(658d) Nano-Scale Block Copolymer Patterning for Selective Area Chemical Vapor Deposition

A self-assembled block co-polymer approach to nanoscale patterning, which offers rapid and cost-effective full wafer patterning at the 20-nm length scale, was used to achieve unique structures or improvements in heteroepitaxial growth of semiconductor materials. This approach uses the self-assembling block co-polymer (PS-b-PMMA) and reactive ion etching to generate nanoscale holes arranged in a nanoscale pattern in a SiO₂ mask layer covering an entire wafer. Specifically, a dielectric SiO₂ thin film (15 nm) on a nominally exact (100) GaAs substrate was prepared by plasma-enhanced chemical vapor deposition. A 1% (w/w) toluene solution of PS-b-PMMA (MW: 46 kg/mol for PS and 21 kg/mol for PMMA, PDI: 1.09) diblock copolymer was spin-coated over the pretreated wafer. The coated wafer was annealed at 180°C for more than 24 h under vacuum to induce the perpendicular cylindrical morphology. The patterned wafer is then used as a substrate for epitaxial growth. The interaction of strain and the growth over the patterned surface was investigated for strained and unstrained epitaxial growth. Growth is initiated in the nanoscale holes and proceeds laterally over the SiO₂ layer. The growth of GaAs, InAs, and GaSb was carried out on the patterned GaAs substrates. InAs and GaSb both have a 7% lattice mismatch with the GaAs substrate. The GaAs epitaxial layer rapidly coalesced with no introduction of additional defects as measured by high resolution x-ray diffraction. The InAs and GaSb, despite a similar lattice mismatch, exhibited different growth behaviors due to the variation in adatom attachment to island of differing strain states. Differences in the surface diffusion rates between In, Ga, and Sb adatoms lead to variations in their growth behavior. The high surface diffusion coefficient of In on SiO₂ allows for the In adatoms to migrate and sample many initial, 20 nm, islands relative to the GaSb. This sampling leads to the subsequent preferential growth of the least strained islands, frustrating the film coalesce and leading to large island formation. The dynamics of this process will be discussed as well as the changes and in some cases improvement in the materials structural properties, in terms of reduced defect density, will be presented.