Fabrication of nanostructured materials using block copolymer based lithography

The main objective of the PhD research activity carried out at MDM Laboratory was the growth and manipulation of nano-objects to be used as building blocks for the fabrication of new generation of nano-transistors, nano-memories and nano-emitters. The first part of the research activity was related to the development of reproducible and controlled protocols for the fabrication of polymeric soft masks for advanced lithographic applications using block copolymers. To this purpose hexagonally packed nanoporous polymeric thin films were fabricated using PS-b-PMMA block copolymers and accurately characterized. Special care was used to the functionalization of the sample surface prior to block copolymer thin film deposition. The effect of the self assembled monolayer of random copolymers conventionally used for surface neutralization was deeply investigated. In particular it was observed that different random copolymer thin films influence the window of thicknesses in which perpendicular orientation of the PMMA domains with respect to the underlying substrate occurs, as well as the characteristic dimensions of the final nanoporous polymeric mask. The possibility of combining “bottom up” self assembly of block copolymers with “top down” patterned templates was then explored to register the periodic domains of the self assembled block copolymer film with the underlying topographic structure. E-beam lithography was used to fabricate trenches in the SiO2 substrate before the deposition of the block copolymer thin films. The nanoporous polymeric mask fabricated during the first part of the research activity was then used as soft mask for patterning the underlying substrate in order to create nanoporous SiO2 hard masks as well as for the fabrication of ordered arrays of Silicon nanodots. The hexagonally packed nanopores of the polymeric mask were transferred to the underlying SiO2 by reactive ion etching. The effects of the etching parameters on the final characteristics of the nanoporous oxide were deeply investigated. The nanoporous SiO2 template was then used as a backbone for the fabrication of tunable nanoporous Al2O3 substrates by atomic layer deposition growth of thin films of Al2O3 on the SiO2 template. Progressive reduction of the pore size down to complete pore filling was obtained by properly adjusting the thickness of the Al2O3 film. This activity demonstrated the feasibility of fabricating periodic nanostructures surfaces with tunable dimensions well below the 20 nm limit. Moreover, since a large variety of oxide materials can be grown by atomic layer deposition, the proposed methodology provided a general approach for the synthesis of nanoporous oxide with accurate control of pore dimension, size distribution and pore frequency. Ordered arrays of Si nanocrystals were fabricated using the nanoporous polymeric film as a lithographic mask to control the formation of the nanodots. Two different approaches were pursued leading to different configurations where nanodots are embedded/deposited in/on the dielectric matrix. The first approach was based on ion beam synthesis and consisted in the implantation of Si ions into the nanostructured polymeric film to locally introduce the desired ion supersaturation in a limited nanosized area. After removal of the polymeric film, a thermal annealing led to the formation of nanocrystals at a depth depending on the ion energy. The second approach was the lift-off process that included material deposition by e-beam evaporation onto the nano-structured polymeric film and on the exposed substrate regions followed by the subsequent removal of the polymeric template and material excess by wet or dry etching. These arrays of semiconducting nanodots are suitable for the fabrication of Si nanocrystals non volatile memories or Si nanocrystals nanoemitters.