A study of the O2-SO2 plasma etch chemistry for top surface imaging photoresist dry development

We have investigated O-2-SO2 plasma chemistry and studied the impact of relevant aspects of the plasma chemistry on top surface imaging (TSI) silylated photoresist dry etch development processes. The studies focused on aspects of plasma chemistry relating to the formation of a polymer deposit postulated to be responsible for profile sidewall passivation. A nonvolatile deposit produced by the etching plasma was observed and analyzed. Mass spectrometry analysis suggested that the deposit exists in the Form of a nonvolatile sulfur oxide. X-ray photoelectron spectroscopy (XPS) analysis of the wafer surface indicated that it is a hydrated sulfur trioxide polymer resulting from chemical reactions of O-2 and SO2 plasma. Our studies also indicated that chamber conditioning produced by sulfur trioxide polymer deposition causes no observable variation in critical dimension (CD) and profile results when evaluated on a Lam TCP 9400SE plasma etcher, nor does our in situ dry cleaning method used to prevent excessive build-up of the deposit. Subsequent pattern transfer to underlying polysilicon layers from the TSI silylated photoresist was found to be little affected by this aspect of O-2-SO2 plasma chemistry. Based on these observations. we postulated that the observed sulfur trioxide polymer deposit may be responsible for profile sidewall passivation and etch anisotropy enhancement associated with SO2 addition to the O-2 plasma for photoresist dry development.