Paper PS+AS+EM+SS+TF-MoA6
Chemical Reaction Probabilities in the Etching of Si by Fluorine Atoms Produced in a Mixture of NF­₃/SF₆ Plasma

Monday, October 21, 2019, 3:20 pm, Room B130

Reaction probabilities in the absence of ion bombardment, defined as the number of silicon atoms removed per incident fluorine atom, have been investigated in mixtures of NF₃ and SF₆ plasmas in an inductively-coupled plasma reactor. Fluorine atom densities were measured by optical emission actinometry, and isotropic etching rates were measured by the degree of undercutting of SiO₂-masked silicon, using cross-sectional scanning electron microscopy (SEM). In addition, atomic force microscopy (AFM) was used to examine surface roughness after etching. The F atom reaction probabilities derived from these measurements indicate ~30-fold higher reaction probability in SF₆ plasma compared with values in NF₃ plasma. Surfaces etched in SF₆ plasma were much smoother than those etched in NF₃ plasma. Addition of only 10% SF₆ to an NF₃ plasma produced a much higher reaction probability (~10-fold) than in a pure NF₃ plasma. This surprising enhancement of reaction probabilities for F with Si in SF₆ plasma will be shown to be due to adsorbed sulfur acting as a catalyst to greatly enhance the etching rate of Si. By allowing sulfur in isopropyl alcohol to evaporate on the masked Si samples, sulfur could be preferentially deposited in relatively high concentrations near mask edges in ~2 mm diameter periodic “strings of beads”. When this sample is placed side by side with one not exposed to sulfur, the sulfur dosed sample etched several times faster at the center of each bead, while sulfur-free surface exhibited the expected slower rate.