Study of Ti etching and selectivity mechanism in fluorocarbon plasmas for dielectric etch

Abstract

The authors studied the behavior of Ti hardmasks in CF4/Ar and C4F8/Ar discharges using conditions relevant to pattern transfer processes into organosilicate glass (OSG), a reference low-k material investigated in parallel. The authors examined various material erosion stages and determined the dependencies of etch rates (ERs) and etching selectivities (ESs) on the following plasma parameters: self-bias voltage (50–150 V), processing pressure (20–60 mTorr) and %CF4 (10–30 %) in CF4/Ar discharges, and O2 addition (0–10 %) and N2 addition (0–20 %) to C4F8/Ar discharges. Erosion behavior and ERs were characterized by real-time ellipsometric measurements and multilayer optical modeling. These measurements were complemented by x ray photoelectron spectroscopy to study the surface composition. The impact of plasma parameter changes were investigated by comparing ERs and corresponding ESs (OSG ER/Ti ER). During the erosion of Ti, the initially oxidized film surface was transformed into a TiFx layer (x ∼ 3) covered by a FC film. The FC film thickness strongly depended on the FC feed gas and was significantly thicker for the C4F8-based etch (1.5 nm) than for the CF4-based etch (0.9 nm). Ti erosion was found to be dependent on the energy deposited on the film surface by ion bombardment and to exponentially decrease with increasing FC film thicknesses. For thin FC films (< 1 nm), erosion was ion driven, i.e., “chemical sputtering”, and, for thick FC films (> 1 nm), erosion was limited by the amount of F that could diffuse through the FC layer to the Ti interface. In contrast to organic masking materials, Ti hardmasks have lower ESs for the more polymerizing C4F8-based discharges than for CF4-based discharges. This can be explained by the consumption of the limited supply of F at the OSG surface by C and H impurities, which form volatile CF4 and HF etch products. For thin FC films and low ion energy deposition by ion bombardment, ESs up to 15 have been achieved.