Studying fabrication induced phase errors in CGHs using RCWA

In this paper we discuss the impact of fabrication errors on the generated phase for diffractive null optics. Diffractive null optics are computer generated holograms (CGH) that are used for the testing of high precision aspheres and freeform surfaces. As the trend towards steeper aspheres demands higher line densities, it is no longer sufficient to neglect rigorous effects during the design and the fabrication of the CGHs. The sensitivity of the generated phase for binary gratings and blazed gratings was studied using Rigorous Coupled Wave Analysis (RCWA). Binary gratings with periods of 1 μm and parameter changes of only 1% in line width (DC) and height already showed phase changes above λ/100 (used wavelength 633 nm). To measure the generated phase, ellipsometry was applied and ways to solve the inverse problem of reconstructing the generated phase by intensity measurements were investigated.