Calculation of duty cycle of beam sampling grating mask and analysis on diffraction efficiency uniformity of beam sampling grating

Abstract

Cr mask is applied to fabricate beam sampling gratings (BSG) used in inertial confinement fusion (ICF). There are requirements on both first-order average diffraction efficiency and uniformity diffraction efficiency of BSG, so duty cycles of Cr mask grating must be measured to analyze the diffraction performance. The special Cr mask is one kind of grating with curved fringes and differing periods. In this paper, a method which can calculate duty cycles of Cr mask by measuring the zeroth-order diffraction efficiency is introduced. Based on rigorous coupled wave analysis (RCWA) theory, this method takes curved fringes as straight, and then calculates duty cycles under certain diffraction efficiencies by using the drawing of the change of diffraction efficiencies against the change of duty cycles and periods. A duty cycle can be found at a very diffraction efficiency and period. With duty cycles obtained, average diffraction efficiency and diffraction efficiency RMS can be calculated at certain etch depth. Also an example is given.