Progress In Managing PFCs

-- Semiconductor International, 11/1/1998

Perfluorocarbon emission reduction approaches take many forms including abatement, capture, recycle, use of alternative chemistries and process optimization. At SEMI's PFC Technical Update in July, parties reported on refined methods of abatement as well as process optimization to obtain short-term reductions in PFC emissions until suitable alternatives can be identified.

The PFCs most commonly used in semiconductor processing along with their global warming potentials (GWPs) are shown in Table 1. Non-PFCs such as NF₃ and CHF₃ are included in the list due to their high GWP, indicating the relative effect on the environment relative to carbon dioxide. CF₄, C₂F₆, C₃F₈ and NF₃ gases are treated in a fixed bed catalytic reactor. Roy Brown and Joseph Rossin of Guild Associates (Dublin, Ohio), reported greater than 95% destruction of the three PFCs at operating temperatures between 600-700°C, with byproducts of CO₂ and HF. Unlike PFCs, NF₃ is completely destroyed at low temperatures, resulting in the near-stoichiometric formation of NO_x. NO_x, both an acid rain precursor and contributor to urban smog, can be reduced using ammonia, yet does not sufficiently reduce NO_x levels. Work in this area is continuing.

Because the catalyst is readily deactivated by SiF₄, it must be scrubbed prior to entering the catalyst bed. NF₃ is the most readily destroyed compound, followed by CHF₃. The three PFCs require processing at 200°C to obtain comparable reductions. Over the course of the test, almost 3.5 kg of C₂F₆ was destroyed using a small amount of catalyst according to a catalyzed hydrolysis mechanism.

Tool optimization is commonly used to reduce PFC emissions, yet often results in the generation of increased volumes of hazardous air pollutants such as fluorine gas (F₂), silicon tetrafluoride (SiF₄), and to a lesser extent, HF and carbonyl fluoride (COF₂). The generation of such pollutants shifts the focus to efficient abatement of byproducts. With the concomitant goal of minimizing the use of resources, Josep Arno of ATMI-EcoSys Corp. (Danbury, Conn.) evaluated abatement efficiency of a point-of-use water scrubber at removing F₂ and SiF₄ while inhibiting the generation of oxygen difluoride (OF₂). Hydrolysis of F₂ produces HF, O₂ and H₂O₂, with small amounts of OF₂. Hydrolysis of SiF₄ produces SiO₂ and aqueous HF. Arno found that a standard water scrubber equipped with a polishing counter-current packed-bed installation at the scrubber exhaust improved F₂ removal efficiency by 60%.

Using effluent with SiF₄ gas concentrations exceeding those normally released during plasma chamber cleans, outlet concentrations of HF and SiF₄ were slightly above the detection limit of the FTIR spectrometer and significantly below their threshold values of 1 ppm for SiF₄ and 3 ppm for HF. For high inlet concentrations of F₂, a proprietary abatement enhancing chemical was injected to discourage the formation of OF₂, while decreasing HF and F₂ outlet concentrations by a factor of 10. Caustic injection (of NaOH), while it does enhance fluorine abatement, generates unacceptable levels of OF₂. Arno found that under all conditions tested, the water scrubber removed over 99% of fluorine delivered, with no compromise in scrubber integrity. Hazardous air pollutants emission levels are expected to increase by a factor of 1.5-3.3 upon transition to 300 mm wafers.

Michael Daniels, Albert Cheng and Janice Chen of Texas Instruments (Dallas Texas) tested C₃F₈ as a drop-in replacement for C₂F₆ for cleaning Applied Materials' Centura dielectric CVD 5200 chambers. As shown in previous studies (see 'Reducing PFC Emissions Using C₃F₈-Based PECVD Clean,' Semiconductor International, Feb. 1998, p.85), C₃F₈ can offer improved gas utilization in the chamber cleaning process, a 47% decrease in consumable costs and a 64% decrease in PFC emissions, while showing no statistical differences in die yield, device parameters and reliability performance. In this study, a Novellus Concept Two Sequel PECVD tool was used. In the TI study, C₃F₈: provides identical or shorter cleaning times, reduces consumable costs by 23%, reduces PFC emissions by 52%, reduces CF₄ emission levels and shows no difference in film properties.