## 3 Results and Discussion
### Silica Sphere Size Control by RIE  
One of the biggest advantages of utilizing SS for the nano-/micro-sphere lithography process is that silica material (i.e., SiO\({}_{2}\)) can provide high etching selectivity to other semiconductor materials, such as Si to adjust the scale of the surface structures. Currently, most reported nano-/micro-sphere lithography (NSL) processes utilize a polystyrene sphere (PS), which reveals a soft material nature that can be problematic to produce repeatable dimensions and well-defined patterns on the lithographic surface. In this report, we successfully demonstrated an enhanced SS size controllability with a RIE process and we believe that this can offer a great opportunity to fabricate SWSS in the desired dimensions. For effective size reduction of SS, two criteria are required. They are: (1) high etching selectivity of SS to Si substrate; and (2) effective horizontal direction etching of SS followed by minimized vertical etching. The high etching selectivity of SS to Si would prevent excess etching of Si surface, which is crucial to produce SWSS fabricated surface geometry in a desired way. In addition, well-controlled horizontal direction etching of SS is required to adjust the dimension of a projected pattern after mask  
Figure 1: Schematic illustration to compare \(N\),\(N\)-dimethyl-formamide (DMF) and water for spin-coating application. Figure 2: (a) Picture of silica sphere (SS) coated Si wafer (2-inch diameter) and (b) scanning electron microscope (SEM) images of SS coated Si surface. layer deposition, but vertical direction etching must be minimized to permit clean SS removal and mask layer lift-off. Based on these considerations, a CHF3/Ar gas mixture is used for the SS etching process and has optimized its etching conditions by changing gas flow rates, RF power, and chamber pressure. After the investigation of each RIE result, we found that the chamber pressure is the most critical knob to produce high SS etching selectivity and a low vertical/horizontal etching ratio. For the experiments, three different chamber pressures were studied, which are 50, 75, and 100 mTorr, and the results are shown in Table 1. From Table 1, we observed that noticeable Si substrate etching was made after etching at 50 mTorr, but as chamber pressure increases to 75 and 100 mTorr, the Si substrate etching was significantly reduced, which is possibly due to the decreased mean-free path length of ionized-species at higher pressure [30]. However, even though significantly reduced Si substrate etching was observed for both pressures of 75 and 100 mTorr, 100 mTorr revealed a higher vertical/horizontal etching ratio of 3.9 compared to 2.6 from 75 mTorr. As mentioned in the earlier section, this excess etching rate in the vertical direction is not ideal for SS lithography because fast vertical diameter reduction of SS compared to that of the horizontal direction restricts our choice on the pattern sizes, which weaken the ability of the SS to pattern the surface as SS size decreases. As shown in Figure 3, for the same SS target horizontal diameter, 800 nm, 100 mTorr chamber pressure produced a reduced vertical SS diameter of 260 nm, compared to 360 nm from 75 mTorr, while the horizontal diameters were sustained at 800 nm for both. Therefore, we decided to use 75 mTorr chamber pressure for SS size reduction with RIE. In Figure 4, we show 1.57 um diameter SS after different RIE process times (10-min, 13-min, 16-min, 19-min) under our optimized RIE condition. From Figure 4a-d, we confirmed that highly effective SS size reduction is possible with minimal Si substrate etching. In addition, from Figure 4e, the measured inter-particle spacing with different etching times reveals that SS size can be easily reduced in a desired scale due to its reasonably linear relation between decreased SS diameter and etching time.