thin film

A thin film subject to compressive strain can either bend (for large strain gradient) or wrinkle (for small strain gradient). The bending is traditionally used in thermostats (bimetal stripes), but couple of years ago, it was extended to the nanoscale thin films which can bend and roll-up to tubes with defined number of rotations. The wrinkles are also rather common in macro- and microscale thin films.
Here, we developed an equilibrium phase diagram for the shape of
compressively strained free-hanging films by total strain energy
minimization.

An electronic device integrates diverse materials, and inevitably contains sharp features, such as interfaces and corners. When the device is subject to thermal and mechanical loads, the corners develop intense stress and are vulnerable sites to initiate failure. This paper analyzes stress fields at corners in flip-chip packages. The stress at a corner is a linear superposition of two modes of singular fields, with one mode being more singular than the other. The amplitudes of the two modes are represented by two stress intensity factors of dissimilar dimensions.

In this paper, we report comprehensive experimental and theoretical
studies of bending in structures relevant to inorganic flexible electronics.
Different from previous mechanics models of related systems, our analysis does not
assume the thin film to cover the entire substrate, thereby explicitly
accounting for effects of edges and finite device sizes, both of which play
critically important roles in the mechanics and bending properties. These
thin-film islands give nonuniform stress, with maxima that often appear at the
edges and spatially non-uniform shear and normal stresses along the film/substrate
interface. Although these results are generally applicable to all classes of

POST-DOCTORAL RESEARCH POSITION IN ENVIRONMENTAL ENGINEERING AND MATERIALS SCIENCE



School of Engineering and Applied Sciences, Harvard University