Excimer laser-assisted etching of polysilicon at 193 nm

Abstract

Excimer laser-assisted etching of polysilicon at 193 nm was studied in the presence of CF₃Br, CF₂Cl₂, and NF₃. In the presence of 193 nm radiation, CF₃Br showed some propensity to etch polysilicon, while CF₂Cl₂ did not show any appreciable etching. In the presence of NF₃, maximum etch rates of 0.6 Å/pulse were obtained for pressures greater than 500 Torr and fluences exceeding 200 mJ/(cm² pulse). The etch rate increased with both fluences and pressure to a limiting value of 0.6 Å/pulse. An adsorptive etch mechanism was proposed, where NF₃ molecules diffuse to the surface, adsorb, and then react after absorbing laser radiation. Thermal effects enhance this process and appear to dominate at lower pressures (<400 Torr) and higher fluences. Etching caused by the gas phase formation of F atoms is minimal due to the low absorption cross section of NF₃ at 193 nm. Etching of submicron profiles in polysilicon was also examined. Polysilicon samples masked by patterned SiO₂ were exposed to NF₃ and 193 nm ArF radiation. Subsequent scanning electron microscopy (SEM) analysis demonstrated directional etching with some surface roughening.