AVS 56th International Symposium & Exhibition
    Vacuum Technology Monday Sessions
       Session VT-MoM

Paper VT-MoM3
Modeling Decontamination of Vacuum Chambers by Downstream Plasma Cleaning

Monday, November 9, 2009, 9:00 am, Room C1

Session: Vacuum Contamination and Pumping
Presenter: C.G. Morgan, XEI Scientific, Inc.
Authors: C.G. Morgan, XEI Scientific, Inc.
R. Vane, XEI Scientific, Inc.
Correspondent: Click to Email
Downstream plasma cleaning is an effective means for removing carbon contamination from vacuum chambers. The downstream plasma cleaning device is mounted on an unoccupied port on a vacuum chamber. When in use, the device creates oxygen radicals using a small leak of oxygen containing gas and a low power (5-20 W) radio frequency (RF) plasma. The oxygen radicals then flow through the vacuum chamber, ashing hydrocarbons. The rate of carbon decontamination has been shown to be dependent on a number of factors: RF power level, chamber pressure and geometry, distance between radical source and contamination, speed of the pumping system, and type of oxygen containing gas used. Decontamination rates can be accurately estimated if chemical models of both the oxygen radical reactions within the chamber and on the contaminant surface can be developed. These models are validated by physical data from two experiments varying the parameters listed above.

Data is presented from two experiments with oxygen radicals. Both experiments use a quartz crystal microbalance (QCM). In the first, a silver-coated QCM is placed in the vacuum chamber and subjected to the plasma cleaning process. Oxygen radicals will incorporate themselves into the QCM and increase its mass. The flux of oxygen radicals impinging on the QCM surface can be calculated using the Deal-Grove model of surface oxidation. By locating the silver-coated QCM in different locations of a vacuum chamber, a map of oxygen radical concentrations as a function of distance from the plasma can be made. In the second, a gold-coated QCM is contaminated with hydrocarbons. Test contamination is achieved by heating a small amount of hydrocarbon in a vacuum chamber and allowing the evaporation to recondense on the gold-coated QCM. The cleaning process is then initiated and an experimental trace showing mass loss from the gold-coated QCM as a function of cleaning time is obtained.

A chemical box model which assumes that once the plasma is lit there is a steady-state oxygen radical concentration within each box can be compared to the data from the silver-coated QCM experiments. The chemistry within each box is obtained by using a standard database of gas phase reaction rates. The second model focuses on the gas surface chemistry of decontamination. The results of the second model are compared with the data from the mass loss traces of the gold-coated QCMs. The combination of both models will provide a means to estimate rates of downstream plasma cleaning for any contaminated vacuum system.