To study the effect of HF concentration, both bare silica substrates and Au NP (size G 2 ) coated silica substrates were etched with different concentrations of HF ranging from 0 to 50% (vol%) for 30 min. From the preferred size range of Au NPs suitable for nanoparticle-masked HF etching, the higher particle size (G 2 ) was selected for the present study. Transmittance at 550 nm and surface roughness vs. HF concentration are shown in Figure . SEM images of the etched substrates are shown in Figure and the width of fabricated surface structures are plotted in Figure . It is observed that the transmittance decreases while surface roughness increases with increasing HF concentration. It is known that higher HF concentration results in intense etching and leads to higher surface roughness. The same is observed from the SEM images (Figure ) that relatively smaller size surface structures are formed at 10% HF concentration and the widths of the surface structures increase with HF concentration. Well-arranged arrays are observed at 30% HF concentration whereas short arrays with larger pit width are observed at 40% and 50% respectively. The width of smaller (circular) pits varies from 0.59 ± 0.13 to 4.81 ± 1.89 µm, whereas elongated pits vary from 0.87 ± 0.21 to 9.59 ± 3.34 µm with HF concentration. Similarly, the width of the arrays changes from 1.97 ± 0.29 to 11.32 ± 2.31 µm. The difference in transmittance between the bare silica and Au NP masked silica after etching is evident for HF concentrations of 30% and beyond. Hence, it is concluded that a minimum HF concentration of 30% is preferable for the etching of bare silica substrate masked with Au NPs of size 35 nm.