AVS 52nd International Symposium
    Plasma Science and Technology Thursday Sessions
       Session PS-ThA

Paper PS-ThA10
Characterization and Modeling of a Transformer-Coupled Toroidal Plasma Source for Remote Chamber Cleaning

Thursday, November 3, 2005, 5:00 pm, Room 304

Session: Plasma Sources and Equipment
Presenter: B. Bai, M. I. T.
Authors: B. Bai, M. I. T.
J.J. An, M. I. T.
H.H. Sawin, M. I. T.
Correspondent: Click to Email
The transformer-coupled toroidal plasma source is widely used for remote cleaning of Chemical Vapor Deposition(CVD) chamber, in which fluorine containing gases are dissociated to form fluorine atoms for downstream etching. It was found that with the addition of a small amount of N2(0.5% of the total flow) into the fluorocarbon plasmas(C2F6 or C4F8), the etching rate of silicon dioxide film was doubled, making the etching rate comparable to NF3 plasmas under the same conditions. The increase of etching rate of silicon dioxide when N is added is due to the surface modification of the transfer tube between the source and downstream chamber, where N atoms or N containing radicals block the recombination site to form COF2 and favors the formation of CO2 thereby causing more fluorine atoms to be delivered to the downstream chamber. The toroidal plasma source was experimentally characterized. Rovibrational bands of N2 and C2 were fitted to obtain the neutral gas temperatures for NF3 and C2F6 plasmas, and they were found to be in the range of 3000-6000K, which is consistent with the high power density(>15W/cm3) coupled into the plasmas. The electron temperatures were found to be 1-4 eV, as determined by the line ratios of atomic argon spectrum. The concentrations of fluorine and oxygen atoms in the plasma source were measured by the actinometry technique and other species were detected by both the Mass Spectrometer and the FTIR. It was found that NF3, C2F6, C4F8 all had nearly 100% dissociation while CF4 had only about 50% dissociation, which makes CF4 unfavorable for the remote cleaning. Global and 1-D models were set up to simulate the toroidal plasma source as an infinite cylindrically symmetrical DC positive column. Detailed plasma chemistries were considered and first three moments of the Boltzmann's equations were applied to decide the equilibrium condition of the plasmas. The model was found to explain the experimental trend very well.