Fourier-transform infrared absorption spectrometry measurements of a CF4 discharge in an electron cyclotron resonance reactor

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Abstract
Knowledge of the neutral gas composition in a discharge is important for understanding the chemical processes involved in both etching and deposition environments. We have performed Fourier-transform infrared absorption spectrometry measurements of a CF4 plasma in an electron cyclotron etching tool. Spectral bands from both CF4 and CF3 were observed. The densities of these species were calculated from the strength of the measured absorption. It was found for typical discharge powers that the CF4 density was approximately 20% of the total density and depended only on the plasma density. This data is used to show that recycling of the daughter species (C, CFx, Fx, etc.) on the chamber walls is the major source of the CF4 observed. Thus one finds that wall conditions play a very important role in determining the plasma chemistry in this high density, ≳1011 cm−3, and low neutral pressure, <10 mTorr, discharge. Additionally, it was found that the CF3 density was approximately 20% of the total density and depended on the product of the plasma density and the CF4 density. This implies that the CF3 is produced primarily through an e−+CF4→e−+CF3+F type of reaction. Finally, it was found that the upper limit of CF2 density was 1.6×1013 cm−3 over the entire parameter range explored.

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