MR Mannex - Semiconductor Safety Association

WAFER CARRIERS AND FIRE SAFETY
M.R. Mannex, PE
Semiconductor Specialist
FM Global
1000 SW Broadway, #1740
Portland OR 97068
503 228 3650
ABSTRACT
Wafer carriers (i.e., boxes and pods) have historically been constructed of plastic materials with
high heat and smoke release rates when burning. Modern semiconductor fabrication plants
include materials management systems that can result in large concentrations of wafer carriers,
such as in stockers. In case of a fire in a stocker, the resulting property damage and business
interruption can have a very serious impact to fab operations. The fab layout schemes now
under development for 300m fabs offer different challenges, and tend to increase inherent fire
hazards as compared with present fab layouts. The concept of the fire risk as associated with
wafer carriers is explored. The interaction of work in progress automatic transport systems,
vertical and horizontal fab layout schemes, and stockers are considered. Mitigation strategies
are discussed, including the development of carriers constructed of inherently fire safe
materials, passive fire protection, active fire suppression systems, and fire detection methods.
INTRODUCTION
The research and development of ‘fire safe’ plastic materials associated the construction of semiconductor
manufacturing equipment has been very active and successful in the past few years. These materials are capable of
resisting fire, emitting little if any smoke, and producing little if any corrosive by-products. As a result, wet benches,
tube washers, spin rinse dryers, and a wide variety of similar plastic tools are now offered in fire safe construction,
and do not require on-board fire suppression systems.
However, wafer carriers are currently primarily constructed of plastic materials that produce high heat and smoke
release rates when burning. As a result, the consequences of a fire involving concentrations of combustible carriers
could be catastrophic to the continued operation of a fab. Typical property damage estimates for such a fire in a
modern semiconductor wafer fab can range from US$50 million for an unprotected stocker, to US$10 million for a
stocker with active fire suppression, to less than US$1 million if the stocker contained carriers constructed of a
material that would not propagate fire. Related business interruption periods are estimated at six months, one month,
and one week, respectively. As large tools are successfully transitioned into fabs in fire safe construction, wafer
carriers remain the single largest hazard exposing modern semiconductor clean rooms to a catastrophic fire.
The term ‘wafer carrier’ as used in this paper encompasses boxes, pods, boats, and reticle containers. Also included
are integral components, such as cassettes. Pods include SMIF (Standard Mechanical InterFace), and FOUP (Front
Opening Unified Pod). The SMIF is widely used in 200mm automated wafer production, while the FOUP is the
carrier expected to be used in the majority of 300mm wafer manufacturing.

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FIRE SAFE MATERIALS
Noncombustible materials are the ideal from a fire risk viewpoint, as they will not support combustion at all (under
normal atmospheres). Examples of noncombustible materials are steel, aluminum, and ceramics. For many reasons,
however, these materials are not generically suitable for some semiconductor process needs.
‘Fire safe materials’ are plastics that have sufficiently low flame and smoke characteristics to be considered
acceptable in regards to business risk management goals. These plastics are qualified under a third party testing
regime, such as the Factory Mutual Research Test Standard 4910, “Cleanroom Material Flammability Test Protocol”
(FM 4910). This test procedure quantifies two indicators of a material’s behavior in a fire: FPI and SDI. The Fire
Propagation Index (FPI) represents the rate at which the surface of the material is involved on fire. The Smoke
Development Index (SDI) is an indicator of the smoke contamination of the environment expected during a fire.
Materials which have a FPI of less than the threshold value of 6.0, and a SDI of less than the threshold value of 0.4
are considered qualified as fire safe.
Materials that meet FM 4910 require high heat fluxes to be ignited. Once ignited, these materials may burn locally in
the ignition area, but they will not propagate a fire beyond the ignition zone. Smoke and corrosive products
generated from the combustion of these materials is reduced, minimizing smoke contamination damage.
A wide range of materials has passed FM 4910, including forms of PVC, CPVC, PVDF, polypropylene, and other
plastic types. As most of the current FM 4910 materials were developed with tool construction in mind, carrier
manufactures are also investigating other plastics which are not currently qualified, which are more suited to carrier
production due to their inherent characteristics.
CARRIER FIRE RISK
Wafer carriers are now primarily constructed of plastic materials that have a relatively high FPI and SDI. Boxes are
primarily comprised of polypropylene, which has a FPI of approximately 30. Pods are mainly constructed of
polycarbonate, which has a FPI of approximately 14. The wafer cassettes inside the carriers are constructed of many
types of materials, including polypropylene and PEEK, with process and pricing requirements usually driving the
selected material.
The fire resulting from the ignition of a small quantity of carriers would not normally be cause for concern to meet
most fab’s risk management goals. However, where many carriers are concentrated in one small area, such as in a
stocker, the resulting fuel density would result in a very large fire if ignited. The high heat release and large amounts
of smoke resulting from a stocker fire are not theoretical. They have been measured in full-scale fire tests at the FM
Global Test Center, verifying the extreme consequences of such a fire.
A fire involving photolithography reticle storage boxes and holders can have a uniquely high consequence. Loss of a
portion of a fab’s reticles can produce a business interruption “choke point”, essentially stopping all production until
replaced. Not only are reticle sets becoming very expensive, the reticle stockers are usually located in the same
physical area as the photolithography equipment. Steppers are especially costly and vulnerable to smoke
contamination, and can again become a critical production choke point until the optics are cleaned, repaired, or
replaced.
The arrangement of the carriers can have an important influence on the speed of fire growth, and ultimate fire size.
The vertical arrangement of carriers in stockers promotes quick fire growth, which can be enhanced by the forced
ventilation present in most units. AMHS (Automatic Material Handling Systems) can include vertical transport units
or other concentrations of carriers. Perhaps the simplest form of carrier storage is on carts in fab aisles. The carts
usually contain several shelves, and if placed continuously adjacent to one another, can produce a significant fire,
although usually not as large as in a stocker.
The property loss prevention consequences of a fire are measured in terms of two parameters: property damage and
business interruption. Property damage includes physical damage to the carriers, wafer work in progress, stockers,
tools, cleanroom, and the fab building. Business interruption is the revenue lost to the fab after the fire for cleanup,
facility repairs, cleanroom re-qualification, tool replacement and repairs, and process and manufacturing requalification.

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Risk can be defined as the combination of hazard and frequency. Hazard indicates the severity of the consequences
of an event. Frequency indicates the likelihood of the future occurrence of such an event. A stocker fire is essentially
a high hazard event, with a relatively low frequency. For a fire in a fab, the hazard is measured in terms of property
damage and business interruption. As mentioned earlier, a stocker fire can result in property damage in the tens of
millions of dollars, given the high values of today’s semiconductor equipment. The frequency is relatively low as
compared with other risks that expose clean rooms to fire, such as heated liquid baths in combustible wet benches,
but that doesn’t mean that a stocker fire won’t occur.
One of the points often made during discussions on this subject is, “The industry has not had a stocker fire.” As far
as has been reported, this is true, but this does not by itself provide an objective measure of the frequency of this
event. There are many ignition sources capable of starting a stocker fire. Some are inherent to the operation of the
stocker itself, such as electrical controls, electric motors, and mechanical friction. Other ignition sources are
introduced to the stocker environment, such as a worker using a portable light, or while performing a repair
requiring soldering. All of these ignition sources have regularly produced fires in many other occupancies. Stockers
are not immune to these exposures.
MITIGATION STRATEGIES
Mitigation efforts can be classified into four areas: inherently safe commodity, passive fire protection, fire detection,
active fire protection, and combinations of these methods.
Inherently safe – An inherently safe commodity is one that will not burn (i.e., is noncombustible), or has such
limited combustibility that the resultant damage is very low (i.e., fire safe, or FM 4910 materials). This is the
optimum condition in terms of fire safety, and is superior to all other mitigation strategies, for several reasons. The
main strength of this approach is in its avoidance of constraints inherent with all other methods. Flexibility in
arrangement is maximized, as carriers may be stacked horizontally or vertically, and with no maximum size
restrictions. All fire suppression and detection systems associated with carrier fire mitigation are eliminated, as well
as the related periodic maintenance and testing. Flexibility in future facility and tool renovations is also maximized,
as there is no requirement for reconfiguration if the processes are changed.
Passive fire protection – Passive fire protection consists of separating the burning commodity from other
combustibles to limit the size of the fire. It also includes separating the burning commodity from high value tools in
order to minimize property damage and business interruption. Separation can be achieved by the construction of
stockers with fire rated walls, fire doors, and fire dampers at HVAC connections. This strategy tends to effectively
limit direct fire thermal damage to the stocker’s surroundings, but does not control smoke contamination to
acceptable levels, as the fire doors and dampers do not activate early in the fire growth. The integrity of the barriers
can also be difficult to maintain over time. Restrictions in overall carrier pile size also fall within this category.
Fire detection – Fire detection systems are regularly recommended as a minimum in all stockers, and are an integral
sub-system of gaseous and fine water spray fire protection systems. The lone exception is stockers in which fire safe
carriers are installed. Fire detection methods include heat detection, smoke detection, and optical flame detection.
Heat detection is usually not utilized in stockers, as the relatively high levels of ventilation tend to dissipate
organized collections of heated gases necessary for prompt activation.
For the same reason, spot type smoke detectors are not recommended. Smoke detector operation should be
unaffected by the relatively high air stream velocities inside stockers. This can be accomplished by means of HSSD
(High Sensitivity Smoke Detection) systems, which sample the air stream by means of a vacuum tube. Alternately,
beam type smoke detectors can be used which operate on the principle of smoke obscuration.
Optical flame detection is a technology that is also independent of air stream velocities. These units view the interior
of the stocker, and are activated if specific frequencies of ultra-violet or infrared light are encountered.
Active fire protection – Active fire protection consists of fire suppression systems traditionally associated with
extinguishing fires, such as sprinklers, gaseous agents, and fine water spray systems. Sprinklers have been
successfully shown through full scale testing to effectively extinguish stocker fires, but only in limited stocker sizes.
However, automatic sprinklers are thermally activated, and must allow the fire to grow appreciably prior to
activation. The resulting fire and smoke damage to the stocker, its contents, and the surrounding area can still be
serious.

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Gaseous agents are separated into two main classes: oxygen depleting, such as CO2 and Intergen; and halogenated,
such as FM200 (as a reference, Halon is a halogenated agent, but its use is in decline). However, the advantages and
disadvantages of all of these gaseous agents are similar. Advantages include clean discharge, minimum suppression
agent clean up, and relatively quick activation due to the use of optical flame detectors or HSSD systems. Relative
disadvantages include high cost, precise and costly maintenance and testing requirements, and the difficulty to
maintain stocker gas tight integrity, both initially and over time. Low leakage doors and HVAC dampers are usually
required to seal off the stocker from the surrounding atmosphere. Overall coordination between detection,
suppression, and ancillary systems can be challenging.
Fine water spray systems offer a mix of pros and cons of sprinklers and gaseous systems. Utilizing a very fine water
mist, these systems have been successfully shown through full scale testing to effectively extinguish stocker fires,
but again only in limited stocker sizes. Fine water spray systems exhibit performance that is not related to stocker
integrity, and are quickly activated similarly to gaseous systems, but also share in the complexities and high costs of
gaseous systems.
CARRIER MATERIAL CONSIDERATIONS
Boxes are comprised of few individual parts, and the outer shell is often constructed of one material. Pods, however,
are surprisingly complex, and are constructed of many individual components. The design and performance
constraints for SMIF pods and FOUPs are linked with AMHS parameters, including tool interfaces. Due to this, the
development efforts for pods must consider many more input variables than boxes.
However, both boxes and pods must meet the performance criteria of the semiconductor fabrication process. The
process parameters for wafer container materials are different than those for tool materials, as are the user’s needs.
Some of the design parameters for carrier materials selection are:
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contamination (e.g., outgassing, trace metals, anions, particle shedding)
process chemical compatibility
dimensional stability
ESD (Electro-Static Dissipation)
cost
weight
molding compatibility
temperature ratings
transparency
color
abrasion resistance
scratch resistance
moisture absorption
oxygen permeability
ultra-violet light transmission
Because of these requirements differ from those for tool construction, the fire safe material solutions which are now
prevalent will not in most cases be suitable for wafer carriers.
FIRE SAFE CARRIER DEVELOPMENT
The fire protection industry relies heavily on the concept of ‘listing’ or ‘approving’ a material or product for
acceptability by means of a third party independent testing laboratory, such as Factory Mutual Research, or
Underwriters Laboratories. These groups test items for suitability to a specific utilization or application, and are
often referenced in industry standards and building codes. However, the proper term to be used – listed or approved
– is related to the specific testing laboratory being referenced. The remainder of this paper utilizes Factory Mutual
Research terminology. In that respect, there is a difference between a Factory Mutual Research ‘Listed’ plastic, and
a Factory Mutual Research ‘Approved’ product, such as a carrier. Factory Mutual Research ‘Approved’ applies to
end use products used for specific purposes. Factory Mutual Research ‘Listed’ applies to testing of materials, which
are done according to specific procedures (e.g. ASTM E-84, or FM 4910). Therefore, FM 4910 plastics are not
Factory Mutual Research Approved, but they are Factory Mutual Research Listed.

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Additionally, some formed plastics, made from a resin that has passed the FM 4910 test, may not pass the same test.
The manufacturing process of the plastic, including additives (e.g., color, ESD, etc.) and manufacturing procedural
differences can affect the flammability properties of the plastic. Additionally, all Factory Mutual Research Listed
plastics are subject to production quality control audits by Factory Mutual Research, in which the manufacturing
process is verified to be identical to that used during the listing process. It is for this reason that Factory Mutual
Research Listing only applies to specific companies that have submitted their formed plastic, as opposed to a generic
listing of plastics.
The goal of any fire safe carrier testing program is to determine if a group of pods as arranged in a stocker will
produce a propagating fire. There are two testing methodologies that may be used to determine this.
The first method can be termed the ‘Listed Materials’ approach. In this approach, the final end use component – the
pod – is not itself Approved, but the materials utilized in constructing the carrier are each individually FM 4910
listed. This approach can be successful, but presents some challenges in determining if the whole pod is truly fire
safe, as some prototype pod designs currently are not constructed totally of fire safe materials.
Due to the complexity of the SMIF pods and FOUPs, most of the recent designs have included some portions of the
pod constructed of proven fire safe materials, with other sub-components being constructed of combustible
materials. This has been done to maximize the acceptance of critical components due to their unique process
requirements, such as ESD. This results in the question: What percentage of the pod needs to be fire safe in order for
the entire pod to be fire safe in its final configuration? The answer is indefinite at this time. The final configuration
of the carrier – the types of different plastics, their portion of the carrier’s total mass, and their location in the
carrier– is relevant, as some of the materials will propagate a fire. It is generally accepted that the presence of small
parts constructed of combustible materials - such as hinge pins, fasteners, labels, etc. – will not materially affect the
overall fire propagation properties of the pod. Due to the inherent unknowns in this approach, defining an answer
entails the assumption of some risk by all stake holders in the outcome, including the fab owner, the carrier
manufacturer, and insurance providers.
The second method can be termed the ‘Approved Component’ path. In this approach, the entire final end use
component – the pod – is Factory Mutual Research Approved for limited fire propagation. With this method, the
configuration of the individual components of the carrier is irrelevant, as the pod as a whole is proven to be
acceptable. The individual materials utilized in constructing the carrier do not necessarily even have to be formally
listed as fire safe. This approach incorporates full-scale fire testing of a number of carriers. Factory Mutual Research
has designed such a fire test for Approval of carriers, which utilizes several carriers in an open-faced array to
simulate a stocker configuration. This methodology avoids the question of “how much fire safe plastic is enough”,
and minimizes the risk going forward for all concerned parties.
Several carrier manufacturers currently have fire safe SMIF and FOUP designs in preliminary development.
However, significant challenges exist before a final product is offered on a routine basis to the semiconductor
industry. Most notable of these is the development and selection of fire safe materials appropriate for all of the
different components which make up a pod. Rather than re-engineer a developed product, carrier manufacturers
have initially expressed a preference to integrate fire safe materials into the initial development of a new product.
Therefore, the FOUP has been the main focus of carrier manufacturers’ research and development efforts. To date,
no carrier manufacturer has applied for Factory Mutual Research Approval of their pod or box. Once an acceptable
product is developed, it is expected that fire safe carriers will be gradually introduced into existing fabs as the
existing pods in use are discarded due to wear and obsolescence. Although carrier manufacturers are intimately
familiar with process and wafer manufacturing requirements, most fab owners will, prior to integration of new
carriers into the main stream of wafer production, perform their own performance testing or pilot production runs in
order to maximize compatibility with site specific processes, tools, and AMHS.
NEXT GENERATION FABS
The fire risk associated combustible carriers in current fabs is significant, but is relatively small compared with the
increased potential fire size and heightened frequency inherent in 300mm manufacturing facilities. This amplified
risk is due to several considerations. The FOUP is physically larger, and therefore contains more plastic than 200mm
and 150mm boxes or pods. This is compounded by potentially radical changes in AMHS configurations.

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Several next generation fab designs have been proposed which include carrier storage systems that extend from the
sub-fab up through the plenum space above the clean room ceiling. In order to differentiate the new designs from
those in use today, the single term ‘stocker’ is not appropriate in characterizing these new arrangements - perhaps
‘super stocker’ is more descriptive. The increased vertical height, overall volume, and fuel loading in these
arrangements combine to produce an environment in which a fire involving combustible carriers would be fast
growing and very large. Additionally, these arrangements introduce a large open vertical component to the overall
fab layout, extending through possibly three floors, which is a significant reduction in fire separation assemblies
employed in today’s fab designs (i.e., passive fire protection).
The frequency of possible fires involving super stockers will also be amplified. Due to the increased size and
complexity of the AMHS, personnel access and maintenance requirements will be greatly increased as compared to
today’s stockers. Similarly, larger physical spaces will result in more widely distributed motorized and automated
transport systems, utilizing higher power requirements. All of these factors indicate more possible ignition sources,
with higher energy and likelihood for ignition.
No current suppression system types – sprinklers, fine water spray, or gaseous – are known to be effective in these
configurations. Extensive fire protection research, including full scale suppression testing, will be required prior to
the fire protection industry being able to provide fire loss mitigation techniques for which fire control can be
determined with adequate reliability. This is not expected to be performed in advance of the first of these
installations, due to the cost and complexity of the tests. However, the provision of fire safe carriers would nullify
all of the previously stated concerns, and allow maximum flexibility in facility design.
CONCLUSIONS
Wafer carrier stockers present a significant fire risk to current 200mm and 150mm semiconductor fabs. Several
mitigation strategies are available to reduce this risk, but the provision of combustible carriers together with fire
protection and detection systems do not always limit the size of a fire loss to levels that are acceptable to fab
management and owners. Only the provision of fire safe carriers offers the combination of low hazard - property
damage and business interruption resultant from a fire - and operational flexibility. This will be even more important
in 300mm installations, as both the stockers sizes grow, and the need for flexibility increases due to innovative
AMHS schemes.
The development of fire safe carriers is ongoing, but is not at a mature stage. Although the final path for component
acceptance is not determined, full scale fire testing offers the least risk in development efforts. Increased
communication is needed between all stakeholders in the semiconductor industry – fab owners, carrier
manufacturers, AMHS manufacturers, testing laboratories, insurance concerns, Architect/Engineering firms, and
building and fire officials - in order to facilitate a smooth industry transition from combustible carriers to low fire
propagation products.

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ABOUT THE AUTHOR
Mark is a semiconductor specialist with FM Global, the world’s largest commercial and industrial property
insurance organization specializing in property protection, of which Factory Mutual Research is an affiliate.
He is responsible for providing field servicing services to several semiconductor fabs in the Northwest, and
also serves as a consultant for other engineering specialists worldwide on FM Global’s Fab Team. He has a
B.S. in Mechanical Engineering from the University of Florida, and is a registered Professional Engineer in
the fire protection and mechanical disciplines. He is a member of SSA, and a full member of the Society of
Fire Protection Engineers. He currently resides in Portland, Oregon.