Fracture and Diffusion in Nanoporous Organosilicate Thin Films

Nanoporous organosilicate thin films are attractive candidates for a number of emerging technologies from biotechnology to optics and microelectronics. However, integration of these materials has been challenged by their fragile nature and susceptibility to environmentally assisted cracking as well as anomalous diffusion. Damage evolution in the form of delamination and cohesive cracking during processes and in-service operation is a principal concern that threatens the reliability and yield of device structures containing these materials. The objective of this research is to investigate strategies for improving mechanical properties by post-deposition UV curing and optimizing process chemistry to suppress crack growth in nanoporous organosilicate thin films. Diffusion of organic molecules under nanoscale confinement is also investigated. Firstly, it was demonstrated that depth dependent UV curing of organosilicate thin films can occur which has significant implications for the variation of mechanical properties through the film thickness. An oscillating elastic modulus depth profile following UV cure was measured by force modulation atomic force microscopy (FM-AFM). The oscillation was associated with UV light interference, which was modeled using a newly developed standing wave equation that accurately predicts the resulting depth dependent cure and includes the film absorption and shrinkage, the reflectivity of the underlying layers, and changes in the film refractive index. By careful selection of the UV spectrum and the underlying layers, cured film can be produced with selected curing profiles optimized for high adhesion and graded physical properties through the film thickness. Secondly, it was shown that small changes in electrolyte chemistry and surfactant additions in process aqueous environments can have dramatic effects on crack growth rates in the films. Crack growth rates were sensitive to the type of electrolyte and decreased in the presence of electrolytes that caused crack tip blunting. Growth rates were also sensitive to nonionic surfactant additions where molecular structure and weight were demonstrated to be important variables. An optimized blend of surfactants and electrolytes can significantly retard defect evolution due to molecular bridging. Surfactant self-assembly and resulting molecular bridging were characterized by in situ AFM and used to quantify the molecular bridging observed. Lastly, it was revealed that the mobility of organic molecules when confined at nanometer length scales differs greatly from properties in the bulk. Unentangled surfactant molecules in the bulk get entangled with adjacent molecules when diffusing through interconnected nanopores in hydrophobic nanoporous organosilicate thin films, exhibiting signatures of reptation. It was also shown that the conventional mobility/free volume relationship in the bulk explained by the free volume theory of diffusion breaks down for linear alkanes in hydrophobic nanoporous organosilicate thin films. While alkane mobility decreased with chain length, the activation energy for diffusion decreased and the free volume increased under nanoscale confinement, which is opposite to the trend in the bulk. The effects of molecular polarity and pore size on diffusion were also demonstrated. Molecular mobility was found to be suppressed with increasing molecular polarity and decreasing pore size.