The fabrication process is schematically illustrated in Fig. 1. A
silicon substrate is first cleaned in piranha solution (3:1, H2SO4
98%:H2O2 30%) at 80 C for half an hour, followed by rinsing repeatedly
with ultrapure water and is subsequently dried with nitrogen.
Image reversal photoresist AZ 5214-E from MicroChemicals GmbH
is dispensed to cover 40% of silicon substrate area and then spuncoated
at 2000 rpm to obtain a 2 lm thick photoresist layer. This
process is followed by pre-bake at 110 C for 120 s. Five microliter
of 10 wt.% aqueous suspensions PS nanospheres with 500 nm
mean diameter (Brookhaven Instruments Ltd.) is diluted in distilled
water and the mixture is deposited on the photoresist surface
using a micropipette and spread evenly. After forming the closepacked
arrays of nanospheres over the photoresist, the sample is
exposed to UV light using Canon PLA-501FA mask aligner with
400 nm wavelength at low exposure energy. The PS nanospheres are then washed off in distilled water using ultrasonic bath and followed
by developing process of photoresist in AZ 351 developer,
rinsing by DI water and drying by nitrogen. All aforementioned
steps were preformed in class-100 clean room.