Oriented colloidal-crystal thin films of polystyrene spheres via spin coating

  • Korea Research Institute of Standards and Science, Daejeon-305340, Korea

Key words: monolayerPS spheresoxygen plasma

Abstract: We developed a simple and inexpensive synthesis of a large-scale close-packed monolayer of polystyrene sphere arrays, which have a variety of applications. The influence of three step spin speeds, spinning time, solution quantity and relative humidity is studied in order to achieve a large area close-packed monolayer. A relatively high surface coverage and uniform monolayer of PS spheres in the range of 85%—90% are achieved by appropriate control of the preparative parameters. Also the effect of the oxygen plasma etching process on the reduction of PS spheres has been studied. We conclude that it can be useful in industrial applications, because of the fabrication speed, surface coverage, control over PS spheres and cost of the process.

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1.   Introduction
  • Two-dimensional colloidal crystals have been attracting a great deal of attention in science and technology because they have been applied in multiple fields such as biosensors[1], nanolithography[2], data storage[3], photonics[4] and plasmonics[5]. The monolayered nanometer-scale patterns have been required in a variety of devices such as electrical, magnetic, optical and biological devices. A large number of methods have been developed to create 2D colloidal crystals, such as dip-coating[6], vapor-liquid-solid growth of nanowires[7], electric field assisted self-assembly[8], Langmuir-Blodgett deposition[9], defined area evaporation induced assembly[10] and floating-transferring[11], electron beam lithography[12], and spin-coating[13]. X-ray lithography and electron-beam lithography have been used to make such fine structure patterns; however, these techniques have some limitations in economical applications to large areas. The use of nano-sphere lithography has attracted attention as a promising process in order to fabricate ordered nanostructures over broad scopes; it is very advantageous because of its high-throughput, materials availability and the capability of producing well-ordered 2D arrays. Vogel et al.[11] demonstrated the formation of binary colloidal monolayers over a wide range of size and number ratios using a Langmuir trough. Hulteen et al.[14] reported the fabrication of single and double layers of polystyrene(PS) spheres using spin coating. Dimitrov \textit{et al}. developed monolayer PS spheres useful for industrial applications via dip-coating[15].

    The large area substrates would require the use of a large amount of spheres and large baths, which in turn would increase the length of the process and complicate the fine control of the different parameters involved. Spin coating is a simple and versatile process for constructing the wafer-scale colloidal monolayer required for the semiconductor industry. Here, we report on the fabrication of a highly oriented close-packed monolayer of colloidal PS spheres with high surface coverage on a p-silicon wafer using the three-step-spin coating process. The influence of surface treatment, spin speeds, spinning time, and concentration of PS particles in the solution on the surface coverage and the uniformity of monolayers on the wafer is investigated. The effect of oxygen plasma etching on the reduction of PS spheres is also studied. The distribution of particles has been analyzed.

2.   Experimental
  • The p-type Si wafer was cut into 2 $\times$ 2 cm$^{2}$, and then the wafer was treated for ultrasonication in acetone, ethanol and deionized (DI) water for 20 min, 10 min and 5 min respectively. After that, the wafer was taken out of the solution and dried by ambient air flow. This procedure was used to remove the surface contamination of silicon wafer. Prior to use, the ultrasonicated silicon wafer was immersed into the oxidant solution containing a mixture of H$_{2}$SO$_{4}$ (97 %) and H$_{2}$O$_{2}$ (35 %) with a volume ratio of 3 : 1 (v/v) for 12 h for surface treatment. Then the wafer was rinsed with DI water sufficiently and dried with air flow. Subsequently, the wafer is transferred into the solution of H$_{2}$O : NH$_{4}$OH : 30 % H$_{2}$O$_{2}$ (5 : 1 : 1 v/v/v) at 80 C to render the surface hydrophilic, washed with DI water several times and dried gently by air flow.

    After the above treatment, the wafer was placed on a spin coater and allowed to spin under controlled conditions. The three-step spin coating process was used in this experiment, as shown in Figure 1 schematically. The polystyrene particles (size $=$ 284 nm) in our investigation were purchased from microparticles GmbH as a 10 % wt/v in solution with standard deviation 0.01 $\mu $m. The PS spheres monolayer was self-assembled on substrates by a spin coating process (1 : 1 v/v %, silica colloidal solution : ethanol) and then 200 $\mu $L suspension was dropped on the substrates fixed on a spin coater at atmospheric pressure. The various spin coating preparative parameters (such as first step spinning rate 100-500 rpm and spin time 10-40 s, second step spinning rate 500-2000 rpm and spin time 20-80 s, third step spinning rate 2000-8000 rpm and spin time 5-20 s, concentration of colloidal silica particles 3-10~wt %) were varied in order achieve a highly oriented monolayer of close-packed thin films of PS particles. The oxygen plasma etching time (0-50 s), DC power (10-30 W), and oxygen flow (20-50 sccm) were varied for size reduction analysis. The structure and morphology of the resultant colloidal crystals were examined by the field emission scanning electron microscope (SEM, Hitachi S-4800).

3.   Results and discussion

    3.1.   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.

  • 3.2.   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.[16] so that the mobility of the particles near the substrate surface decreases rapidly.

  • 3.3.   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.

  • 3.4.   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.

4.   Conclusions
  • We have demonstrated a size-controllable NSL technique based on spin-coating of polystyrene nanospheres for large area monolayer successfully. The polystyrene nanospheres will be used as a mask for the fabrication of different nanostructured II-VI, III-V semiconductors. By adjusting the etching time, oxygen flow, and DC power, we could obtain the desired bead sizes and hence control the shape and the on/off ratio on a nano-meter scale. This method is expected to have various applications in science and technology.

Figure (11)  Reference (16) Relative (20)

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