Selective sidewall wetting of polymer blocks in hydrogen silsesquioxane directed self-assembly of PS-b-PDMS.

We show the importance of sidewall chemistry for the graphoepitaxial alignment of PS-b-PDMS using prepatterns fabricated by electron beam lithography of hydrogen silsesquioxane (HSQ) and by deep ultraviolet (DUV) lithography on SiO(2) thin films. Density multiplication of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) within both prepatterns was achieved by using a room temperature dynamic solvent annealing environment. Selective tuning of PS and PDMS wetting on the HSQ template sidewalls was also achieved through careful functionalization of the template and substrate surface using either brush or a self-assembled trimethylsilyl monolayer. PDMS selectively wets HSQ sidewalls treated with a brush layer of PDMS, whiereas PS is found to selectively wet HSQ sidewalls treated with hexamethyldisilazane (HMDS) to produce a trimethylsilyl-terminated surface. The etch resistance of the aligned polymer was also evaluated to understand the implications of using block copolymer patterns which have high etch resistance, self-forming (PDMS) wetting layers at both interfaces. The results outlined in this work may have direct applications in nanolithography for continued device scaling toward the end-of-roadmap era.

Mechanical constraint and release generates long, ordered horizontal pores in anodic alumina templates.

We describe the formation of long, highly ordered arrays of planar oriented anodic aluminum oxide (AAO) pores during plane parallel anodization of thin aluminum 'finger' microstructures fabricated on thermally oxidized silicon substrates and capped with a silicon oxide layer. The pore morphology was found to be strongly influenced by mechanical constraint imposed by the oxide layers surrounding the Al fingers. Tractions induced by the SiO(2) substrate and capping layer led to frustrated volume expansion and restricted oxide flow along the interface, with extrusion of oxide into the primary pore volume, leading to the formation of dendritic pore structures and meandering pore growth. However, partial relief of the constraint by a delaminating interfacial fracture, with its tip closely following the anodization front, led to pore growth that was highly ordered with regular, hexagonally packed arrays of straight horizontal pores up to 3 µm long. Detailed characterization of both straight and dendritic planar pores over a range of formation conditions using advanced microscopy techniques is reported, including volume reconstruction, enabling high quality 3D visualization of pore formation.

Study on the combined effects of solvent evaporation and polymer flow upon block copolymer self-assembly and alignment on topographic patterns.

Microphase separation of a polystyrene-block-polyisoprene-block-polystyrene triblock copolymer thin film under confined conditions (i.e., graphoepitaxy) results in ordered periodic arrays of polystyrene cylinders aligned parallel to the channel side-wall and base in a polyisoprene matrix. Polymer orientation and translational ordering with respect to the topographic substrate were elucidated by atomic force microscopy (AFM) while film thickness and polymer profile within the channel were monitored by cross-sectional transmission electron microscopy (TEM) as a function of time over a 6 h annealing period at 120 degrees C. Upon thermal annealing, the polymer film simultaneously undergoes three processes: microphase separation, evaporation of trapped solvent, and mass transport of polymer from the mesas into the channels. A significant volume of solvent is trapped within the polymer film upon spin coating arising from the increased polymer/substrate interfacial area due to the topographic pattern. Mass transport of polymer during this process results in nonuniform films, where subtle changes in the film thickness within the channel have profound effects on the microphase separation process. The initially disordered structure within the film underwent an orientation transition via an intermediate formation of perpendicular cylinders (nonequilibrium) to a parallel (equilibrium) orientation with respect to the channel base. Herein, we present a time-resolved study of the cylinder reorientation process detailing how changing film thickness during the annealing process dramatically affects both the local and lateral orientation of the observed structure. Finally, a brief mathematical model is provided to evaluate spin coating over a complex topography following a classical asymptotic approximation of the Navier-Stokes equations for the as-deposited films.

Pore directionality and correlation lengths of mesoporous silica channels aligned by physical epitaxy.

Herein we report on the alignment of mesoporous silica, a potential host for sub-10 nm nanostructures, by controlling its deposition within patterned substrates. In-depth characterization of the correlation lengths (length of a linear porous channel), defects of the porous network (delamination), and how the silica mesopores register to the micrometer-sized substrate pattern was achieved by means of novel focused ion beam (FIB) sectioning and in situ SEM imaging, which to our knowledge has not previously been reported for such a system. Our findings establish that, under confinement, directed deposition of the sol within channeled substrates, where the cross-sectional aspect ratio of the channels approaches unity, induces alignment of the mesopores along the length of the channels. The pore correlation length was found to extend beyond the micrometer scale, with high pore uniformity from channel to channel observed with infrequent delamination defects. Such information on pore correlation lengths and defect densities is critical for subsequent nanowire growth within the mesoporous channels, contact layout (electrode deposition etc.), and possible device architectures.