Optical properties of thin film CrxNy–CrOxNy multilayer films for 157 nm optical lithography

Abstract

Cr x N y and CrxOyNz thin films and multilayer CrxNy–CrxOyNz thin films have been investigated as a binary masking material for 157 nm lithography. The chemical compositions of commercial photolithography masks were determined to be a bilayer structure of Cr0.85N0.15 with a top anti-reflection layer of graded CrxOyNz with a total thickness of ∼100 nm. To simulate the commercial structures, CrxNy and CrxOyNz thin films were reactively rf sputter deposited from a metal chromium target in various Ar–N2 and Ar–N2–O2 environments and their optical properties examined. After replicating the standard ∼100-nm-thick bilayer structure, the process was scaled to ∼50 and 80 nm by: (1) thinning the CrxNy layer and (2) by thinning both the CrxNy and CrxOyNz layers proportionally. The vacuum ultraviolet optical properties of the thinned masking materials were evaluated and the 80 nm features were determined to have the desired optical density >4 and the reflectivity of <15%. Modeling of the optical properties agreed well with the experimental results.