Fabrication of photonic band gap structures in As40S60 by focused ion beam milling

We report on the use of focused ion beam (FIB) milling to produce photonic band gap (PBG) structures in As40S60 for applications in the near/mid-infrared (IR) spectral range. Kinetic milling parameters such as FIB current and spot-size have been adjusted in combination with chemically increased etching using iodine (I2) to increase sputtering yield and improve sidewall verticality of the square holes. FIB milled cross-sections show that square holes deeper than 8 μm with aspect ratios ∼6:1 and a lattice spacing ∼2 μm can be milled with this technique. These structures are of dimensions suitable for operation in the wavelength range ∼4.5–5.33 μm where the interior absorption coefficient of As40S60 is approaching its minimum. Sub-micron holes with widths 0.66 μm and depths greater than 3.77 μm were milled using FIB currents of 12 pA with I2 to facilitate chemically enhanced sputter yields of up to ∼60.