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SEMI F5-1101 © SEMI 1990 , 2001 19 A2-5.1.4 Limitations — Fill material can also react with oxygen or moistu re so hot bed sy stems are not always suited to w et processes or wh ere air is used. A2-5.2 Hot Catalytic Bed …
SEMI F5-1101 © SEMI 1990, 2001 18
filled with adsorbent granules. These are commonly
based on activated charcoal. Some gases are
physically adsorbed into the granules while the non-
adsorbed gases pass through into the facility exhaust.
A2-4.1.2 Flow Capacity and Removal Capacity —
Both capacities of these systems are a function of the
container size (mass of granules) and surface area.
A2-4.1.3 Efficiency — For a single gas, efficiency can
approach 100% until the bed capacity is reached.
However, since the gas is only held physically it can be
displaced by another compound, causing the efficiency
to drop significantly.
A2-4.1.4 Limitations — Adsorption traps hazardous
compounds and does not destroy them. Gases are
adsorbed at different rates and the capacity of the bed
for different gases will also vary. A more strongly
adsorbed gas entering the bed after a less strongly
adsorbed compound can displace the first gas re-
releasing the first gas to the exhaust. The waste material
must be kept sealed in the container and evaluated for
hazards. Exposure to air, water, or heat can cause gases
adsorbed in the material to be released.
A2-4.2 Chemisorption
A2-4.2.1 Principle of Operation — Exhaust gas is
passed through a container of granules that consist of
either:
• adsorbent granule coated with some reactive
chemical;
• reactive porous medium e.g., soda lime; or
• a resin matrix coated with some reactive chemical.
A2-4.2.1.1 Gases are adsorbed into the porous matrix
and then chemically react either with the granules
themselves or with the coating. These reactions are
designed to convert the gas into a solid material
(typically a salt) that remains attached to the granules.
A2-4.2.1.2 Reactive chemicals are typically either
bases (metal hydroxides) or oxidizers (e.g.,
permanganates, or metal oxides).
A2-4.2.2 Flow Capacity and Removal Capacity —
Both capacities of these systems are a function of the
container size (mass of granules) and surface area.
A2-4.2.3 Efficiency — Acid/Base and Redox reactions
are most readily accomplished. Therefore, with acidic
gases and hydride gases, efficiency can be high; with
removal below OEL. With other gases, e.g., PFCs and
halogenated carbon compounds, efficiency is often
minimal or none.
A2-4.2.4 Limitations — There may be adsorption as
well as chemisorption, so the spent material must be
evaluated for hazards, since gases can desorb from it.
This is especially true for chlorine (Cl
2
) when the bed
uses hydroxides. These will form OCl compounds that
on exposure to air or moisture will regenerate chlorine.
A2-4.2.4.1 Materials used for chemisorption can
themselves be hazardous (e.g., heavy metals or caustic
(KOH/NaOH)) therefore, unused canisters may be
classified as hazardous items.
A2-4.3 Adsorption with Subsequent Air Oxidation
A2-4.3.1 Principle of Operation — These systems are
designed and offered mainly for high flows of arsine
(AsH
3
) and phosphine (PH
3
), used in the deposition of
III-V materials (such as gallium arsenide (GaAs),
indium phosphide (InP)). These gases are adsorbed
into a carbon bed, into which air is subsequently fed at
a controlled rate to oxidize the adsorbed hydride into
oxides that will remain in the granules.
A2-4.3.2 Flow Capacity and Removal Capacity—
Both capacities of these systems are a function of the
container (drum) size (mass of granules) and surface
area.
A2-4.3.3 Efficiency — Capable of removing hydrides
to below OEL.
A2-4.3.4 Limitations — Hydride is adsorbed into the
carbon bed, the oxidation of this is exothermic. When
air is fed in to oxidize the hydride, it is possible to start
combustion of the carbon bed, unless the flow rate is
well controlled.
A2-5 Hot Chemical Bed Systems
A2-5.1 Hot Bed Reactors
A2-5.1.1 Principle of Operation — Exhaust gases are
passed through a steel container (cartridge/drum) that is
filled with reactive material. The material is
maintained at an elevated temperature, typically 250 to
550 degrees C. The gases chemically react with the fill
material and are converted into inorganic salts that
remain fused into the matrix of the fill material. There
are versions with a single fill material that are specific
to abatement of one gas, and versions with two or more
zones of fill material that can abate a wide range of gas
compounds.
A2-5.1.2 Flow Capacity and Removal Capacity —
Both capacities of hot bed systems are a function of the
cartridge size (mass of granules) and surface area.
A2-5.1.3 Efficiency — Efficiency of these systems is
generally high, with gases removed to below OELs.
Stable PFC gases are not removed by all systems.
SEMI F5-1101 © SEMI 1990, 200119
A2-5.1.4 Limitations — Fill material can also react
with oxygen or moisture so hot bed systems are not
always suited to wet processes or where air is used.
A2-5.2 Hot Catalytic Bed
A2-5.2.1 Principle of Operation — Gas is passed into
a steel cartridge, which is held at an elevated
temperature, that contains granules of a catalyst. The
target-input gas is catalytically converted to other gases
[e.g., ammonia (NH
3
) to nitrogen (N
2
) and hydrogen
(H
2
), and oxides of nitrogen (NOx) to nitrogen (N
2
) and
oxygen (O
2
)].
A2-5.2.2 Removal Capacity — In theory, if not
poisoned, a catalyst bed should last for a long time.
Units available are claimed to be able to operate for
one-year before bed change.
A2-5.2.3 Efficiency — Efficiency of hot catalytic bed
systems is normally high, with removal of the target gas
to below OEL levels.
A2-5.2.4 Limitations — Hot catalytic bed units are
designed to be gas specific. Catalysts can be sensitive
to other compounds; so for complex gas mixtures in
process exhausts they may not be suitable on their own.
A2-5.3 Hot Reactor Beds with Gas Inputs
A2-5.3.1 Principle of Operation — Exhaust gases are
passed into a hot bed as in hot bed reactors above, but
concurrently with the process gas another gas is
introduced (e.g., air or steam). The gases then react
together in the bed, either with each other or with the
bed material. Hazardous gases are converted either to
solid salts that remain fused into the bed matrix, or
other gases [e.g., carbon dioxide (CO
2
)] that pass on
into the exhaust.
A2-5.3.2 Flow Capacity and Removal Capacity —
Both capacities of these systems are a function of the
container (cartridge) size (mass of granules) and surface
area.
A2-5.3.3 Efficiency — Process gases [e.g., phosphine
(PH
3
), carbon monoxide (CO)] are normally removed to
below the OEL.
A2-5.3.4 Limitations — These systems are suited only
to processes where the end products will be either
stable solids or inert gases.
A2-6 Reactor Systems (Plasma, Microwave,
etc.)
A2-6.1 Principle of Operation — Gases are abated in
the vacuum line (before or after the vacuum pump) by
passing them through a reaction chamber, containing
plasma, to enhance reaction of gases such as silane into
a solid material. For high efficiency PFC conversion, it
is necessary to inhibit recombination by the addition of
a material (e.g., moisture, or a hydrogen/oxygen
mixture). Two types of reactors exist - large plasma
volume/small surface area, in which a downstream
particle trap is utilized; and small plasma volume/large
surface area, where solid films are deposited. Other
reactor systems (e.g., microwave systems) are also
being developed.
A2-6.2 Flow Capacity and Removal Capacity —
Reactor systems are designed for the process system
effluent and are sized to handle specific process flows.
A2-6.3 Efficiency — With inputs of up to 300 sccm of
silane, removal efficiencies of greater than 99% have
been documented. At higher flows efficiency can be
reduced. High removal efficiencies of PFCs have also
been demonstrated.
A2-6.4 Limitations — Silane will form solids in the
plasma. Gaseous by-products will pass into the
exhaust. Solids transmission into the vacuum pump
will be reduced, but additional abatement devices may
be required downstream if by-products need to be
removed. Formation of hydrogen (H
2
) as a by-product
should be considered to ensure that the H
2
outlet
concentration is below the lower flammable limit.
A2-7 Traps/Filters/Cyclones/Precipitators
A2-7.1 Principle of Operation — Designed to remove
the particulate component in exhausts. Some processes
produce solid powders (e.g., silica) and other processes
generate condensable vapors. These vapors, on
compression in the pump and cooling in the exhaust,
condense into solid materials. Numerous designs exist
to collect these solids out of the gas stream. These
include filters (paper/bag etc.), cooled condensation
chambers, cyclones, and electrostatic precipitators.
A2-7.2 Removal Capacity — Removal capacity is
normally measured as the mass of solid the unit can
collect before it partially blocks. As they are blocked,
these systems generate a backpressure, which can
prevent effective removal of material or create a
process disturbance.
A2-7.3 Efficiency— Efficiency for these various units
is difficult to measure. It is usually assessed
pragmatically in terms of increased interval between
exhaust clean out.
A2-7.4 Limitations— These various units must be used
with care and only when not associated with hazardous
gases. Often solid exhaust materials are mixed with
hazardous gases. If these gases are present in the
trapped solids a dangerous situation can result. Where
gases and solids are mixed it is safer to keep material
volatile and abate all by a POU abatement technology
SEMI F5-1101 © SEMI 1990, 2001 20
from the sections above. If only non-hazardous gases
are left these systems can provide cost-effective
abatement to keep ducts clear. They can also be used in
combination with other technologies.
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representations as to the suitability of the standards set
forth herein for any particular application. The
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herein. These standards are subject to change without
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