3,4].

One methodological focus lies in the use and development of the “Elastic Finite Integration Technique (EFIT)” capturing the behavior under strong damping, dispersion and transmission in complex microelectronics components.

Eigenstresses in thin layers take on very high values (in the order of GPa). The stored elastic energy can constitute a major crack driving force. In Leoben the “Ion Layer Removal (ILR)” method was developed, which enables the determination of local and depth resolved eigenstress measurements [5]. By supportive finite element modeling (FEM), stress reconstruction is also possible in the presence of anisotropy and plasticity.

FEM is also used in combination with Raman measurements to reconstruct the complex stress-tensor field within microelectronics components, e.g. for 3D integration [6]. The simulation of layer per layer deposition, including intrinsic and thermal stress, was used to understand the stress concentrations at the scalloped wall of a tungsten Trough Silicon Via (TSV) (because of the Bosch process), which was measured by X-ray nanodiffraction [7].

Fracture and Interface toughness is strongly dependent on the microstructural design. To investigate this topic further, simulations and micro-cantilever fracture experiments are done. For this task the “Configurational Force” concept is used (see also “Damage in multi-material composits”) [8].