Modification of poly(styrene) thin films and enhancement of cryogenic plasma etching resistance by ruthenium tetroxide vapor staining

Abstract

Poly(styrene) thin films, typically 80 nm thick, were stained using ruthenium tetroxide vapors and studied using AFM, XPS, ToF-SIMS, ellipsometry and tested as masks for cryogenic plasma etching. Staining induces the formation of two layers in the films. In the surface layer (less than 20 nm thick), Ru–O–C bonds are mainly observed at low staining times whereas CO and Ru–O bonds are present with longer staining times. This surface layer contains an atomic concentration of Ru up to 9% and exhibits low etch rate (less than 3 nm min−1) which accounts for the good etching resistance of the stained film. In the inner part of the film, ruthenium is also present, but within a concentration less than 1% at. which is not high enough to significantly modify the etch rate in comparison to unstained PS film. The use of Ru stained poly(styrene) film as masks for cryogenic plasma etching can be beneficial since at the staining saturation, the overall etch rate is divided by 7 compared to unstained film and the selectivity Si:PS increases from 10:1 to 70:1.