Insight into the dry etching of fence-free patterned platinum structures

Abstract

An investigation was undertaken to characterize the time progression of the Pt etch process during the dry etching (DE) of fence-free patterned structures. The goal of this study was to clarify the mechanism of fence-free Pt etching. In particular an explanation was sought that could account for the origin of the extremely shallow sidewall angles typically reported in the literature for fence-free Pt etching. The experiment consisted of coprocessing two oxidized Si wafers possessing identical 250 nm thick Pt film layers, but different photoresist (PR) mask thicknesses. Etching was suspended at 20%, 40%, 60%, and 80% of the full etch process in order to cleave off small pieces of wafer for analysis by scanning electron microscopy (SEM). Using Cl2-based DE conditions known to produce fence-free etching for 250 nm thick film layers, we found that a severe fence actually coats the PR mask during the first 20% of the etch process! As the etch continues the fence structure evolves, achieving a maximum height and width followed by progressive recession until disappearing completely prior to process endpoint. SEM micrographs clearly reveal how the existence of these transient fence structures account for the formation of shallow Pt sidewalls. The data also shows that the final profile of an etched Pt structure possesses a functional dependence on the initial thickness and slope of the PR mask as well as on the initial thickness of the Pt layer. Finally, we believe that the observed behavior of the transient fence provides the strongest evidence to date supporting the existence of a chemically assisted physical sputtering component associated with the DE of Pt films in halogen-based plasmas.