NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 1 Greenhouse Gases and Global Warming Prof. Karen K. Gleason, Department of Chemical Engineering, MIT Source materials contributed by : Mr. Simon Karecki & Prof. Rafael Reif Department of Electrical Engineering & Computer Science, MIT © 1999 Massachusetts Institute of Technology. All rights reserved

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 2 Outline l The greenhouse effect and greenhouse gases l Global warming and Global Warming Potential (GWP) l Using GWP to evaluate processes

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 3 Greenhouse Effect l Little of the sun’s incoming EM energy is absorbed by the atmosphere. These visible and UV photons are too weak to break up air molecules, but too strong to excite vibrations. l The clouds and earth’s surface reflect the light, while the the air and particles scatter it. The energy of the reflected light is lost to space. l The energy that is not reflected or absorbed by the atmosphere hits the surface, where most is absorbed. from M. A. K. Khalil, Oregon Graduate Institute Global Warming Symposium, 1994.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 4 l The surface gets warm and radiates EM energy (heat). Without radiation, the earth’s temperature would rise continuously. l The energy radiated from the earth is absorbed by atmospheric constituents (80%). This radiation has the correct energy quanta ( i.e., is in the IR part of the spectrum). l The constituents radiate the energy, half of which goes down to the surface and the other half to space. Thus, the surface gets warmer. Greenhouse Effect (continued) from M. A. K. Khalil, Oregon Graduate Institute Global Warming Symposium, This is a natural phenomenon.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 5 Net outgoing IR & Radiation Forcing l The net outgoing IR from the atmosphere is the difference between the IR leaving the earth surface and that absorbed by the atmosphere. l The absorption of outgoing IR by naturally occurring greenhouse gases (H 2 0, CO 2, O 3, CH 4, N 2 O) in the upper atmosphere is estimated to keep the earth’s surface 20 to 33°C warmer that it would otherwise be. Without the greenhouse effect, most water on the earth would be frozen, vastly changing the ecology. l A change net outgoing radiation from the atmosphere is termed radiation forcing. Sources of such change include –Greenhouse gases created by the activities of man (i.e. anthropogenic) – Atmospheric changes due to volcanic activity – Changes in the sun’s activity

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 6 Greenhouse Gases l The molecular vibrations of greenhouse gases are at the frequencies required to absorb IR. l Changing the atmospheric concentration of a greenhouse gases produces radiation forcing. l Greenhouse gases have both anthropogenic and natural origins. l The dominate natural greenhouse gases are H 2 0 and CO 2.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 7 Contrasting the Impact of Greenhouse Gases l Carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxides released in large volumes but have relatively low global warming potentials. l Other gases, such as perfluorocarbons, are released in much smaller volumes but have much higher global warming potentials.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 8 Global Warming l The Earth’s temperature increases as result of the build-up of heat- trapping (i.e. “greenhouse”) gases. l A higher frequency of extreme climatic events (drought, floods) is predicted. l Coastal flooding due to rising sea level: potential salinization of fresh groundwater supplies l Change in plant and animal populations –increase of some species, potentially “pests & pathogens” –loss of agricultural crops

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 9 Predictions Based on Present Rate of Global Warming Gas Emission l Global Mean Temperature Rise of 0.3 °C per decade l Non-uniform temperature rise (greater increase in polar regions) l Sea-level rise of 6 cm per decade l Non-uniform increase in rainfall

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 10 Greenhouse Effect vs. Global warming l The greenhouse effect is large (tens of °C) and has a firm scientific basis. (Venus has a HUGE greenhouse effect.) l Global warming results from perturbing the greenhouse effect: It is a smaller (tenths of °C) effect which requires careful measurement over several decades. As a result, it is still debated in the scientific community. (Note that the mean global temperature 10,000 years ago during the last ice age was only 4°C less than it is now) l Dilemma –Invest resources now but later determine the that the global warming effect is negligible OR –Do nothing now, but later find that small steps taken today would have had a dramatic improvement on the earth’s climate

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 11 Global Warming Policy today IPCC - “..emissions resulting from human activities are substantially increasing concentrations of greenhouse gases..” US Climate Change Action Plan - “..control emissions of HFCs and PFCs..” UN Rio Conf. - “..levels of CO 2 to be reduced to earlier levels by ” DuPont’s Sale Policy - phase out supply of C 2 F 6 if solutions to reducing emissions of this PFC are not sufficiently mature by year-end EPA MOU - commits signatories to (company-blind) reporting of estimated PFC emissions to the EPA on an annual basis and striving to reduce their PFC emissions.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 12 Atmospheric Lifetime,  l Compounds are eventually broken down through chemical reactions in the atmosphere. l Typically, the rate of loss is proportional to concentration, c, l LIFETIME = TIME TO REDUCE CONCENTRATION BY 37% (1/e) l Unstable compounds are highly reactive -> short lifetimes l Stable compounds are unreactive -> long lifetimes (first order kinetics)

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 13 Global Warming Potential (GWP) l For a given gas, the GWP is –an index of the potential to cause radiation forcing –relative to releasing the same mass of CO 2 –cumulative, integrating between the present and a future time

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 14 Calculation of GWP l The total outgoing IR absorbed by compound i –at any instant, is proportional to a i *c i l a i is the radiation forcing efficiency per molecule i l c i is the concentration of molecule i –over time, c i will decrease –defined such that GWP CO2 = 1 l The integrated time horizon (ITH) is typically chosen as 100 years. Sometimes 20 or 500 year horizons are considered.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 15 Comparison of Lifetimes & GWPs data on NF 3 from Air Products all others from IPCC ‘95 *CHF 3 is technically not a perfluorinated compound, but is often included on this list because of its widespread use and PFC-like atmospheric behavior.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 16 Observations l PFCs are potent greenhouse gases which are long-lived and strong infrared radiation absorbers. l The semiconductor industry uses PFCs with high GWPs. l Lifetimes for some PFCs approach that for known human civilization. For all intents and purposes, they are permanent. These compounds may have other effects in addition to global warming. l Tabulated lifetimes and GWPs are updated periodically as atmospheric measurements and models improve. The 1994 IPCC values typically changed 10 to 35 % from the 1992 report. Uncertainty in 1994 values is estimated to be ±35%. l Releasing one molecule of C 2 F 6 today has the same GWP as the releasing of 9,200 molecules of CO 2 when considered over a 100 year integrated time horizon!

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 17 Source Reduction for Existing Processes l Reduce feed gas flows used l Improve rates & thus decrease overall process time – Decreasing flow rate by 80% and overall processing time by 80% – Will use only 64% of the gas as compared to the original process l Develop good endpoint monitoring to avoid overetching l Use Design of Experiment (DOE) to optimize process parameters such as flow rates, pressure, temp., power, etc. l Side benefits from design for the environment: –reduced material costs (less gas purchased) –increased wafer throughput

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 18 Comparing Processes Using Different Gases l The two processes differ in –Flow rates, F1 & F2 –% Conversion efficiencies, E1&E2 –GWP1 & GWP2 of the gas utilized l If none of the reaction products contribute to global warming: Relative impact = {(F1)(100 -E1)(GWP1)}/{(F2)(100 -E2)(GWP2)}

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 19 Example Comparison l Chamber cleaning using NF 3 versus C 2 F 6 –F1/F2= 0.25 (lower volume of NF 3 needed) –E1=80% and E2=40% l easier to break an N-F bond (~59 kcal/mole) than a C-F bond (~127 kcal/mole) –GWP1=13,100 and GWP2=9,200 l note per molecule, NF 3 has move of impact on global warming than C 2 F 6. l Relative impact = {(0.25)(20)(13,100)/(1)(60)(9,200)) }= 0.12 l The NF 3 process reduces global warming by an order of magnitude. Do not judge a molecule by its GWP alone! (Remember this calculation assumes reaction products have negligible GWPs. Be aware that higher molecular weight PFCs can break down to smaller PFC molecules in the plasma.)