Antireflective "Moth Eye" Structure for Use at Visible Wavelengths: Fabrication and Test Results

Abstract

Previous experimental (1) and theoretical (2) work has shown that onedimensional gratings having high spatial frequency and simple surface relief profile can produce extraordinary antireflective properties (very low reflectivity over a broad waveband). Such structures are said to show the "moth eye" effect. In previous work, actual devices were made by using laser interferometry in photoresist to define the surface regions for subsequent etching. We have taken a binary optics approach to fabricate a doubly periodic, two-dimensional array of square pits that acts as an effective antireflective structure over the visible spectrum. We used direct-write electron-beam lithography to generate the two-dimensional pattern of squares in high-contrast photoresist deposited directly on fused quartz substrates. The squares were nominally 250 nm on a side and spaced at 350 nm intervals in both x and y directions. After development, the exposed square regions were etched to a depth of 110 nm by reactive ion etching. A SEM photo of the structure is shown in the figure. Areas as large as 1 cur have been fabricated for testing. According to the theoretical model of Gaylord and Moharam (2), our structure should have zero reflectivity at 540 nm and less than 0.5 percent reflectivity at 480 nm and 620 nm. Although our samples show some scatter in the blue, we present measured reflectivity data which agree well with the predicted values. We have also fabricated test devices containing controlled variations in pit depth or width, and measurements of these are presented with comparisons to theoretical values.