Plasma etching of sputtered Mo and MoSi2 thin films in NF3 gas mixtures

Abstract

Plasma etching characteristics of sputtered molybdenum and MoSi₂ thin films using various NF₃ gas mixtures in a planar reactor are presented. Anisotropic (vertical‐to‐lateral etch ratio of ∼3) edge profiles were obtained. The etch rates of Mo, MoSi₂, doped poly‐Si, and SiO₂ were determined as a function of rf current, reactor pressure, and NF₃ concentration. At 1 A and 100 mTorr in 100% NF₃, etch rates of 2490, 3440, 14000, and 670 Å/min were measured for Mo, MoSi₂, doped poly‐Si, and SiO₂, respectively. Also, the etch rate ratios of Mo, MoSi₂, doped poly‐ Si over SiO₂ were 1.5–4, 4–8, and 12–24, respectively. Diluting the NF₃ plasmas with argon or helium decreased the etch rates for all the materials studied here. At 200 mTorr and 1 A, the Mo etch rate dropped from 1800 Å/min in 100% NF₃ to 540 Å/min in 20% NF₃/80% Ar, while the corresponding silicide etch rate decreased from 7850 to 1130 Å/min. Auger spectroscopy measurements inferred that the desorption of molybdenum products, but not the silicon ones, may be the rate‐limiting step in the etching process. Furthermore, dc voltage measurements on the rf electrode indicated a higher degree of ion bombardment at low pressures (3 plasmas diluted with inert gases.