A study of the work-hardening behavior of metallic thin films

Joost J. Vlassak, Harvard University, DMR 0906892

Discrete dislocation modeling Experiments

The objective of this project is to develop experiments and models to understand the mechanical behavior of materials in small, constrained volumes and on the dislocation processes that take place in the vicinity of a free surface or a hard passivating layer. Many micro-electronic and micro-mechanical devices consist of dissimilar materials put together in complex architectures. Components of these devices are often subjected to very high stresses. Insight in the mechanical behavior of materials at these small length scales and accurate models to describe this behavior are essential to the design and fabrication of reliable devices.

(a) Micro heater sample attached on the in-situ SEM tensile tester (b) Freestanding gold (Au) film on the fabricated micro tungsten (W) heater

(a) (b)

(a) SEM image of the sample during tensile testing(b) Preliminary stress-strain curve of a 1 µm thick Au film – the cause of strain softening at large strains is currently under investigation

Schematic representation of the thin-film model passivated on one surface

Stress distribution in 1 µm two-surfaces passivated film with 1.5 µm grains after a plane-strain deformation of 1% when dislocation climb is enabled

Stress-strain curves for films. Figure (a) shows the effect of passivation layer(s); figure (b) shows the effect of the film thickness for films with passivation layers on both surfaces. Dashed curves are pertinent to the case of glide only.

These figures have been published in Journal of Applied Physics 2012, Vol. 111

A study of the work-hardening behavior of metallic thin films

Joost J. Vlassak, Harvard University, DMR 0906892

Broader impacts

Images illustrating the work of Gidong Sim (KAIST, South Korea/Harvard University) on a new highly sensitive tensile tester that is capable of both uniaxial and fatigue testing inside an SEM. Micro heater sample are fabricated which enables measurement at elevated temperatures.

In-situ tensile tester for use in SEM

• The project has contributed to the training of several graduate students, including PhD students Kamyar Davoudi and Li Han (now graduated), MS student Dongwoo Lee, and visiting PhD student Gidong Sim. Dr. Sim graduated this year and will continue this research as a post-doctoral scholar.

• Collaboration with Professor JunHyub Park (Tongmyong University, South Korea) has resulted in the development of a very sensitive thin-film tensile tester for use inside an SEM that is suitable for both uniaxial and fatigue testing.

• Collaboration with Professor Soon-Bok Lee from KAIST (South Korea) on the mechanical behavior and reliability of thin metallic films for use in stretchable electronics.

• Collaboration with Professor Lucia Nicola (TU Delft, Netherlands) on discrete dislocation dynamics

• Simulations of the work-hardening behavior of passivated thin films.

• Implementation of discrete dislocation dynamics software at Harvard University and extension of the software to include creep by dislocation climb.