The fabrication of the patterned substrates was carried out using conventional contact alignment optical lithography and dry etching processes. Thermally grown SiO2 (250 nm thick) on standard 100-mm-diameter, (100) Si wafers (University Wafer) were used. To pattern the silicon dioxide, an adhesion promoter, MicroPrime MP-P20 (Shin-Etsu MicroSi Inc.), was spin-coated at 4000 rpm, followed by the photoresist coating (MEGAPOSIT SPR955-CM 0.7, MicroChem Corp.). Coated wafers were then soft-baked on a hot plate at 90°C for 90 s, exposed to 40 mJ/cm2 of ultraviolet radiation using a SUSS MicroTec mask aligner, and postexposure-baked at 115°C for 90 s. The development process was carried out in a CD-26 developer (<5% tetramethylammonium hydroxide, MicroChem Corp.) for 1 min, rinsed with deionized (DI) water, and dried with nitrogen. After 1-min exposure to oxygen plasma at 100 W to remove any residual resist on the exposed area, the patterned SiO2 was etched in an inductively coupled plasma (ICP) ion etching system (Oxford Plasmalab 100). The etch process was carried out in a mixture of 45-sccm (standard cubic centimeters per minute) SF6 and 2-sccm O2 at substrate temperature of 15°C. The etch depth or the height of the features was determined by the etching time. An etch time of 3 s generally resulted in an approximately 20-nm etch depth. Removal of the photoresist was carried out by exposure to oxygen plasma for 10 min, consisting of 100 sccm of oxygen at 10-W radio frequency power and 2000-W ICP power.

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