Laser-induced stress wave technique for thin film adhesion and high strain rate materials research

Thin films are crucial components in a wide range of multilayer microelectronic and optical devices. Among the many properties, interfacial adhesion between the thin film and substrate is one of the key parameters influencing the overall reliability and durability of integrated thin film devices. However, due to the critical dimension of thin films, conventional techniques face challenges to reliably evaluate the thin film interfacial properties. To address this challenge, laser-induced stress wave techniques have been developed to quantitatively investigate the intrinsic strength of a planar thin film/substrate interface. Based on the previously developed tensile and mixed-mode loading technique, this dissertation further extended the laser-induced stress wave technique to pure shear loading of thin film interfaces. The technique was also applied to study the interfacial adhesion of various types of interfaces including the adhesion strength of ultra-thin nano-porous zeolite low-k films on silicon substrates, the adhesion between biological cells and inorganic substrates, and the dynamic interfacial properties of tungsten and tungsten heavy alloys. The application background of this study spans the wide field of semiconductor industry, biomedical devices and defense applications.; In addition to the further development and applications of laser-induced stress wave technique in various bi-material interfaces, this dissertation also investigated the theoretical bases for the formation and evolution of super sharp shock waves in fused silica which are important for measuring the adhesion of ultra-thin films. Due to its negative non-linear elasticity, fused silica has the special capability of developing decompression shock wave from an original Gaussian stress wave. Experimentally measured shock is typically around nanosecond wide, while one-dimensional wave analysis predicted a shock width of tens of picoseconds. The discrepancies may be due to the diffraction effect because of a finite source size as well as intrinsic wave absorption. Numerical simulation based on KZK equations was carried out by taking into account the interrelated effects of diffraction, nonlinearity and attenuation. The results confirmed the existence of a sharp shock of about 10 ps wide with nonlinearity effect only, about 20-30 ps wide with nonlinearity and attenuation effects in regardless of the existence of the diffraction effect.