Surface sulfurization of amorphous carbon films in the chemistry of oxygen plasma added with SO2 or OCS for high-aspect-ratio etching

Etching of oxygen-based plasmas with sulfur dioxide (SO2) or carbonyl sulfide (OCS) can form high-aspect-ratio (HAR) features of amorphous carbon films as carbon hard masks (CHM). The etched profiles showing shapes such as bowing or tapering are essentially dependent on the partial pressures of SO2 or OCS in the O2 plasma. The surface treated after the OCS-added plasma exhibited strong sulfurization by S2 and CS species in S 2p of the X-ray photoelectron spectroscopy (XPS). The gas-phase interactions in the sulfur-oxygen-carbon system generated atoms and molecules, such as O, O+, and O2+, which etched at the bottom and, conversely, SO, CO, CS, CS2, and S2, which inhibited isotropic etching at the sidewalls of the HAR features. The chemical reactions of the CS sulfurizing precursors in the gas phase were monitored by comparing their optical emission intensities at a wavelength of 257 nm with those of SO2 at approximately 320 nm. The optimization of the HAR profiles of the CHM can be controlled by sidewall sulfurization of the CHM to obtain desirable profile shapes for the HAR features.