No More PFC Emissions in Plasma Chamber Cleaning?
Maria A. Lester, Associate Editor -- Semiconductor International, 1/1/2000
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Researchers at the University of Illinois (Urbana, Ill.) studied PFC abatement in inductively coupled plasma reactors. Here the scaling of plasma abatement from etching and chamber cleaning was analyzed using a burn-box model. The plasma etching chamber and the downstream plasma burn-box were simulated using O2, H2 and H2O additive gases. Results indicated further PFCs were generated in the form of CF4 during C2F6 abatement using an O2 additive. The remaining oxidation products were COF2, CO and CO2. But when an H2 additive was used in place of O2, the F and C atoms were converted to HF and C, thus breaking up the CF4 gas. And when H2O was used as an additive, the primary products were HF, CO, COF2 with minimal amounts of C and CO2. Therefore, PFCs from plasma etching effluent can be minimized with H2 or H2O additives to a downstream plasma burn-box.
Air Products and Chemicals Inc. (Allentown, Pa.) presented research on the operation of high-pressure fluorinated gas plasmas used for CVD chamber cleaning applications. Researchers investigated NF3 and C2F6, which are used predominantly for CVD chamber cleans because they offer the potential for the lowest environmental impact. Air Products characterized the way in which the discharges use the supplied energy to dissociate the PFC source gases and etch the deposited residue from the reactor. The use of optimized C2F6 cleans was shown to reduce emissions by greater than 60%. But again, higher-power C2F6 cleans led to more CF4 formation. Whereas the use of NF3 plasma cleans resulted in dissociation of the source gas with minimal byproduct formation. According to John Langan, OEM business development manager for Air Products, mixing NF3 with gas diluents altered the nature of the discharge to achieve lower impedances, higher-power coupling efficiencies and faster etch rates. Collaboration with the process equipment manufacturer ultimately can lead to reduction in the amount of influent PFC gas, maintenance in cleaning rate and reduction in PFC byproduct generation.
Applied Materials (Santa Clara, Calif.) developed a cleaning technology, the Remote Clean process, for its CVD reactors that uses a remote plasma source. NF3 gas is introduced into a separate, remote chamber where it is dissociated by a high-density plasma into charged and neutral species. Mainly neutral species are sent to the CVD reactor, where they provide a soft clean. The remote NF3 clean was shown to reduce PFCs by two orders of magnitude compared to the C2F6 clean (see Figure). Researchers performed a case study comparing in situ C2F6 cleans to remote NF3 cleans. The in situ clean caused corrosion of chamber components at high power densities, limited etch rate and a longer clean time. The limited PFC utilization removal efficiency (URE) was 20% to 70%. In contrast, the NF3 remote clean produced near complete URE with a high etch rate, 20-60% reduction in clean time. No ion bombardment was found in the main chamber.
These papers demonstrate the achievement in the understanding of PFC dynamics, a requirement to identify viable solutions for chamber cleans. For more information on the AVS symposium, visit www.vacuum.org. •
