Environmental Health
and Safety (EHS) of
Gas Boxes/Cabinets
Joseph B. Barsky, MS, CIH and Russ McDonald
October 27, 2011
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Objectives
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Regulations and Terminology
Best Engineering Practices
Safety Controls are not "Customer Options"
EHS considerations when buying used equipment
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Regulations and Terminology
• Regulations and Terminology
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Definitions
Gas cabinet: An exhausted enclosure used to contain cylinders of process gases that
contains leaks without risk of affecting people. A well-designed gas cabinet can also
accommodate panel-mounted manifolds and gas handling systems that provide precise
control over operating parameters and total purging of all equipment and lines. They
protect personnel with a safe, efficient and cost-effective means to organize gas
distribution.
Gas box: A subsystem of the factory gas delivery system located at point-of-use, typically
internal to a semiconductor process tool, and utilized to control delivery of process gases
into the processing chamber. Also referred to as gas interface box and gas jungle.
Mass flow controller (MFC): A self-contained device (consisting of a transducer, control
valve, and control and signal-processing electronics) commonly used in the
semiconductor industry to measure and regulate the mass flow of gas.
Mass flow meter (MFM): A self-contained device, consisting of a mass flow transducer and
signal-processing electronics, commonly used in the semiconductor industry to measure
the mass flow of gas.
Pressure transducer: A device commonly used in the semiconductor industry to measure
gas pressure. Typically consisting of a sensor and signal-processing electronics, this
device allows for remote indication of gas pressure.
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Definitions
Purifier: An in-line device used for the removal of homogeneous impurities from gases,
typically consisting of a packed-bed of active solids contained in a stainless steel housing.
The active purification media may remove impurities such as moisture, oxygen, CO, CO2,
hydrocarbons, hydrogen, or nitrogen from specific gases using a variety of mechanisms.
Point-of-use purifiers often contain a particle filter within the same housing.
Regulator: A valve designed to reduce a high incoming pressure (for example, from a
cylinder) to a lower outlet pressure by automatically opening to allow flow until a desired,
preset pressure on the outlet side is reached, then automatically throttling closed to stop
further pressure increase.
Valve: (1) A device that controls the flow or pressure of a gas. Valve functions can include
shutoff, metering, backflow prevention, and pressure relief. (2) Any component designed
to provide positive shutoff of fluid media with the capability of being externally operated.
Newer systems should have lockable valves to meet Lockout/Tagout requirements.
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Typical Gas Systems
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Regulations and Standards
Fire and Storage requirements may differ from location to
location.
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California Fire Code, Chapter 27
International Fire Code
Uniform Building Code
Uniform Fire Code
Cal/OSHA / Federal OSHA
NFPA 704
SEMI
CGA
ANSI
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Some Pertinent SEMI Standards
• SEMI F1- Specification for Leak Integrity of High Purity Gas Piping Systems and
Components
• SEMI F3- Guide for Welding Stainless Steel Tubing for Semiconductor
Manufacturing Applications.
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SEMI F4- Specification for Pneumatically Actuated Cylinder Valves
SEMI F5- Guide for Gaseous Effluent Handling
SEMI F6- Guide for Secondary Containment of Hazardous Gas Piping Systems
SEMI F13- Guide for Gas Source Control Equipment
SEMI F14- Guide for Design of Gas Source Equipment Enclosures
SEMI S2- Environmental, Health, and Safety Guideline for Semiconductor
Manufacturing Equipment
• SEMI S4- Safety Guideline for the Segregation/Separation of Gas Cylinders
Contained in Cabinets
• SEMI S6- EHS Guideline for Exhaust Ventilation of Semiconductor Manufacturing
Equipment
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Gas Cabinets and Gas Boxes
Gas boxes are a typical 1/4-inch delivery system, with
assumed that regulated gas of no greater than 50 psi will be
used in furnaces, chemical vapor deposition (CVD) tools, and
etch reactors. Systems are designed for flows no greater
than 30 slm.
Cylinder storage problems are simplified because the use of
gas cabinets encourages separation of gases according to
their hazard classification. Separation of gases becomes
standard procedure, as opposed to indiscriminate storage
and grouping. For example, corrosives, oxidizers,
flammables and toxics can be separated and grouped into
separate cabinets. Not only will this satisfy both the national
and local fire and building codes, but it is also safer in the
event of component failure or leakage.
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Gas Cabinet Objectives
A properly designed gas cabinet system will fulfill the following
objectives:
• To control air flow around compressed gas cylinders and
manifolds - isolating hazardous gases from operators
• To minimize hazards from compressed gases in event of a
fire external or internal to the gas cabinet
• Maintenance of gas integrity
• Automatic shutoff of gas in the event of catastrophic failure
• Effective control of residual gas during cylinder changeout
A gas cabinet with
panel-mounted
manifolds.
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Materials of Construction
Use of 12 gauge steel or thicker for the cabinet and door will ensure sturdiness and
also provide a half-hour or more of fire protection.
If poisonous gases are to be kept in the cabinet, an access port or window must be
provided so the cylinder valves can be closed and leaks detected without opening
the cabinet door and compromising the exhaust system.
Must have self-closing doors, self-closing limited access ports or fire rated windows.
Gas cabinets, cylinders and delivery lines must be seismic braced.
Provide welded connections or exhausted enclosures for all Class I & II gases.
For corrosive gases, primary piping shall be constructed of inert materials, or
secondary containment shall be provided.
Provide an approved manual activation control for all Class I & II gases.
Material compatibility is an important consideration in selecting equipment for gas
applications. An improper selection can result in problems ranging from corrosioncaused contaminants to catastrophic failure. It is recommended that users be
aware of any potential hazard before using a gas and take the proper precautions.
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Ventilation
In order to provide containment of potentially dangerous gases,
cabinet exhaust systems should be designed with the capability to
allow an average of 200 linear feet per minute (61 linear meters per
minute) of air to pass through the cabinet with the access window
open with a minimum single point of 150 linear feet per minute (45.7
linear meters per minute). This is roughly equivalent to 13 air
changes per minute. A ventilated gas cabinet facilitates the safe
management of the gases stored within it.
To ensure ventilation is working as specified, a photohelic gauge is
used to monitor static pressure in the duct.
Gas cabinets must be connected to an abatement (treatment) system.
No exterior storage area shall be within 75 feet of a fresh air intake.
Cabinets shall be constructed to sweep the exhaust by the cylinder
connection and gas manifold, preventing pocketing of gases or dead
spaces.
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Fire Protection
As an extra measure of fire protection, all gas cabinets shall be
equipped with an integral sprinkler system. While exact
requirements may vary with the specific application, a typical
sprinkler would have a fusible link rated at about 135°F
(57°C) and a flow capability of approximately 40 GPM
(2.524 L/s). If used with corrosive gases, they must be
coated for protection from corrosion.
“No smoking” signs must be posted within 25 feet of outdoor
storage, use, and handling areas.
Provide a local manual emergency alarm, emergency
telephone, or signaling device where Class I & II gases are
transported through exit corridors or exit enclosures.
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Chemical Protection
Gas cabinets with toxic gases shall have a continuous gasdetection system to monitor for potential leaks. If a leak is
detected, it should initiate local and area audible and visual
alarms and automatically shut down the delivery system.
Monitor the room or area where the gas is stored at or below
the Permissible Exposure Limit (PEL)
Monitor the discharge from a treatment system to ensure it is
at or below Immediately Dangerous to Life or Health (IDLH)
A special leaker gas cabinet should be provided so a leaking
cylinder can be put into this cabinet. The leaker cabinet
should have a higher exhaust flow rate.
The number of cylinders contained in a single gas cabinet
shall not exceed three, one of which must be an inert purge
gas.
Pressure relief valves must go to the abatement system.
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Gas Detection
Choosing the correct gas detection system for your application
takes skill and understanding of the hazards, potential
interferences, and conditions of the area to be monitored.
Toxic gas detectors may be electrochemical, metal oxide
semiconductor (MOS), chemically impregnated paper tape,
photoionization, pyrolysis, galvanic cell, catalytic combustion,
and infrared sensing elements.
They can be in the exhaust stream, inside the gas cabinet, or
sample draw type to prevent dust and dirt from affecting the
sensors.
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Some Advantages/Disadvantages
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Most Important
Calibration with a zero air standard and a certified
calibration standard concentration of the gas being
monitored.
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Automatic Shutdown for All Class 1 Gases
Provide an automatic "fail-safe to close" shut-off valve for the
following:
1. Gas detection
2. Remote location alarm
3. Failure of emergency power
4. Seismic activity
5. Failure of primary containment
6. Activation of manual fire alarm
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Restricted Flow Orifice and Treatment Systems
Restricted Flow Orifice (RFO). All gas cylinders
with TGO Class I gases (except for lecture
bottles) having a vapor pressure greater than 29
psia shall have a restricted flow orifice.
All cylinders shall be marked with RFO size.
Treatment system must be capable of reducing the
maximum allowable discharge concentration of
the gas to one-half the IDLH at the point of
discharge.
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Maintenance
Annual maintenance is required for all safety control systems.
Ex: Monitoring system, scrubbers, seismic detectors,
ventilation, etc.
Records must be maintained to prove adequate maintenance
of all safety control systems.
The ventilation rate of every mechanical ventilation system
used to prevent harmful exposure shall be tested after initial
installation, alterations, or maintenance, and at least
annually, by means of a pitot traverse of the exhaust duct or
equivalent measurements. Records of these tests shall be
retained for at least five years (Cal/OSHA Title 8, §5143.
General Requirements of Mechanical Ventilation Systems).
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Emergency Response
An emergency response plan must be developed and submitted to the
AHJ.
Submit or update the emergency response plan component of the
Hazardous Materials Business Plan (HMBP),
Quarterly emergency drills must be conducted and maintain records of
same for three years.
Gas alarms must transmit an alarm signal to a constantly attended
control station for two or more cylinders.
Provide a minimum of two breathing apparatus and other appropriate
protective equipment for Class I or corrosive gases. Ensure they are
placed nearby, but in a safe location to put them on.
Must provide emergency power for the following:
1. Exhaust ventilation and treatment system
2. Gas detection system
3. Emergency alarm system
4. Temperature control systems
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Permits and Regulated Gas Releases
Permits must be submitted for storing, use, or handling of new
regulated gases, new gas cabinets, relocations, and
decommission and closures.
Submit report of incident and immediate notification of the fire
department for any unauthorized releases.
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TGO Gases
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TGO Gases
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TGO Gases
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TGO Gases
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Valves
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Valve Specs
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Regulators
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Regulator Specs
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MFC/MFMs
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MFC/MFM Specs
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Pressure Transducers
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Pressure Transducer Specs
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Filters/Purifiers
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Filters/Purifiers Specs
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Resources
SEMATECH Design Guidelines for Gas Box Components,
Technology Transfer 96063137A-ENG
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Best Engineering Practices
• Best Engineering Practices
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Photohelic Gauge vs Magnahelic Gauge
Gas cabinets must be equipped with an
exhaust failure monitor alarmed and
interlocked to shut off the gas by the
cylinder.
Both gauges are indicators of static pressure
relative to the amount of exhaust flow in
the duct.
• A photohelic has two contacts for
shutdown and alarm controls (orange bars)
which are set by the customer for upper
and lower safe operating setpoints upon
system exhaust certification.
• A magnahelic has no contacts and is used
as an indicator only.
• Various models are available and the
proper scale must be specified for each
use.
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Safety Controls are not "Customer Options"
• Many manufacturers today are under great cost pressures
when dealing with customers in Asia. Many have requested
stripped-down versions of the standard product offerings with
safety built-in.
This is not like ordering a new car with a radio-delete. More
like ordering a car without brakes and no air bags.
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EHS considerations when buying used
equipment
• Do you know for sure what the prior gases used were and are they
compatible with what gases you want to use
• Pump purge is not sufficient to run incompatible gases
• Was the gas box/cabinet properly decommissioned?
• Are there any parts missing?
• Do the automated controls work properly? Pump purge? EMO?
• Is there evidence of prior “dusting” (silane) or corrosion from past
leaks (HBr)?
• Do the doors still close automatically?
• Any physical damage to the gas box/cabinet?
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Exhaust considerations
For best exhaust flow, use the largest duct diameter possible, eliminate
as many 90 degree turns as possible, use 45 degree “Y” connections
for lowest loss of static pressure, don’t use square duct from HVAC
folks.
Ensure that the fan is placed as close to the outlet of exhaust as
possible as the duct will be positive pressure after going through the
fan and leaks before the fan will be into the duct and after the fan will
be out of the duct. Avoid positive pressure ducts in occupied areas.
Ensure that there is sufficient flow to prevent fallout of contaminants prior
to abatement.
Ensure exhausted materials are compatible on the way to the abatement
device.
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Exhaust considerations
All non-welded connections for toxics and flammables must be within the
exhausted gas cabinet. That means purifiers, MFCs, etc.
If duct depositions are possible, plan for removable sections for ease of
cleaning with periodic preventive maintenance.
Do not presume prior tracer gas tests meet today’s requirementsexample the Arsine Threshold Limit Value- Occupational Exposure
Limit changed in 2007 from 50 ppb to 5 ppb. Under SEMI S6
requirements tracer gas testing must show leakage from the enclosure
to be less than 1.25 ppb from the worst case release scenario.
Safe Delivery Systems (SDS) while under vacuum in normal use may
not be in a fire situation. Many jurisdictions are adding additional
requirements.
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