Gratings reduce reflection from silicon substrates

Thin-film coatings can suppress Fresnel reflection for devices such as solar cells and electro-optical instruments, but they can have problems with adhesion, thermal mismatch, and the stability of the thin-film stack. Researchers at Tohoku University (Sendai, Japan) have fabricated a two-dimensional subwavelength structured surface on a crystal silicon substrate that dramatically decreases the reflectivity and is more stable than a multilayered thin film. The researchers started with a 200-µm-thick crystal silicon substrate and coated it with a 400-nm-thick electron-beam resist. They drew a pattern on the resist with an electron beam, then developed it to form a tapered resist. The pattern was then transferred through fast atom-beam etching with sulfur hexafluoride and the resist removed. That produced a conical grating with a period of 150 nm and grooves 350 nm deep. The researchers measured reflectivity of wavelengths from 190 to 2500 nm and found a marked decrease, particularly between 200 and 1000 nm, encompassing the visible region. At 400 nm, for example, reflectivity dropped from 54.7% for the silicon substrate to 0.5%.