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SEMI F5-1101 © SEMI 1990 , 2001 7 reduce th e size of the scrubber, usu ally at the cost of increased energy consumption). NOTE 5: See A p pend ix 1 for absorber d esign criteri a. 8.2 Group 2 — Acid Aerosols 8.2.1 Part …

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SEMI F5-1101 © SEMI 1990, 2001 6
form sub-micron solid aerosols of ammonium chloride,
ammonium fluoride, ammonium nitrate, or ammonium
sulfate, respectively.
8.1.5.10 Any exhausts carrying ammonia should be
discharged to a separate scrubber or a point-of-use
ammonia abatement used at the process discharge.
8.1.6 Chemistry of Pollutants
8.1.6.1 Effective abatement of any compound requires
careful consideration of the chemistries and physical
states of the specific compounds being abated.
8.1.6.2 The packed-bed scrubbers commonly employed
should generally be adequate for absorption of readily
soluble gases, if appropriate operating conditions are
also maintained.
8.1.6.3 Misuse can negate the potential performance of
the scrubbers.
8.1.6.4 As scrubbers are low-energy devices (pressure
drops on the order of 2.5 to 10 cm (1-4 inches) of water,
they are incapable of effectively collecting fine aerosols
such as the ammonium salts mentioned above.
8.1.6.5 The most economic design approach, therefore,
is to arrange the exhaust system and use POU
abatement devices to avoid formation of such aerosols,
where possible.
8.1.6.6 The practice of using only recirculated water in
the scrubber, to absorb acid vapors and to neutralize the
water bleed stream should be reviewed to ensure that
scrubber efficiency is not affected.
8.1.6.7 Acid gases, such as hydrogen chloride and
hydrogen fluoride, are readily soluble in water. The
partial pressure of acid gases in equilibrium with the
solution, which increases with increasing solute
concentration, reduces the absorption of additional gas.
Reduction in absorption can be minimized by
increasing the rate of water blow-down (i.e., using more
fresh water make-up).
8.1.6.8 However, increasing the rate proportionately
increases the consumption of water and the volume of
water that must be treated before discharge as
wastewater.
8.1.6.9 The introduction of an alkaline solution (e.g.,
sodium hydroxide) in the scrubber can assist in
removing compounds from the gas stream if the use of
water alone does not provide adequate removal
efficiencies. The use of additives in the scrubber can, in
itself, create safety and environmental concerns.
8.1.6.10 Some of the neutralized absorbent must be
bled off from the recirculated stream to prevent the
brine concentration from reaching an excessively high
level, but water consumption and the volume of waste
can still be reduced by alkaline solution.
8.1.6.11 Aqua regia is a reactive mixture of nitric and
hydrochloric acids. The reaction produces nitrosyl
chloride and elemental chlorine:
HNO
3
+ 3HCl è NOCl + Cl
2
+ 2H
2
O
8.1.6.12 The nitrosyl chloride vapor is readily
decomposed by absorption in an alkaline solution:
NOCl + 2NaOH è NaNO
2
+ NaCl + H
2
O
8.1.6.13 Elemental chlorine has only a limited
solubility in water, and an alkaline solution (pH > 10)
must be used to attain adequate absorption efficiencies:
Cl
2
+ 2NaOH è NaOCl + NaCl + H
2
O
8.1.6.14 During the initial reaction of the nitric and
hydrochloric acids, hydrogen chloride gas and nitric
acid vapors are volatilized along with the nitrosyl
chloride and chlorine.
8.1.6.15 Nitrogen dioxide may also be produced by
decomposition of nitric acid.
8.1.6.16 All these compounds can be absorbed, to
some degree, in an alkaline solution.
8.1.6.17 If a pollutant is in a particulate form (solid or
liquid), its collection is only slightly affected by its
chemical nature or solubility.
8.1.6.18 The dominant factor determining particulate
collectability is particle size.
8.1.6.19 Abatement of particulate compounds requires
a high-energy method (usually creating an extremely
high pressure-drop across the abatement device) which
can be costly in both energy and abatement device size
requirements.
8.1.7 Scrubber Design and Selection
8.1.7.1 A variety of commercially available scrubbers
are applicable to absorption of readily soluble gases.
8.1.7.2 These include packed-bed counter-current, co-
current, and horizontal cross-flow scrubbers, horizontal
spray chambers, vertical spray towers, and gas-
atomizing scrubbers such as the venturi scrubber.
8.1.7.3 To some extent, the choice of scrubber type is a
matter of preference, as it is usually possible to obtain
equivalent performance on the soluble gases with
different types of devices; however, more packing or
higher liquid loading may be required on some devices.
8.1.8 The choice is likely to be limited by a
consideration of practical or economic factors such as
cost, available space, and weight (e.g., it is possible to
SEMI F5-1101 © SEMI 1990, 20017
reduce the size of the scrubber, usually at the cost of
increased energy consumption).
NOTE 5: See Appendix 1 for absorber design criteria.
8.2 Group 2 — Acid Aerosols
8.2.1 Part of acid emissions may be in the form of
mists (fine droplets or aerosols) rather than gases.
These may be composed of relatively non-volatile acids
such as phosphoric and sulfuric acids.
8.2.1.1 The mechanisms of removal of particulate
matter (including liquid aerosols) from gas streams are
entirely different from those involved in the absorption
of gases (See Sections 13.1 through 13.3).
8.2.1.2 The critical factor in the collection process is
the particle size of the mist. If the mist is relatively
coarse, as apparently is the case in most instances, the
same types of scrubbers that are used for gas absorption
should collect the mists with adequate efficiency.
8.2.1.3 If a mist having a particle size in the
micrometer or sub-micrometer range is encountered, it
will be necessary to use a high-energy scrubber or a
scrubber with a high-pressure-drop, sub-micron filter to
obtain adequate collection efficiency.
8.2.1.4 Whenever such a fine mist or other aerosol is
encountered, the gas stream from the source should be
abated at point-of-use or separately from the other
exhaust gas streams in the plant so as to minimize
energy costs.
8.2.1.5 Abatement systems utilizing a scrubber with a
high-pressure-drop, sub-micron filter are being used in
the semiconductor industry to reduce emissions from
aqua-regia processes, hot nitric baths and spray etchers
using nitric acid (entrained in exhaust).
8.2.2 It has been determined experimentally that the
collection efficiency of a scrubber on a given mist or
dust is a function of the energy consumed in the
exposure of the particle to the liquid in the scrubbing
process.
8.2.2.1 The relationship between energy consumption
and efficiency is little affected by the geometry or size
of the scrubber or by the method by which the energy is
applied to making contact between the gas and the
liquid (See Sections 13.1 through 13.3).
8.2.2.2 The energy consumption required to attain a
given efficiency increases with a decrease in the size of
the aerosol (e.g., mist or dust). In most of the scrubbers
used in the semiconductor industry, the energy
consumed is drawn from the gas stream in the form of
pressure drop.
8.2.3 The energy/efficiency relationship provides a
convenient and practical method for particle and
aerosol scrubber design. It is essentially independent of
the size of the scrubber, at least down to a very small
size (perhaps 150 to 300 L/min (5–10 ft
3
/min) capacity,
and possibly even smaller). Hence, a small pilot unit
can be used to determine performance on an actual
plant exhaust stream.
8.2.3.1 Tests can also be made under laboratory
conditions, using synthetic aerosols generated for the
purpose.
8.2.3.2 The major problem in using synthetic aerosols
is in replicating the aerosols actually encountered in
practice.
8.2.3.3 The pilot plant scrubber should be tested over a
range of pressure drops to give a well-defined pressure
drop/efficiency correlation.
8.3 Group 3 — Ammonia
8.3.1 Ammonia gas (NH
3
) can be either a process gas
or evolved from ammonium hydroxide used in wet
chemical cleaning of wafers. As previously discussed,
ammonia (NH
3
) exhausted through the acid system will
react with the acids present producing an ammonium
salt aerosol that is not easily abated.
8.3.1.1 Packed-bed scrubbers are not very efficient at
removing sub-micron ammonium halide aerosols, even
though they are water-soluble.
8.3.2 Wet scrubber technologies, as listed above, are
suitable for removal of ammonia provided they are
operated and maintained at low pH (e.g., 3–5).
8.3.2.1 This is normally achieved by the use of sulfuric
acid dosing.
8.3.3 Processes that emit both ammonia and acid gases
(such as nitride deposition) should be fitted with a
point-of-use ammonia scrubber, prior to being
exhausted to the acid exhaust.
8.4 Group 4 — Volatile Organic Compounds (VOCs)
8.4.1 VOCs can represent a large proportion of
exhausted air streams within typical semiconductor
operations. The pollutants are produced from processes
such as solvent cleaning, and photoresist application
and stripping. Such processes typically contribute
flammable, hazardous, and/or environmentally harmful
compounds (e.g., VOCs, which participate in ozone
formation in the atmosphere) to the exhausted air
stream.
8.4.2 There are three primary abatement technologies
used at end of pipe for this category: adsorption,
recovery and oxidation. Adsorption generally uses
hydrophobic zeolite or activated carbon. The oxidation
process usually is thermal or may be catalytic.
SEMI F5-1101 © SEMI 1990, 2001 8
8.4.3 Emission Sources and Chemicals Emitted
8.4.3.1 VOCs usually have high vapor pressures and
can be hazardous, flammable, or may tend to form
photochemical smog. Processes using them (such as
solvent cleaning, photoresist application, vapor
degreasing and photoresist stripping) are carried out
under local exhaust hoods or in exhausted enclosures.
8.4.3.2 Chemicals emitted may include: acetone, ethyl
benzene, ethyl lactate, hexamethyldisilazane (HMDS),
isopropyl alcohol, methanol, methyl ethyl ketone
(MEK), n-butyl acetate, n-methyl-2-pyrrolidone
(NMP), petroleum distillates (VM&P naphtha),
propylene glycol monomethyl ether acetate (PGMEA)
and xylenes.
8.4.3.3 The chemical species emitted are typically
vapors of the specific VOCs being used. These vapors
are entrained in large volume air streams and are thus
diluted to low levels.
8.4.4 Current Practices
8.4.4.1 New regulations and proposed legislation,
being implemented worldwide, require significant
reductions in total VOC emissions (often measured in
terms of mass discharged).
8.4.4.2 Removal of VOCs from exhaust streams can be
accomplished by adsorption, (e.g., fluidized bed and
concentrator systems), oxidation, or a combination of
adsorption and oxidation systems. Adsorption
technologies, with subsequent desorption and
condensation, can be used to recover VOCs as a liquid.
8.4.4.3 End-of-pipe abatement is most commonly used
for VOCs. Point-of-use systems, using the same
technologies as end-of-pipe, are employed where
exhaust facilities preclude use of an end-of-pipe unit, or
as additional abatement to meet regulatory
requirements.
NOTE 6: See Appendix 1 for VOC abatement design and
selection criteria.
8.5 Group 5 — Pyrophoric Gases
8.5.1 Where separate exhausts for pyrophoric gases
have been installed, it has been practice to fit a large air
dilution chamber at the end of pipe. The reasoning has
been that this will ensure safety by dilution and result in
oxidation of any residual pyrophoric gases. However,
oxidation may not occur since research has shown that
silane, once diluted to <1.5 % in nitrogen, does not
undergo appreciable oxidation when subsequently
mixed with air.
8.5.2 Due to the risks associated with ducting
pyrophoric gases through a facility, it is strongly
recommended that these gases be abated at point-of-
use. This avoids the need for a separate pyrophoric gas
exhaust. If POU abatement is provided, the discharge
from the POU abatement system can be directed to the
acid or general exhaust system.
NOTE 7: See Appendix 2 for POU abatement of pyrophoric
gases.
8.6 Group 6 — See section 10 for Emergency Release
Discharge Exhaust provisions.
8.7 Group-7 — Special/Direct
8.7.1 Recovery of specific gases such as hydrogen (H
2
)
may be necessary to reduce the massive risk of duct fire
when large quantities of flammable gases are
discharged.
8.7.2 PFC reclamation maybe required by
environmental permits in some jurisdictions.
8.7.3 Abatement of Oxides of Nitrogen (NO
x
) may be
necessary to meet environmental regulations in some
jurisdictions.
8.8 Group 8 — General Exhaust
8.8.1 This exhaust group is used to handle exhaust
emissions that include heat and excessive nitrogen flow.
8.8.2 Heat exhaust is used to reduce the heat load on
the balance of the building.
8.8.3 Exhaust of large volumes of nitrogen or other
asphyxiant gas is used to keep potential asphyxiation
hazards away from people.
8.8.4 Discharge from general exhaust should be to a
location where the heat or reduced oxygen will not be a
hazard to personnel.
8.8.5 General exhaust can sometimes be used for
discharge of post-treatment emissions from POU
abatement systems.
8.8.5.1 In such a case, the discharge permit for the
POU abatement system should be consulted for proper
management.
8.9 Group 9 — Highly Toxic Gases
8.9.1 Gases such as arsine, phosphine, diborane,
germane, etc., can be treated in a variety of ways,
depending upon their concentration and their
byproducts (e.g., gas reactor columns, adsorption
systems).
8.9.2 These gases would not likely be allowed in a
burn system (even though many of them might be
pyrophoric) because they could release hydrides from
the air intake.