###### Abstract  
In this paper, we present a novel silicon (Si) subwavelength-scale surface structure (SWSS) fabrication process using the silica sphere (SS) lithography technique, which allows controllable geometries. The process involves a new cost-effective solvent-controlled spin-coating method that deposits SS on large Si surface areas with enhanced SS monolayer coverage and uniformity compared to conventional methods. The larger areas and rapid, low cost processing allow colloidal sphere lithography to be realistically used for solar cells. We successfully demonstrate 1.57 \(\upmu\)m diameter SS on a 2-inch round Si wafer with more than 95% SS monolayer coverage and great uniformity. Using these deposited SS, a SWSS fabrication process was designed and successfully demonstrated Si inverted pyramid structures with dimension on the order of 1.1 \(\upmu\)m, thus potentially providing a new technique for effective light-management of thin crystalline Si solar cells.  
inverted pyramid; silica sphere lithography; sub-wavelength; spin-coating; solar cell Article Sub-Wavelength Scale Si Inverted Pyramid Fabrication with Enhanced Size Control by Using Silica Sphere Lithography Technique  
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In this paper, we describe a novel SWSS fabrication process that enables the processing of Si inverted pyramid structures in a controlled scale on large Si surface areas. The Si surface patterning has been done with a silica sphere (SS) lithography technique to replace the conventional multi-step photolithography method. We have developed a solvent-controlled SS spin-coating method that enables deposition of a highly uniform SS monolayer on large surface areas [13]. Previous SS spin-coating methods based on water as a solvent require additional surfactant mixtures for (1) enhanced SS dispersion in solution, and (2) delayed solvent evaporation rate because of the properties of water [14]. Although surfactant addition offers significantly enhanced solution properties compared to pure water for uniform SS monolayer spin-coating; it is successful only for small area applications (e.g., 1-inch \(\times\) 1-inch). Other conventional SS deposition methods including Langmuir-Blodgett coating and substrate surface functionalized spin-coating are not suitable for manufacturing due to their slowness and/or sensitivity to the processing environment [15; 16; 17]. To realize large area SS lithography with a simple and low-cost spin-coating approach, we use \(N\),\(N\)-dimethyl-formamide (DMF) as a solvent and confirm that DMF can provide (1) a highly uniform close-packed SS monolayer with (2) minimal sensitivity to the spin-coating environment (e.g., vibration, humidity, and temperature), in addition to (3) large area SS deposition capability. Here, we demonstrate more than 95% SS monolayer coverage with outstanding uniformity on a 2-inch round Si wafer just with a simple spin-coating process performed in a non-environment controlled atmosphere.  
One major advantage of SS lithography for SWSS fabrication is that SS offers highly flexible dimensional control of the structures. The period of fabricated structures can be easily changed by introducing different size of SS. In addition, the size of structure itself also can be adjusted by SS size reduction with dry etch process after SS spin-coating. In this work we utilized an optimized reactive-ion etching (RIE) condition designed mainly to etch the SS (i.e., SiO\({}_{2}\)) based on a CHF\({}_{3}\) plasma assisted RIE process. We further demonstrate effective SWSS fabrication, especially, a well-arrayed inverted pyramid structure with dimension on the order of 1.1 \(\upmu\)m and conclude that this will provide a more simplified and large-area applicable SWSS fabrication approach compared to conventional methods.