Effect of spin rate
The optimization of spin rate, time and other preparative parameters is an important parameter to achieve large area uniform coating. In this report, we followed the three steps spin rate process to obtain a monolayer of close-packed PS spheres. Figures 1-3 show the SEM images of the effect of the first, second and third steps spinning rates on the surface coverage and growth of the close-packed monolayer. It is seen that the close packing and uniform surface covering of PS spheres increases with the first step spin rate up to 200 rpm and then decreases (irregular arrangement) for higher spin rates with the generation of multilayer's of PS spheres. It is observed that the second step spinning rate plays an important role in the orientation and compactness of monolayered PS spheres (actual growth process). The well grown monolayer of PS spheres with surface coverage enhances up to 1000 rpm (Figure 2) and then decreases for the higher rate. Figure 3 shows that the effect on surface coverage and close packing is strongly enhanced during the third step spinning rate. The periodic arrangement is increased with rates up to 6000 rpm and then it decreases. At low spin rates, the growth of a bilayer and incomplete growth of PS spheres is observed. For higher rates, more PS spheres were repelled out from the substrates. In general, for the spin coating, centrifugal forces, capillary force and solvent evaporation, immersion capillary force is a dominant factor to organize well-ordered silica particles in the first, second and third step spin rates respectively. The lowest surface roughness is observed at the optimal spin rate 200-1000-6000 rpm. As the thickness of the layers of microsphere dispersion become thinner, the spheres start to protrude from the water, giving rise to a water flux from thicker areas towards protruding spheres. This water flux assembles the spheres into a crystal. This approach is very convenient for preparing two-dimensional colloidal crystals.
Effect of spinning time
Figures 4-6 show the SEM images of PS spheres fabricated at different spinning times (ranging from 10-40 s, 20-80~s, and 5-20 s first, second and third step spinning time respectively). The monolayer of PS spheres is well grown at 30, 60 and 10 s for first, second and third step time respectively. As we increasing the growing time of the first step, it is seen that a close-packed monolayer is achieved up to 30 s and then it grows randomly. In the case of the second step, it is the time required to organize and grow the well ordered layer of particles. In this experiment, the spin time enhances the surface coverage and monolayer periodic arrangement up to 60 s and then it disturbs. At the third step, the extra precursor solution is repelled out, which makes the layer have a good periodic arrangement with high compactness. The maximum surface coverage achieved is about 90 %. It is seen that, as the spinning time enhances for the three steps growth process, solvent evaporation increases, particles near the substrate surface are fixed to the substrate because of the increasing solution viscosity, as pointed out by Deckman et al. so that the mobility of the particles near the substrate surface decreases rapidly.
Effect of concentration of colloidal PS spheres
Figure 7 shows the SEM images of PS spheres thin films developed at various PS spheres concentrations in the 3-10~wt % solution by keeping FSR-200 rpm, SSR-1000 rpm, TSR-6000 rpm and time FST-30 s, SST-60 s, TST-10 s respectively. The surface coverage as a function of solution concentration increases from 3 to 7 wt % and then decreases. However, when a 3 wt %, 284 nm particles suspension is spin coated, a uniform but low surface coverage is obtained. Further increasing the concentration of the 284 nm particles suspension to 7~wt % increased the average surface coverage in the range of 70 %-90 % with a total average of 80 %. Figure 7 shows that the monolayer surface coverage steeply increases with increasing solution concentration. To explain these results, it must be taken into account that centrifugal immersion capillary forces act when the liquid surface is comparable to the particles size and the solution viscosity increases with solution concentration. Therefore, it can be assumed that for the 3 wt % concentration, centrifugal forces are not sufficiently strong in the radial direction, therefore a large number of particles remain suspended in the solution; moreover, immersion capillary forces acted much later than in the case of the 7 wt % concentration. For the 10 wt % solution concentration causes a decrease in the mobility of the particles near the solution/substrate interface, which grows in an irregular arrangement. The polystyrene monolayer deposited by spin-coating shows a dense hexagonal packing structure over a large area of the substrate, although line defects or empty spaces are occasionally observed between domains, as seen in the figures.
Effect of oxygen plasma etching
In order to form periodic different structures with different diameters, we coated a single-layered polystyrene bead array on the surface of the substrate and then applied oxygen RIE to the array to tailor the size of polystyrene beads as a mask. Figures 8-10 show the SEM images of oxygen plasma etched PS spheres films and its particle size distribution. It is seen that the size and diameter (284-79 nm) of the particles decreases with the oxygen etching time (0-50 s). The variation of diameters with oxygen etching time is shown in Figure 10. It varied as 284, 252, 172, 147, 105, and 79 for 0-50~s etching time and keeping oxygen flow at 50 sccm and DC power at 20 W. The effect of DC power during oxygen etching on the diameter change has been shown in Figure 11. It is seen that diameter variation completely depends on preparative parameters like oxygen flow, etching time, DC power etc. The well organized reduced size PS structure will be useful for the generation of various nanostructures of different II-VI, III-V semiconductors. We have demonstrated a size-controllable nanosphere lithography (NSL) technique based on spin-coating of polystyrene nanospheres useful for industrial applications.