Gas-Phase Wafer Cleaning Technology

This chapter describes the activation of a gas-phase or a gas-surface reaction using heat or light. The atoms and molecules that take part in the chemical reactions are neutral. Neutrals produced by a downstream plasma source are not considered here. The pressures span atmospheric pressure to the mTorr range, with concentrations and temperatures at or close to the condensation point. After discussing the processing window for gas-phase processes and comparing them to similar liquid-phase processes, several enabling applications are listed. The chemistries and mechanisms are described first in general for any gas-surface reaction followed by details of the primary processes in use today: oxide removal with HF vapor, organic and resist removal with ozone, and metal removal with chlorine activated by ultraviolet light. The gas-phase cleaning processes are those a process engineer is likely to encounter: front-end-of-line including isolation, gate and prefurnace, and pre-epi modules; middle of line contact; and back-end-of-line and hard mask trim and nanostructure formation, polysilicon emitters, and pre-thin film measurement. Finally, the gas-phase processing equipment in use today is described. Although gas-phase cleaning has not displaced liquid-phase for integrated circuit manufacturing steps as initially anticipated, the selectivity that is possible in the gas-phase and its vacuum compatibility have enabled several niche applications.