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Self-assembly of block copolymers by solvent vapor annealing, mechanism and lithographic applications
Block copolymers (BCP) are a unique class of polymers, which can self-assemble into ordered microdomains with sizes from 3 nm to about 50 nm making BCPs an appealing meso-scale material. In thin films, arrays of BCP microdomains with long-range lateral order can serve as ideal templates or scaffolds for patterning nano-scale functional materials and synthesizing nanostructured materials with size scales that exceed the reach of photolithography. Among many annealing methods, solvent vapor annealing (SVA) is a low-cost, highly efficient way to annihilate defects in BCP thin films and facilitates the formation of highly ordered microdomains within minutes. Directing the self-assembly of BCPs could, in principle, lead to the formation of domains with near perfect lateral ordering. The mechanism of SVA of BCPs, however, is still ill-understood, albeit it has been widely adopted in research laboratories around the world for the past decade. In the first part of this thesis, the ordering process of BCP thin films during annealing in neutral solvents was investigated mainly by in situ synchrotron X-ray scattering. Briefly, the solvent molecules impart mobility to the BCP and enable a marked improvement in the lateral ordering of the BCP microdomains. Both, BCP concentration in the swollen film and the rate of solvent removal play a key role in obtaining films with well-ordered microdomains. The amount of swelling in a BCP thin film during SVA depends on the chemical nature of the blocks, the quality of the solvent, and the molecular weight of the BCP. A high degree of swelling - still low enough to prevent solvent-induced mixing (disordering) of BCP microdomains,- provides a high chain mobility, and thus results in the formation of arrays of ordered microdomains with large grain sizes after SVA in neutral solvents. The rate of solvent removal is another critical parameter for obtaining long-range lateral order in BCP thin films after SVA in neutral solvents. While in the swollen state ordered structures form with exceptional order, removal of the solvent results in a deterioration of order due to the confinement imposed to a BCP in a thin film by the rigid silicon substrate. It was found, however, that an instantaneous solvent removal can minimize disordering to preserve the order formed in the swollen state. Self-assembled BCP microdomains also serve as ideal template to pattern other materials with exceptional lateral resolution. In this thesis, two examples of BCP lithography was also demonstrated. A reconstruction process was used to enhance the etch contrast between two organic blocks. In one example, a BCP pattern was transferred to a silicon substrate to form high aspect ratio, 5:1, sub-10nm silicon lines or holes with high fidelity. While in a second example, I demonstrated the fabrication of silicon oxide dots with an areal density as high as 2 Tera dots per inch2 by BCP templates, which has the potential to serve as etch mask for bit pattern media applications.^
Polymer chemistry|Physics|Condensed matter physics
Gu, Xiaodan, "Self-assembly of block copolymers by solvent vapor annealing, mechanism and lithographic applications" (2014). Doctoral Dissertations Available from Proquest. AAI3615416.