Nano and microscale silica chemistry in block copolymer templates using supercritical carbon dioxide as a reaction medium
Abstract (Summary)The ability of block copolymers (BCPs) to self-assemble into well-defined arrays of nanoscopic structures has enabled them to be used as templates and scaffolds for the fabrication of nanostructured materials. Such materials find applications in several fields including microelectronics, photonics and sensors. In this dissertation nanostructured silicate films were fabricated by performing phase selective silica chemistry within self-assembled BCP templates using a discrete two step replication process: (i) template formation and (ii) supercritical fluid assisted silica deposition. The use of supercritical CO2 as a reaction medium enabled facile transfer and diffusion of silicate precursor within the BCP film without disturbing its order. The sensitivity of the chosen precursor to acid, helped to control the silica condensation at nanoscopic and at microscopic length scales. Removal of templates yielded mesoporous silicate films in which the porous geometry can be completely controlled over multiple length scales. The use of BCP films with cylindrical domains oriented normal to the substrate as templates for phase selective silica deposition yielded arrays of perpendicular nanochannels in silica films. Obtaining such morphology in mesoporous materials has proven to be challenging, although they are promising candidates for applications ranging from catalysis to sensors and the separations. To fabricate directly patterned mesoporous silicate films, a photo acid generator was added in the BCP templates. Before performing phase selective precursor condensation, the templates were exposed to UV radiation through a photomask that has microscopic features. Photo-lithographic exposure triggered area selective generation of acid, which in turn led to patterned formation of silicate network. Because the acid generated in UV exposed field segregate further into hydrophilic domains of the BCP, precursor condensation is controlled in micro and nano-scopic length scales. De-templating via calcination yielded patterned mesoporous silicate films. These mesoporous silicate films inherited two levels of porosity. Microscopic features were inherited from the photomask and the nanoscopic features were inherited from the phase separated block copolymer template. Direct definition of the former followed by replication of pattern obviated the need for additional etching-cleaning steps and offers a cost-effective and compressed process routine for device level structures as small as 90 nm.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2008