Analysis of the impact of different additives during etch processes of dense and porous low-k with OES and QMS

The focus of this paper is the impact of CF"4 based plasma etch processes with the additives argon and C"4F"8 on material properties and geometrical parameters of etched trenches using dense and porous SiCOH. Argon and C"4F"8 were added to change the radical to ion composition and to shift the carbon to fluorine ratio, respectively. With several techniques, FTIR, spectral ellipsometry and contact angle measurements, modifications in the structure of the materials and their surface conditions were analyzed. To understand the influences of the additives on the plasma conditions, optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS) were used to estimate the composition of the plasma insitu. For the additive argon, a slightly enhanced etch rate and an increased refractive index due to serious plasma damage for porous SiCOH was observed. At higher Ar flow rates peaks of Si"2O"4H"x clusters in the QMS spectra and increased CO and O lines, measured with OES, indicate a higher sputter yield on the SiCOH network. SEM cross-sections show, that argon has no effect on the sidewall geometry of etched trenches. A higher CH/CN line in the OES spectra indicates an enhanced sputter effect of the SiCN films in via bottoms. For C"4F"8 addition results of spectral ellipsometry show a decreased etch rate and refractive index. Using FTIR the formation of a polymer film on the surface was observed. Higher C"2 lines in the OES spectra are indications of enhanced polymerization efficiency. Finally, the addition of C"4F"8 decreases the etch rate in the trench sidewalls and therewith assumedly the sidewall damage.