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 (<150 mTorr) and in NF3 plasmas diluted with inert gases.