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Green and Clean Using a catalytic technique to abate PFC emissions in a 300-mm etch toolJames Cox and Sig Koenigseder, Hitachi America; and Tim Decker, Intel The testing of a catalytic-based point-of-use abatement system found that it is capable of destroying >99% of PFC gases from the exhaust of a 300-mm dielectric etch process tool. Perfluorocompounds (PFCs) are used extensively by the semiconductor industry in etch processes and chemical vapor deposition (CVD) chamber cleans. Common PFCs used in these processes include perfluoromethane (CF4), perfluoroethane (C2F6), trifluoromethane (CHF3), perfluoropropane (C3F8), perfluorocyclobutane (C4F8), and perfluorocyclopentene (C5F8). These compounds were originally adopted for use in IC production because they are relatively stable, nontoxic, noncorrosive, easy to handle, and low in cost. However, because their stability and strong infrared absorption make them potentially significant contributors to global warming, the industry is now striving to achieve reductions in fabrication facilities' PFC emissions. At 80%, carbon dioxide (CO2) accounts for the largest share of global-warming gases in the earth's atmosphere, while PFCs make up only 2% of the global total of such gases. However, when rated against CO2, which has been given an arbitrary global-warming-potential val-ue of one, PFCs have po- tential values that are from thousands to tens of thousands of times higher. The semiconductor industry is responsible for approximately 5% of the PFC emissions in the United States, equivalent to 0.1% of the global-warming gases in the earth's atmosphere. Although this percentage may seem trivial, the industry's PFC emissions have nearly tripled since 1990, and a steady increase is expected to continue as the industry continues to expand.1 To combat this trend, government and industry experts have worked cooperatively to create a voluntary PFC emissions reduction plan. Under the resulting agreement, known as the memorandum of understanding, the U.S. semiconductor industry will unilaterally reduce PFC emissions over the next decade. In addition, by 2010 the Semiconductor Industry Association (SIA) seeks to reduce PFC emissions to 10% below the output levels of 1995 for its members in the United States, Japan, and Europe; the levels of 1997 for its members in South Korea; and those of 1998 for its members in Taiwan. To meet these targets, the industry is considering four primary PFC emissions reduction methods: process optimization, alternative chemistries, recovery and reclaim, and point-of-use (POU) abatement. All of these techniques have had some degree of success, but most also have drawbacks. Often quite challenging to achieve, optimization strives to reduce emissions by tuning processes so that they will consume less chemicals, yet still maintain critical processing targets. Alternative chemistries with a low global-warming potential have been used successfully in CVD chamber clean applications, where process control for wafer production is not the primary concern. However, this technique has not fared well in dry etch processes, which require tight processing tolerances. Recovery and reclaim methods have also been studied but have not been proven economically feasible. In contrast, POU abatement has been found to be practical and is becoming the most widely accepted method of eliminating PFC emissions.2,3 POU abatement systems use combustion, plasma discharge, or catalytic technologies to reduce or eliminate PFC emissions by changing the chemicals' molecules into non-PFC by-products that are atmospherically safe and pose little or no global-warming potential. Because combustion systems have been used by IC fabs for many years to abate other harmful process emissions, it was a logical step to evaluate the effectiveness of this technique for PFC abatement, even though very high temperatures would be required to destroy these stable compounds. However, a 1997 Sematech report found that combustion units require fuel gases in order to reach the necessary high temperatures, may not completely destroy some particularly stable PFC molecules, and often generate large emissions of nitrogen oxide gases.4 A second type of PFC abatement system uses a plasma discharge to destroy molecules in a process tool's foreline. A source of hydrogen (normally from injected water) ensures that the PFCs are converted into hydrogen fluoride (HF) by-products so that they do not recombine to form CF4. Tests have shown that this technology achieves PFC destruction rates of higher than 90%, but those rates decrease with increased PFC gas-flow rates. Additionally, the placement of this type of system between the process chamber and the dry pumps restricts the diameter of the foreline in the plasma discharge unit. Also, all PFC by-products must travel through the downstream dry pump. 5–7 The third POU abatement technology uses a catalytic reaction process to destroy PFCs in gaseous exhausts. A catalyst is a substance that aids in promoting a chemical reaction but is not itself altered by that reaction. Catalysts also permit reactions to occur at lower activation energies than would otherwise be possible. This article presents the results from a study of a catalytic PFC abatement system offered by Hitachi America (Carrolton, TX). The study was conducted jointly by Hitachi and Intel at International Sematech (Austin, TX). Designed to be installed downstream from etch and CVD process chambers and their associated dry pumps, the supercatalytic decomposition system (SCDS) utilizes a proprietary catalyst to completely convert PFCs to CO2 and an aqueous HF solution. Catalytic PFC Abatement Two typical CF4-and-water reactions are diagrammed in Figure 1. In this generic example, the yellow curve shows the uncatalyzed reaction pathway, which features a large activation-energy "hill" that must be climbed before the reaction can proceed. In contrast, the red curve, depicting a catalyzed reaction pathway, is much lower, indicating that less energy is needed to activate the reaction. This low energy requirement, in turn, increases the reaction rate and helps drive the reaction to completion.
The catalytic abatement system that was studied takes advantage of these properties of a catalytic reaction by using a proprietary catalyst that destroys PFCs at low temperatures. In addition, a second catalyst in the system converts hazardous carbon monoxide (CO) to CO2. Process-tool exhaust gases containing PFCs enter the system through a three-way valve, which provides a bypass option so that maintenance can be carried out safely (i.e., without hazardous gases in the system). The PFCs are then removed from the exhaust via a three-stage process. The exhaust gases first pass through the system's prereaction scrubber tower, which consists of a column packed with small plastic diffusers that serve to greatly increase the exposed surface area. The purpose of this process step is to remove any water-soluble gases and particulates from the exhaust gases by exposing them to water. Fresh facility water and recirculated wastewater enter through the top of the column, cascade through the packed column, and exit at the bottom. The exhaust gases enter at the bottom of the column and travel up, to exit at the top. Because any water-soluble components will be flushed out of the column with the wastewater, only non-water-soluble gases, such as PFCs, will continue to the system's reaction stage. During the second stage, the exhaust gases pass through an electrically heated prereactor, an electrically heated catalytic reactor, and a quench station. Upon entering the prereactor, the PFC gases are mixed with injected air and demineralized water while being heated to the appropriate reaction temperature (typically 750°C). The PFC/water/air mixture then enters the upper section of the catalytic reactor, where the PFC components react with the water and air in the presence of the catalyst to form carbon monoxide and HF. The CO is subsequently converted to CO2, a safer compound, in the lower section of the reactor. Finally, the hot gaseous reaction products—CO2, HF, nitrogen as N2, argon (Ar), and oxygen (O2)—enter the quench station, where a water spray cools them so that they can be processed further. After being cooled, the gases enter a postreaction tower, where they are separated in a second packed column that is similar in form and function to the one in the prereaction tower. The insoluble gases (primarily CO2 and N2) are pulled out of the system by a venturi pump and exhausted into the facility's acid exhaust system. The water-soluble HF becomes an aqueous solution that is collected in a wastewater tank contained within the abatement system. This wastewater is partially recirculated through the system for use in the quench station and prereaction tower before eventually being discharged to the industrial wastewater system. Reusing this water reduces the system's overall water consumption to approximately 0.5 gal/min.8 Experimental Procedures and Results For this study, the catalytic POU system was installed to abate the exhaust of a Hitachi 700-series 300-mm dielectric etch system that uses C5F8, C4F8, CF4, CHF3, O2, Ar, helium (He), and N2. PFC destruction data were collected during wafer processing using a Fourier transform infrared spectroscopy (FTIR) instrument to sample and measure emissions from both the dielectric etcher and the catalytic abatement system. The data were collected in real time and stored in a PC for later analysis. The experimental setup is shown in Figure 2.
In order to ensure accurate measurements of PFC concentrations before and after treatment of the exhaust by the catalytic abatement system, two different-sized FTIR sample cells were used. Because untreated etcher exhaust has a very high concentration of PFCs, a sample cell with a 10-cm path was sufficient to achieve precise measurements upstream of the catalytic reactor. However, because the PFC concentrations after treatment by the abatement system were very low, a 10-m cell was required for those samples. Before taking any measurements, the FTIR system was calibrated to a known standard of ethylene. Four experimental runs were performed to evaluate the ability of the abatement system to remove PFCs from the etcher exhaust. The first set of two tests used C5F8 as the primary etch gas. (It is important to abate C5F8 not only because it contributes to global warming, but also because it is considered hazardous.) The second set of two tests used C4F8 as the primary etch gas. The secondary etch gases used in both cases were CF4, CHF3, O2, Ar, and N2. For each of the primary gases, experimental runs were made at both low (20-std cm3/min) and high (80-std cm3/min) flow rates. In comparison, a typical 300-mm dielectric etch process uses a 15-std cm3/min flow of the primary etch gas (either C5F8 or C4F8). Table I indicates the flow rates of all the process gases used in the four experimental runs.
The purpose of the first run was to determine the abatement system's PFC destruction removal efficiency (percent DRE) for a 20-std cm3/min C5F8 dielectric etch process. Data were collected with the etcher in both plasma-on and plasma-off positions. The FTIR samples were taken at three locations: immediately downstream of the etcher's dry pump, downstream of the abatement system's prereaction scrubber, and downstream of the abatement system, where the sampled gases had been treated fully. Figure 3 shows three plots of the resulting data. The left-hand plot shows the concentration profiles of the various chemical compounds in the etcher exhaust prior to the generation of a plasma reaction. In this case, the gas levels supplied to the etcher were detected in the exhaust with little or no change. The middle plot shows how the concentration profiles of each chemical in the etcher exhaust changed after plasma had been generated. Once the etching phase begins, the primary etch gas disappears as it breaks down in the plasma. However, lower-molecular-weight PFC by-products begin to increase. It is noteworthy that in this plot CF4, which is a common by-product from the breakdown of larger PFC molecules, became the dominant PFC because it is extremely stable and thus is very difficult to abate completely. Finally, the right-hand plot shows the chemical concentration profiles measured after treatment of the exhaust by the catalytic abatement system. At this point, all of the PFC gases had been abated with a DRE of >99.9%. The only remaining emissions were HF at below 1 ppm, CO2, and the inert makeup gases from the etch process.
Data illustrating the efficiency of the abatement system's prereaction scrubber are given in Figure 4. Using an exploded scale, the figure shows the FTIR data collected while plasma was being generated in the etcher. The left-hand plot depicts the data from samples taken downstream from the dielectric etcher, and the right-hand plot shows data from samples taken immediately downstream of the prereaction scrubber. As the second plot shows, two etch by-products—carbonyl fluoride (COF2), a hazardous gas, and silicon tetrafluoride (SiF4)—were completely removed by the prereaction scrubber before the exhaust gases entered the catalytic reactor.
The second experimental run was similar to the first in that the primary dielectric etch gas used was C5F8; however, the C5F8 flow rate was increased to 80 std cm3/min. This rate was an extreme value chosen to test the abatement tool's capacity to handle higher-than-normal PFC flows. The resulting FTIR data are presented in Figure 5 in three plots that show gas concentrations in the etcher exhaust emissions with the plasma off and with the plasma on, and in the catalytically abated exhaust. As in the first experimental run with C5F8, the catalytic abatement tool was able to achieve a PFC DRE of >99.9%.
For the third and fourth experimental runs, the primary etch chemical was C4F8. Experiment 3 used a 20-std cm3/min flow rate for this gas, while experiment 4 used an 80-std cm3/min flow rate. The resulting FTIR data are plotted in Figures 6 and 7, respectively. As the figures show, the catalytic abatement system achieved a PFC destruction rate of >99.9% at both the 20- and 80-std cm3/min C4F8 flow rates.
It should be noted that during the period of these four experiments, the catalyst had been installed and operating at temperature in the abatement tool for 8 months. Although the catalyst itself is not consumed by the reactions it helps to initiate, it does have a limited lifetime. The two mechanisms that may reduce its effectiveness over time are poisoning, which occurs when nonreactive species are absorbed onto active catalyst sites and block those sites, and sintering, whereby active catalyst sites are lost when high temperatures cause grain size changes. The testing described above indicates that no catalyst efficiency degradation had occurred after 8 months, and a follow-up catalyst lifetime evaluation at 14 months also revealed no efficiency degradation. Conclusion Because PFCs are potentially significant contributors to global warming, cost-effective methods to reduce the emissions of such chemicals generated by the semiconductor manufacturing industry are needed. But because the stringent process requirements of the industry make it difficult, if not impossible, to convert to the use of PFCs with lower global-warming potentials or non-PFC gases, the focus has shifted to techniques that are able to abate PFC emissions at their source. One promising abatement method is to use a catalytic system to remove PFCs from the exhausts of etch and CVD tools. The experiment discussed in this article illustrates the capability of such a catalytic system to handle PFCs emitted by a dielectric etch tool at very high flow rates with a destruction efficiency of >99.9%. The abatement system also destroys other hazardous gases (carbonyl fluoride, carbon monoxide, and hydrogen) that are by-products of the etch process or of the breakdown of the PFCs, thereby creating a safer work environment while helping IC fabs to meet targeted PFC emissions reductions. References
James Cox is a product marketing manager at Hitachi America (Dallas, TX), where he is focusing on the development of a wide range of equipment for markets in the United States. Previously, he worked at Texas Instruments as a lithography engineer and 300-mm process development engineer. He received a BS in chemistry from Louisiana State University in Shreveport and an MS in chemical engineering from Texas A&M University in College Station. (Cox can be reached at 972/615-9037 or jim.cox@hal.hitachi.com.) Sig Koenigseder is a senior process engineer at Hitachi America. He has spent more than 20 years in the semiconductor industry working in all areas of the fab. He received a BS in physics from Southwest Texas State University in San Marcos. (Koenigseder can be reached at 972/615-9061 or sig.koenigseder@hal.hitachi.com.) Tim Decker is a member of the equipment selection team for next-generation products at Intel. He has been with the company for 18 years. Recently he completed a three-year assignment at International Sematech (Austin, TX) as program manager for 300-mm etch processes and Hitachi's SCDS program. (Decker can be reached at 512/288-1887 or timothy.k.decker@intel.com.)
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