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SEMI F5-1101 © SEMI 1990 , 2001 4 7.10 Many types of proces s equipmen t use and exhaust incompatible gases sequentially (e.g., silane and then nitrogen trifluoride (NF 3 ) as elements of a dep osition and cleaning cycle…

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SEMI F5-1101 © SEMI 1990, 20013
may be as low as 10%). Additional by-products
will have different and sometimes more hazardous
properties.
7.2 Considerable research has been conducted into
various hazardous by-products of processes, from
explosive solids to organochlorine compounds with
mutagenic properties. (See Sections 13.1 through
13.3.)
7.3 In considering selection of exhaust systems and
abatement technology, it is essential to review all the
materials likely to be present in the exhaust.
7.4 Contributions from the materials on the wafer must
also be considered, as they often cause blocked
exhausts (e.g., aluminum etches produce aluminum
chloride (a by-product of removal of aluminum from
the wafer with chlorinated gases)).
7.5 Processes that produce specific hazardous by-
products should be evaluated for appropriate point-of-
use abatement technology before discharge to an
exhaust system.
7.6 Gaseous and particulate contaminants emitted from
semiconductor manufacturing can be divided
conveniently into nine major groups, each of which
might justify its own exhaust system.
7.6.1 Group-1 Acid — Easily hydrolyzable acids, (e.g.,
hydrogen chloride hydrolyzes into hydrochloric acid.)
7.6.2 Group-2 Acid aerosols (“difficult–to-scrub”) —
This group includes acids that generate aerosols and
that are not easily removed by simple water scrubbers
(e.g., aqua regia, sulfuric and nitric).
NOTE 2: High concentrations of easily soluble acids will also
fall into the “difficult-to-scrub” category.
7.6.3 Group-3 Ammonia — Exhaust streams
containing ammonia
7.6.4 Group-4 Volatile Organic Compounds (VOCs)
— Exhausts containing volatile organic compound
vapors or flammable gas discharge requiring
destruction.
7.6.5 Group-5 Pyrophoric — Exhausts containing
pyrophoric gases.
7.6.6 Group-6 Emergency Release Discharges —
Enclosures are provided with additional exhausts to
contain sudden unanticipated releases of gas from
cylinders, etc.
7.6.7 Group-7 Special/Direct — Additional exhausts
used to keep specific gases separate for recovery
purposes (e.g., hydrogen (H
2
) reclamation from EPI or
PFC reclamation from CVD tools, Oxides of Nitrogen
(NO
x
)).
7.6.8 Group-8 General Exhaust Systems without End-
of-Pipe Treatment — Exhaust of heat or post-treatment
exhaust from POU abatement systems.
7.6.9 Group-9 Highly Toxic Gas Exhaust — Highly
toxic gases especially those with poor warning
properties (e.g., arsine, phosphine, diborane, germane).
The end user has a primary responsibility to prevent
discharge of these gases above a regulated level (e.g.,
usually one-half (1/2) of Immediately Dangerous to
Life and Health (IDLH) levels).
NOTE 3: Facility exhaust systems may be called by a number
of different names such as: Acid, Corrosive, Ammonia,
Scrubbed, VOC, Solvent, Incinerated, Highly Toxic, Toxic,
Dedicated, Special, PFC, General, Heat, Treated, or others.
The actual facility exhaust system name, and what groups are
routed to each system, will vary by factory and by country.
7.7 The groups listed in Section 7.6 are not a definitive
division, but represent a summation of common
industry practice for separation. The division is based
on three basic principles:
• avoiding mixing of incompatible gases in the
exhaust,
• potential need for different end-of-pipe abatement
technologies, and,
• keeping materials separate for recovery.
7.8 The number of separate exhaust systems used is
entirely a decision for each facility, based on the
following:
• processes and chemicals used,
• the number of each type of process,
• whether recovery of gases is required,
• whether point-of-use (POU) abatement is installed,
and
• compatibility of emissions with each other or the
effluent handling system.
7.9 In theory, if POU abatement systems are fitted to
all sources of gaseous effluent all exhaust could then be
combined into a single exhaust system.
7.9.1 In practice most facilities operate a compromise
between the two extremes with two to four separate
exhaust systems and point-of-use abatement systems
fitted only to certain sources. The decision (as to which
combination to use) will often be based on economic
and space constraints.
7.9.2 Safety and environmental factors listed below,
should always be considered first when determining
specific gaseous sources that require POU abatement
systems.
SEMI F5-1101 © SEMI 1990, 2001 4
7.10 Many types of process equipment use and exhaust
incompatible gases sequentially (e.g., silane and then
nitrogen trifluoride (NF
3
) as elements of a deposition
and cleaning cycle).
7.10.1 With more than one of these processes being
discharged asynchronously into a common exhaust
duct, undesired reactions may occur.
7.10.2 It is possible to avoid this problem by fitting the
equipment outlet with three-way valves to divert the
discharge to the correct exhaust system, depending on
equipment process step. Three-way valves may not be
necessary if pump ballast-gas flow is increased to dilute
gases.
7.10.2.1 Switching must accurately account for the gas
transmission times from process chamber inlet to the
exhaust valve.
7.10.2.2 Introduction of additional mechanical devices
could create major hazards, reliability, and service or
maintenance issues.
7.10.2.3 Valves must incorporate failsafe designs to
ensure pump outlet is not shut off.
7.10.3 An alternative solution is to fit a local POU
abatement system that treats at least one component and
is not adversely affected by the other components.
7.11 Point-of-Use (POU) Abatement Devices
7.11.1 POU abatement devices are designed to operate
at the process tool or pump outlet, and to remove
hazardous gases BEFORE they enter the exhaust
ductwork.
7.11.2 An end-of-pipe scrubber does not protect the
facility from reactions in the ductwork. Reactions
between process gases, as discussed above, or reactions
with other process by-products within the ductwork
itself are not prevented by end-of-pipe systems either.
7.11.3 POU abatement devices may be appropriate
wherever the exhaust gases can react in the ductwork.
7.11.4 If process systems are not treated with POU
abatement devices, the higher concentrations of
hazardous gases will present a higher risk during a leak
into the facility space.
7.11.5 Improperly functioning POU abatement devices
can create backpressure that can increase the risk of a
leak.
7.12 Four significant consequences can be foreseen
from effluent gas reactions in the ductwork:
7.12.1 Blocked Ducts — Reactions between gases, or
condensation of vapors, can produce solids and semi-
solids (such as crystals, slurries or gels) that will block
the ductwork.
7.12.1.1 Collection of solids can create dead leg
sections that may contain reactive byproducts or unused
raw materials. Catastrophic release of these trapped
materials could generate a significant hazard.
7.12.1.2 Extensive downtime (for clean out, or ducts
collapsing under the weight of accumulated solids) is a
possible undesirable outcome of this condition.
7.12.1.3 Solids deposited (such as from metal etching)
can, themselves, be both corrosive and hazardous,
leading to health risks for people performing cleaning
in ductwork. (See Related Documents 13.1 through
13.3).
7.12.1.4 Blocked ducts may result in toxic process
gases or by products escaping into the work area.
7.12.2 Duct Corrosion — Etching and some chamber
cleaning process emissions are corrosive. Both metallic
(including stainless steel) and plastic ducting can be
attacked, resulting in the release of hazardous gas or
condensed liquids into the workplace.
7.12.3 Duct Fires or Explosions — Flammable and
pyrophoric gases can ignite in the ductwork. If the
ductwork is combustible or has flammable or
combustible deposits in it (e.g., hydrocarbon pump oil),
facility fires can result.
7.12.4 Additional information on protection of
Industrial Exhaust systems from fires may be found in
FM Global Loss Prevention Data Sheet 7-78,
“Industrial Exhaust Systems”.
NOTE 4: Destruction of whole production lines with multi-
million dollar cost impact has resulted from such incidents.
7.12.5 Formation of Ammonium Compounds — If
ammonia is discharged into a duct with acid
compounds, a sub-micron ammonium compound fume
can be generated, that is not easily removed by end-of-
pipe systems.
7.12.5.1 The result is visible discharges.
7.12.5.2 Ammonium compounds produce solids in the
duct.
7.13 If any of the above reactions can occur in the
process exhaust, POU abatement should be considered.
7.14 Exhausts of oil-lubricated vacuum pumps should
be fitted with well-maintained oil mist separators/
filters.
7.15 POU abatement devices may also be needed
upstream of the recovery device for the recovery of
some gaseous compounds (e.g., hydrogen (H
2
), PFC
gases).
SEMI F5-1101 © SEMI 1990, 20015
8 End-of-Pipe Abatement Technologies
8.1 Group 1 — Acid Abatement
8.1.1 The appropriate abatement technology for this
category is commonly called " wet scrubbing."
8.1.2 Many, if not all, of the semiconductor processes
have traditionally been exhausted through centralized
wet scrubbers.
8.1.2.1 This procedure has limited technical
acceptability and may not meet the requirements of all
regulatory agencies.
8.1.2.2 This technology should be targeted at
abatement of acid gases, vapors, mists, and fumes
emitted from CVD, dry etching, and wet chemical
operations.
8.1.2.3 Most wet chemical processes are typically
carried out under local exhaust ventilation devices, such
as fume hoods, or in specially designed rooms that are
totally exhausted. These types of operations include:
• Processes used to clean wafers or clean wafer
handling equipment (such as quartz components,
tubing),
• Processes used in wafer fabrication for surface
treatment such as oxide, silicon nitride and other
surface treatments, (e.g., metal etching), and
• Processes used for development of positive
photoresist.
8.1.2.4 The chemical species emitted depends on both
the chemicals used in the processes and the conditions
of use.
8.1.2.5 Volatile acids (e.g., hydrochloric (HCl),
hydrofluoric, nitric, and acetic) yield corresponding
gases and vapors, especially when heated.
8.1.2.5.1 The reaction of hydrochloric and nitric acids
in aqua regia yields nitrosyl chloride (NOCl) vapor and
elemental chlorine (Cl
2
) gas.
8.1.2.5.2 When sulfuric acid is heated, it yields both
sulfuric acid vapor (H
2
SO
4
) and sulfur trioxide (SO
3
).
8.1.2.5.3 Phosphoric acid (H
3
PO
4
) is not significantly
volatile, but spray may be carried over into the exhaust
system by mechanical entrainment.
8.1.2.5.4 Under appropriate conditions, spray droplets
of any of the chemicals used may be mechanically
entrained into the ventilation air exhausted from the
various processes.
8.1.3 Design and selection of scrubbing equipment for
the acid exhaust systems must take into account both
the chemical and physical properties of the materials
being emitted.
8.1.4 The principles involved in collection of
particulate matter (including liquid aerosols) are
entirely different from those determining absorption of
gases.
8.1.5 Current Practice
8.1.5.1 Exhausts carrying acid compounds comprise
the largest volume of air discharged from a typical
semiconductor manufacturing plant.
8.1.5.2 The current practice is to connect most or all of
the corrosive sources to a central building exhaust
system and to discharge the combined gas streams to
one or more large wet scrubbers.
8.1.5.2.1 A variety of scrubber types might be used,
but the two main types are cross-flow and counter-
current scrubbers equipped with packing material of
some type.
8.1.5.3 The scrubbers usually are fed with a stream of
water (potable, recycled, etc.).
8.1.5.4 A side stream of water is bled from the
scrubber to a wastewater line.
8.1.5.5 It is futile to discharge organic solvents to
water scrubbers even if the organic solvents are, to
some degree, soluble in water.
8.1.5.5.1 Any dissolved organic solvent will eventually
be desorbed from the water and will escape to the
atmosphere, or be discharged as wastewater.
Additionally, these organics could serve to propagate
biological growth in the scrubber.
8.1.5.6 Exhausts from process tools are frequently
discharged to the appropriate building exhaust systems,
either with or without point-of-use abatement.
8.1.5.7 Arsine and phosphine, which have only slight
solubility in water, pass through the water scrubbers so
that the effect of discharging these gases into the
building scrubber system is limited to dilution.
Fluorine and chlorine also have limited solubility in
water and may pass through water scrubbers without
sufficient abatement. POU abatement devices should
be considered for these types of gases.
8.1.5.8 Preliminary oxidation of these and numerous
other process gases results in formation of fine fumes
and other aerosols that will be collected with only low
efficiencies by the typical large, low-energy wet
scrubbers.
8.1.5.9 If ammonia is discharged into a system that
also carries hydrogen chloride, hydrogen fluoride, nitric
acid, or sulfuric acid; it will react, in the vapor phase, to