Transcript No Slide Title

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
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Outline
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The greenhouse effect and greenhouse gases
Global warming and Global Warming Potential
(GWP)
Using GWP to evaluate processes
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Greenhouse Effect
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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.
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.
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.
Earth
Earth
Earth
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Greenhouse Effect (continued)
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The surface gets warm and radiates
EM energy (heat). Without radiation,
the earth’s temperature would rise
continuously.
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).
The constituents radiate the energy,
half of which goes down to the
surface and the other half to space.
Thus, the surface gets warmer.
from M. A. K. Khalil, Oregon Graduate Institute
Global Warming Symposium, 1994.
Earth
Earth
Earth
This is a natural phenomenon.
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Net outgoing IR & Radiation Forcing
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The net outgoing IR from the atmosphere is the difference
between the IR leaving the earth surface and that absorbed by
the atmosphere.
The absorption of outgoing IR by naturally occurring
greenhouse gases (H20, CO2, O3, CH4, N2O) 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.
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
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Greenhouse Gases
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The molecular vibrations of greenhouse gases are at the
frequencies required to absorb IR.
Changing the atmospheric concentration of a greenhouse
gases produces radiation forcing.
Greenhouse gases have both anthropogenic and natural
origins.
The dominate natural greenhouse gases are H20 and CO2.
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Contrasting the Impact of Greenhouse
Gases
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Carbon dioxide (CO2), methane (CH4) and nitrous oxides
released in large volumes but have relatively low global
warming potentials.
Other gases, such as perfluorocarbons, are released in much
smaller volumes but have much higher global warming
potentials.
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Global Warming
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The Earth’s temperature increases as result of the build-up of heattrapping (i.e. “greenhouse”) gases.
A higher frequency of extreme climatic events (drought, floods) is
predicted.
Coastal flooding due to rising sea level: potential salinization of
fresh groundwater supplies
Change in plant and animal populations
– increase of some species, potentially “pests & pathogens”
– loss of agricultural crops
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Predictions Based on Present Rate of
Global Warming Gas Emission
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Global Mean Temperature Rise of 0.3 °C per decade
Non-uniform temperature rise (greater increase in polar
regions)
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Sea-level rise of 6 cm per decade
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Non-uniform increase in rainfall
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Greenhouse Effect vs. Global warming
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The greenhouse effect is large (tens of °C) and has a firm scientific
basis. (Venus has a HUGE greenhouse effect.)
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)
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
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Global Warming Policy
IPCC - “..emissions resulting from human
US Climate Change Action Plan -
activities are substantially increasing
concentrations of greenhouse gases..”
“..control emissions of HFCs and
PFCs..”
1988
1992
UN Rio Conf. - “..levels of CO2 to
be reduced to earlier levels by
2000..”
1993
today
DuPont’s Sale Policy - phase out supply of C2F6 if
solutions to reducing emissions of this PFC are not
sufficiently mature by year-end 1999.
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.
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Atmospheric Lifetime, t
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Compounds are eventually broken down through chemical
reactions in the atmosphere.
Typically, the rate of loss is proportional to concentration, c,
1 
dc
    c
t 
dt
c
 exp( t t )
c0
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(first order kinetics)
LIFETIME = TIME TO REDUCE CONCENTRATION BY 37%
(1/e)
Unstable compounds are highly reactive -> short lifetimes
Stable compounds are unreactive
-> long lifetimes
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Global Warming Potential (GWP)
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For a given gas, the GWP is
– an index of the potential to cause radiation forcing
– relative to releasing the same mass of CO2
– cumulative, integrating between the present and a future
time
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Calculation of GWP
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The total outgoing IR absorbed by compound i
– at any instant, is proportional to ai*ci
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ai is the radiation forcing efficiency per molecule i
ci is the concentration of molecule i
– over time, ci will decrease
– defined such that GWPCO2 = 1
ITH
GPWi 
 ai ci dt
0
ITH
 aCO2 cCO2 dt
0
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The integrated time horizon (ITH) is typically chosen as 100
years. Sometimes 20 or 500 year horizons are considered.
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Comparison of Lifetimes & GWPs
Gas
CF4
C 2 F6
C 3 F8
SF6
NF3
CHF3 *
CO2
Atm. Lifetime
(years)
50,000
10,000
5,600
3,200
740
264
50-200
GWP
(100 ITH)
6,500
9,200
6,950
23,900
13,100
11,700
1
data on NF3 from Air Products all others from IPCC ‘95
*CHF3 is technically not a perfluorinated compound, but is often included on this list
because of its widespread use and PFC-like atmospheric behavior.
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Observations
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PFCs are potent greenhouse gases which are long-lived and
strong infrared radiation absorbers.
The semiconductor industry uses PFCs with high GWPs.
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.
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%.
Releasing one molecule of C2F6 today has the same GWP as the
releasing of 9,200 molecules of CO2 when considered over a 100
year integrated time horizon!
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Source Reduction for Existing Processes
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Reduce feed gas flows used
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
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Develop good endpoint monitoring to avoid overetching
Use Design of Experiment (DOE) to optimize process
parameters such as flow rates, pressure, temp., power, etc.
Side benefits from design for the environment:
– reduced material costs (less gas purchased)
– increased wafer throughput
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Comparing Processes Using Different Gases
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The two processes differ in
– Flow rates, F1 & F2
– % Conversion efficiencies, E1&E2
– GWP1 & GWP2 of the gas utilized
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If none of the reaction products contribute to global
warming:
Relative impact = {(F1)(100 -E1)(GWP1)}/{(F2)(100 -E2)(GWP2)}
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Example Comparison
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Chamber cleaning using NF3 versus C2F6
– F1/F2= 0.25 (lower volume of NF3 needed)
– E1=80% and E2=40%
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
note per molecule, NF3 has move of impact on global warming
than C2F6.
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Relative impact
= {(0.25)(20)(13,100)/(1)(60)(9,200)) }= 0.12
The NF3 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.)
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