Effect of temperature on deposition layer formation in HBr/N2/fluorocarbon-based plasma

The effects of wafer temperature on etching rate and surface composition were investigated to clarify the surface reaction mechanism under HBr/N2/fluorocarbon-based gas plasma for developing a process for three-dimensional NAND flash devices. The etching rates of both polycrystalline silicon (poly-Si) and SiO2 were found to increase at a wafer temperature of 20 °C as compared with those at 60 °C. Comparing the gas combination of fluorocarbon/N2 and HBr/N2 mixtures, the temperature dependence of SiO2 etching rates was considered to relevant to the sticking probability of fluorocarbon polymers. To determine the cause of the temperature dependence of the poly-Si etching rate, surface composition was evaluated by thermal-desorption-spectroscopy and laser-sputtered-neutral-mass-spectrometry analyses. Ammonium bromide was confirmed in the deposition film at a wafer temperature of 20 °C. The observed increase in poly-Si etching rate at lower temperatures was possibly caused by increased amounts of nitrogen, hydrogen, and bromine fixed to the surface with the formation of ammonium bromide.