Plasma etching uniformity control for making large and thick dual-focus zone plates

Abstract

A typical zone plate structure is composed of concentric rings with a radially decreasing feature size such that the path of light through every second zone to the focus differs by one optical wavelength. In order to minimize the wavefront error, the etching depth has to be controlled uniformly across the whole substrate area to a phase difference of one half wavelength between adjacent zones. In this study, an optimized plasma etching process has been developed to minimize the feature size variation effect and a Teflon ring has been made to eliminate the edge effect on thick fused silica. A large phase type dual-focus zone plate with varied concentric ring sizes with good etching profile and better than 2.0% etching depth uniformity was successfully made.