Design, Fabrication and Measurement of Pyramid-Type Antireflective Structures on Columnar Crystal Silicon Lens for Millimeter-Wave Astronomy

Pyramid-type antireflective subwavelength structures for large-diameter (\(> 30\hbox { cm}\)) silicon lenses are promising for broadband astronomical observations. The refractive index and dielectric loss tangent of the lens material, columnar crystal silicon which is manufactured by Mitsubishi Materials Electronic Chemicals Co., Ltd., were measured at around 30 K using a Martin–Puplett-type Fourier transform spectrometer. The measured refractive index and dielectric loss tangent between 200 GHz and 1.6 THz were \(\sim \) 3.42 and 1–\(5 \times 10^{-4}\), respectively. Three different pyramid-type structures with a period of \(265\hbox { }\upmu \hbox {m}\) and depth of \(600\hbox { }\upmu \hbox {m}\) were simulated to obtain their reflectance using an electromagnetic field simulator, HFSS. The structures were fabricated on both sides of a 100-mm-diameter plane-convex lens made of columnar crystal silicon with a 150-mm radius of curvature using a metal-bonded V-shaped blade and a dedicated three-axis machine. The fabrication errors in the period and depth were less than \(10\hbox { }\upmu \hbox {m}\). The reflectance of the lens flat surface was measured using a vector network analyzer to be between \(-8\) and \(-17\) dB in the range of 110–170 GHz, which was consistent with the result from the simulation.