Computational modeling of laser-induced delamination testing of thin films

Thin film adhesion often determines microelectronic device reliability and it is therefore essential to have experimental techniques that accurately and efficiently characterize it. Laser-induced delamination is a novel technique that uses laser-generated stress waves to load thin films at high strain rates and extract the fracture toughness of the film/substrate interface. The effectiveness of the technique in measuring the interface properties of metallic films has been documented in previous studies. The objective of the current effort is to model the effect of residual stresses on the dynamic delamination of thin films. Residual stresses can be high enough to affect the crack advance and the mode mixity of the delimitation event, and must therefore be adequately modeled to make accurate and repeatable predictions of fracture toughness. The equivalent axial force and bending moment generated by the residual stresses are included in a dynamic, nonlinear finite element model of the delaminating film, and the impact of residual stresses on the final extent of the interfacial crack, the relative contribution of shear failure, and the deformed shape of the delaminated film is studied in detail. Another objective of the study is to develop techniques to address issues related to the testing of polymeric films. These type of films adhere well to silicon and the resulting crack advance is often much smaller than for metallic films, making the extraction of the interface fracture toughness more difficult. The use of an inertial layer which enhances the amount of kinetic energy trapped in the film and thus the crack advance is examined. It is determined that the inertial layer does improve the crack advance, although in a relatively limited fashion. The high interface toughness of polymer films often causes the film to fail cohesively when the crack front leaves the weakly bonded region and enters the strong interface. The use of a tapered pre-crack region that provides a more gradual transition to the strong interface is examined. The tapered triangular pre-crack geometry is found to be effective in reducing the stresses induced thereby making it an attractive option. We conclude by studying the impact of modifying the pre-crack geometry to enable the testing of multiple polymer films.

Carbon nanotube films for flexible electronics

Discovering scalable routes to fabricate large scale electronic devices on flexible substrates has been the goal of the newly emerging field of flexible macroelectronics. Thin film transistors (TFTs) have been fabricated on flexible substrates by using organic small-molecule and polymer-based materials, or thin layers of crystalline inorganic semiconductors. Recently, films of carbon nanotubes have been proposed as electronic materials with superior electrical performance due to exceptional electrical and mechanical properties of single-walled carbon nanotubes (SWCNTs). In this thesis, some aspects of recent research efforts on integrating arrays of carbon nanotubes into macroelectronic devices are described. Carbon nanotube films have two major uses for flexible macroelectronics. The first approach uses carbon nanotube thin films as active semiconducting materials in the channel of flexible TFTs. Even though, high-performance carbon nanotube thin film transistors have been realized, the electronic non-homogeneity of the as-grown carbon nanotubes in the film limits the device performance for some applications. In this thesis, the application of electrochemical functionalization on carbon nanotube films to improve the electronic homogeneity of the film is described. The effect of the crystal quartz substrates on the growth rate of carbon nanotubes, and whether this can be used to sort out as-grown carbon nanotubes by electronic type is also discussed. Finally, I argue that high density carbon nanotube films can also be used as highly conducting stretchable interconnects on mechanically flexible electronic circuits. The sheet resistance and the nature of the buckling of carbon nanotube films on flexible substrates are discussed.