A subwavelength structured (SWS) surface, which is the surface-relief grating with the period smaller than the light wavelength, behaves as antireflection surface. The SWS surface with deep tapered shape grating especially suppresses the reflection over a wide spectral bandwidth and a large field of view. Here, SWS surfaces consisting of hexagonal gratings have been fabricated on crystal silicon substrates using an ordered anodic porous alumina (OAPA) mask and fast atom beam (FAB) etching .
The process steps used in the fabrication of the SWS surfaces are shown in Fig.1. For the preparation of the OAPA membrane as the etching mask, we use a two-step anodizing process.
Figures 2(a) and (b) show the fabricated SWS surfaces. The etching times of FAB are 30 min and 50 min for the results shown in Figs.2(a) and 2(b), respectively. The SWS surface in Fig 2 have holes in hexagonal arrangement, the period of the holes is 100 nm and they are about 700 nm deep. The etching rates of silicon and OAPA membrane are 23.3 nm/min and 9.35 nm/min, respectively. Since the hole diameter is 60 nm, the aspect ratio of the etched hole is 12. The 60 nm diameter alumina holes of the mask have been well transferred to the Si substrate. The SWS surface shown in Fig.2(b) is obtained by over etching of that shown in Fig.2(a). In the bottom layer, the etched substrate also consists the circular holes in the hexagonal arrangement. In the upper layer, however, Si surface is tapered periodically at 6-fold rotation symmetry since the thin walls of the hexagonal structure are partially removed by the over etching. Therefore the smallest distance between the elongated structures in the upper layer is 60 nm.
Figure 3 shows reflectivities measured as a function of the wavelength. The incident light is randomly polarized. The dashed and solid curves show the reflectivities of the polished Si substrate and the SWS surface shown in Fig. 2(b), respectively. It is shown that the SWS surface decreases the reflection drastically compared with that of the polished Si surface. The reflectivity of the SWS surface is lower than 1.6 % at the wavelengths between 370 nm and 800 nm. The theoretical calculations were carried out by using the rigorous coupled-wave analysis (RCWA) proposed by Moharam. The calculated reflectivity agrees well with the measured results.