Study on polymeric neutral species in high-density fluorocarbon plasmas

Abstract

Production and extinction processes of polymeric neutral species (CmFn;m⩾2) in electron cyclotron resonance C4F8 and CF4 plasmas have been studied by using a quadrupole mass spectrometer (QMS) employing low-energy electron attachment technique. This technique allows the detection of electronegative CmFn species as negative ions by scanning the attaching electron energy in the QMS typically in the range of 0–10 eV. In addition to the most abundant F− and CF3− signals resulting from dissociative attachment to various fluorocarbon species, pronounced attachment resonances of negative ions corresponding to the series of CmF2m±1− such as C3F7−, C4F9−, and C5F9− were primarily observed especially at low microwave powers and high pressures. The C4F8 plasma contained a large amount of polymeric species and a high fraction of reactive F-stripped species as compared to the CF4 plasma, providing evidence of a high potential of gas phase and surface polymerization in a low F/C ratio plasma. The amount and composition of polymeric species were examined by varying gas residence time and diluted hydrogen or argon concentration. At 20 mTorr, the overall amount of polymeric species was suppressed by enhanced gas flow with decreasing residence time, while a fraction of F-stripped species was increased. The amount of polymeric species was also suppressed with increasing diluted hydrogen, and the different behavior in the two plasmas was interpreted as the result of interactions between H atoms and polymeric species. The results provide insights into the kinetics and chemical activity of polymeric species in a high-density plasma as a practical etching source.