Inductively Coupled Plasmas For Highly Efficient and Low Damage Resist Stripping

A new plasma source using inductively coupled plasma (ICP) excitation has been developed that offers several advantages over more commonly used microwave-based sources, including low generation of damaging species such as charged particles and UV radiation and less heat production by the plasma. The new ICP source is inherently more efficient in producing the neutral reactive (atomic and molecular) oxygen species active in the strip process. A range of damage and contamination tests, including measurements of wafer charging, mobile metal and heavy metal contamination, and minority carrier lifetime degradation, confirm that the ICP source produces less wafer charge damage and contamination than typical microwave systems. This paper discusses the fundamental physics of the ICP source and presents results of computer modeling studies using a Boltzmann code which explain the overall increase in performance. The calculations give the distribution of electron energies in the plasma along with the rate constants for the high energy inelastic collision processes which produce oxygen dissociation, ionization, and UV excitation. These calculations show that in the ICP source, less than 0.3% of the power goes into the production of charged particles and UV radiation, and about 17% goes into the production of heat. The ICP source therefore requires less heavily baffled gas flow and generates a much lower heat load on the plasma chamber walls, making it much more compatible with advanced process chemistries such as those using fluorine.