silicon, water, and propylene filters create an explosive event · 2017-05-08 · silicon, water,...
TRANSCRIPT
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Silicon, Water, and Propylene Filters Create an Explosive
Event
Jamie Rubin
Avago Technologies
970.288.4880
September 2015
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The Incident
� Teaching a class from 9AM to 3 PM
� 10:30 AM waste water technician comes in and
says:
– I was draining the backgrind filters and I came back a
couple hours later to transfer them to a waste drum and
they were really warm to the touch. What should I do?
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What Would You Do?
� Several times in this this presentation asking for
comments and your suggestions –
– Based on the available information
– #1 – The initial incident notification
– #2 – Root cause analysis
– #3 – NIOSH HHE investigation
– #4 – NIOSH HHE report
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The Backgrind Process
� Thinning silicon wafers
– Water and diamond grit “paper”
� The “filters” remove particulate prior to discharging water to POTW (wastewater treatment plant)
� Particulate removed using polypropylene filter material
� Replace filters when “clogged”
– So ground up crystalline silica, water, maybe some grit from the “paper” on polypropylene filters
– Filters heating up after being drained of water
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Clarifications Needed?
� The warm filters had not been reported before
� HOWEVER -- This is a new duty for this waste
water technician, previous technician just retired
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What Would You Do?
� Review:
– Ground up crystalline silica, water, maybe some grit from the “diamond – sand paper” imbedded on polypropylene filters
– Filters heating up after being drained of water
� Your Suggestions?
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What Would You Do?
� Put drum of filters in an over-pack drum and run
cold water into the drum
� Run cold water in the drum to cool the filters
� Take the drum of filters to a safe place
� Evacuate the building
� Contact local HazMat to handle drum
� Join the previous waste water technician and
RETIRE IMMEDIATELY
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What We Did
� Asked Technician to move drum outside to a vacant area and investigate after teaching class
– Technician was a good “hazardous waste technician” –thought we needed to put a lid on the drum because
� “All hazardous waste containers need to be closed except when adding or emptying waste to the drum!”
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What We Did
� What reaction could happen?
– Silica and water – silane gas?
� Experts consulted
– Silane gas manufacturer
– Other EHS professionals with silane safety expertize
� Silane gas can not be produced because it would need an acidic environment.
NOTE: If silane could be generated, plan was to call the local bomb squad to handle the drum
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What is Silane Gas
� Formula SiH4
� Widely used in the manufacturing of silicon wafer
products and solar panels (photo-voltaics)
� Comes in pressurized cylinders
– Pyrophoric – burns in air without an ignition source
– In still atmospheres may not immediately react
– Not very toxic (TLV 5 ppm)
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What We Did
� Because we have a sealed drum:
– Do not have access to the contents in the drum to conduct any air monitoring
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The Drum
� Drum was warm to the touch but not mis-shapen
� Decided to don PPE: nomex suit, SCBA and leather
gloves and to remove the lid from the drum
– Others in the area for “back-up”
� While loosening the drum bolt:
– Observers reported hearing a pop like a “gun shot”
– Drum lid went up in the air 5-7 feet
– No one was injured!
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Scene of the Incident
� Drum was in an unoccupied area
� Picture of drum and where the drum lid landed
� Bolt was loosened but not out!
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Investigation Root Cause
� Determine cause of exothermic reaction
� Determine cause for drum incident
� Eliminate/minimize the hazard
Was this a one time event or likely to occur again with
the same conditions?
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Manufacturing Process
� Avago Technologies in Fort Collins, Colorado manufactures semiconductor devices
– Cell phones / wireless communication
� Circuitry on silicon wafers
� Thin the wafer – backside
� Cut into individual die
� Placed in a package
� Sold to customer
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The Thinning Process
� “BACKGRIND”
� Circuitry on top of wafer –thinning backside
� Thinning silicon wafers
– Water and diamond grit paper
� Remove particulate prior to discharge to POTW (wastewater treatment plant)
– TTS (Total Suspended Solids)
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The Thinning Process
� Removal of the Ground Up Silicon:
– Filter media:� Polypropylene bags
� Polypropylene cartridges
– Replace filters when “clogged”
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Backgrind/Wastewater Process
Back grinder
Lift
station
in
tunnel
1500 gallon
tank
Bag Filter Cartridge Filter
Discharge
Re-circulated
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Tank
Cartridges
Filter bags
cartridges
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Filter Bags
� Filter Bags used to remove
silica particles
– First stage.
� Made of polypropylene
New bag
(white)
Used bag
(black)
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Filter Cartridges
� Filter Cartridges used to
remove silica particles
– Second stage.
� Made of polypropylene
– 0.5 micron filter
Used cartridge
(black)
New cartridge
(white)
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What Would You Do?
� Root Cause Analysis
– Is this a repeatable event?
– Drained filters and monitored them
� Began to heat up again.– Your Suggestions?
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What Would You Do?
� Sample using a multigas meter for flammables
� Sample for silane
� Take a grab sample and send to a lab
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Initial Investigation
� What we did:
� Sampled for silane
– Hydride gas portable
monitor
– Paper tape system
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Paper Tape Monitoring
� Pulls an air sample through a chemically
impregnated “paper tape”
– ChemCassette (trade name)
� If contaminant is present the tape is stained
� Shine a light onto the tape and measure the amount
of light reflecting – darkness of the stain determines
the concentration of the contaminant.
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Paper Tape Monitoring
� General categories
– Hydrides
– Mineral acids
– Oxidizers
– Bases
� Picture of
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What is a Hydride Gas?
– Silane - SiH4,
– Arsine - AsH3
– Phosphine - PH3
– Diborane - B2H6
– And others
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Initial Investigation
� Each dot represents an
separate sampling
period
� Hydride gas
consistently detected
– BUT which hydride gas?
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WHAT IS STAINING THE
HYDRIDE SENSING CHEM-
CASSETTE TAPE?
Manufacturer’s description:
ARSINE - YELLOW-BROWN
PHOSPHINE - YELLOW-BROWN
SILANE - GREY
What color do you see?
What gas might it be?
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Basic Multigas Meter
� Oxygen concentration
� Flammable gas/vapor
� Carbon monoxide
� Hydrogen sulfide
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Initial Investigation
� Samples using Multi-
Gas meter
– Oxygen normal or
slightly reduced levels
– Flammable gas detected
– CO detected
– H2S not detected
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CSU Student Involvement
� Colorado State University has a graduate EHS program.
� Local SESHA members often present to these students
� One of the students asked if he could help conduct investigation as part of SESHA Scholarship Paper
– Semiconductor Environmental Safety and Health Association
– Scholarship winner when presented preliminary data
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Methods/Research
� Testing Performed:
– Hydride ChemCassette monitor
– Direct reading gas instruments
– Additional ChemCassette monitoring
– Detector tube testing
– Analytical Testing
– GC Mass Spectrometer grab sample
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Silane Hazards
� Pyrophoric
– Ignition without ignition source possible if > 2% silane
� Unpredictable
– Releases should be “mixed” to control combustion
� TLV 5 ppm
� DOT class 2.1 (Flammable Gas)
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TLD-1 hydride
Monitor
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ChemCassette Monitor
� Portable paper-tape colorimetric monitor.
� Standard continuous monitor for industry
� Hydrides ChemCassette:
– Silane Key: Exceeded maximum:15 ppm *
* Maximum reading for the TLD-1
instrument is 3X TLV.
NOTE: silane pyrophoric at >2% (20,000 ppm)
These levels are noted within 15 minutes after draining the filters
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Backgrind/Wastewater Process
Back grinder
Lift
station
in
tunnel
1500 gallon
tank
Bag Filter Cartridge Filter
Discharge
Re-circulated
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Backgrind/Wastewater Process
Back Grinder
Lift
station
in
tunnel
1500 gallon
tank
Bag Filter Cartridge Filter
Discharge
Re-circulated
No “hydride” gas detected
“Hydride” gas detected
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ChemCassette Monitor
� Oxidizers ChemCassette
– None detected
� Mineral Acids ChemCassette
– None detected
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Direct Reading Instrument
� Sample drum of filters with plastic cover – 15 minutes
� Multigas Electrochemical Cell
– % LEL – 60%
– CO – 999 ppm (maximum reading on the instrument)
– 02 – 19.5% (Down from 20.8%)
� Consuming oxygen or displacing oxygen?
– H2S – 6.5 ppm
� Different multigas meter
– Similar results
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Hydrogen confirmation
� Literature search indicated hydrogen production is
possible:
– FORMULA
�2 H20 + Si = SiO2 + 2 H2 (exothermic)
– But hydrogen will not stain hydride Chemcassette
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Initial Sampling Results(possible interferences)
Analytical Method AsH3 PH3 SIH4 CO H2 H2S
Other Hydride
Hydride ChemCassette Maybe Maybe Maybe Maybe Maybe
Direct Reading Flammable Maybe Maybe Maybe Maybe Maybe
Direct Reading CO Maybe Maybe Maybe
Direct Reading H2S Maybe Maybe
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Detector Tubes
� Drager detector tubes used:
– Carbon Monoxide
� None detected (Draeger 2a)
– Hydrogen
� Present
– Arsine
� Maybe -- Unusual stain color
– Silane
� Drager does not manufacture a silane detector tube
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CO Detector Tubes
� Dräger-Tubes & CMS-Handbook
– Carbon Monoxide 2/a 2 to 60 ppm
– Carbon Monoxide 5/c 100 to 700 ppm
– Carbon Monoxide 8/a 8 to 150 ppm
– Carbon Monoxide 10/b 100 to 3000 ppm
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Arsine (Colorimetric)
Right Side:
BlankLeft Side:
Pumped seven times
**stain is not the
color indicated by
manufacturer for
arsine (violet) but
turned from white to
gray
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Hydrogen confirmation
� Detector tube indicated hydrogen present.
� Hydrogen could also explain the multigas meter
readings for
– LEL – would have normal response to hydrogen
– Carbon monoxide – reading equipment specifications,
hydrogen is a false positive for electrochemical CO
sensors in multigas meters used.
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Phase 2 Sampling Results
Analytical Method
AsH3 PH3 SIH4 CO H2 H2SOther
Hydride
Arsine DetectorTube NO
CO Detector Tube NO
Hydrogen Detector Tube YES
Odor NO
Literature Search YES
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Silane Sampling
� Impinger sampling for silane
� Detects silica and assumes silane.
� Four Separate Locations
– Additionally, a “control” was included; sample in non-contaminated
area.
� Pre-filtered (remove particles) midget fritted impinger with
10 milliliters of a 0.01 N KOH solution.
� Small quantities of Si detected.
– Lower amounts than indicated by hydride monitor
– Si detected in blank and control
– Could silica source come from glass impinger in corrosive solution?
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Phosphine (NIOSH 1003)
� Four Separate Locations
– Additionally, a sample blank was included.
� Sampled according to NIOSH Method 1003.
� No phosphine present.
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Arsine (NIOSH 6001)
� Arsine was sampled according to NIOSH Method
6001.
� Arsine is not present.
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2nd Silane Sampling
� Eliminate Glassware from sampling and analytical
method
� Small quantities of Si detected.
– Low quantities in control
– Higher quantities in tank off-gassing
– Highest levels with filter off-gassing
� Increases matching hydride ChemCassette
– but analytical results lower quantitatively than
Chemcassette
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GC Mass Spectrometer
� A sample was collected in a large Mylar bag and
analyzed at Colorado State University.
� Nothing was detected.
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Phase 3 Sampling Results
Analytical Method
AsH3 PH3 SIH4 CO H2 H2SOther
Hydride
GC Mass Spec
Silane Analytical Maybe
Phosphine Analytical NO
Arsine Analytical NO
2nd Silane Analytical Maybe
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Summary Sampling Results (SESHA presentation)
� BUT:
– Silane analytical methods concentration less than
the hydride ChemCassette readings.
� Factor >10X
– Unknown gas is staining hydride ChemCassette.
� Hydrogen does not stain ChemCassette.
Analytical Method
AsH3 PH3 SIH4 CO H2 H2SOther
Hydride
SUMMARY NO NO Maybe NO YES NO Maybe
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Summary Sampling Results (SESHA presentation)
� NOTE:
� Hydrogen present
� Silane may be present
– Something is staining hydride gas detection monitor
– Lab results indicate silica is present
� Carbon Monoxide not present
– Detector tube negative and multi-gas meter – hydrogen
interference
Analytical Method
AsH3 PH3 SIH4 CO H2 H2SOther
Hydride
SUMMARY NO NO Maybe NO YES NO Maybe
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What Next?
� Talking to colleagues and they suggested this might
warrant a NIOSH Health Hazard Evaluation (HHE)
� Wrote to NIOSH
– They looked at previous data
– They were interested in investigating this further.
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What Should NIOSH Do?
� Your Suggestions?
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What Should NIOSH Do?
� Determine hydride generation issue (silane?)
� Confirm hydrogen main hazard
� Review and verify preliminary investigation
results
� Identify other health issues
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NIOSH HHE
� The day before NIOSH arrived:
– Removed filters and placed in drum
– Covered drum (with plastic) to concentrate off-
gassing constituents
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NIOSH HHE Sampling
� Collected samples in
“blood draw” vacuum
tubes
� Monitored for temperature
change
� NIOSH contract analytical
lab used
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NIOSH HHE Sampling
� Direct reading AND
personal monitor
– All instruments
detected CO
– Could be
interference from
hydrogen?
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NIOSH HHE Sampling
� Temperature rise
documented
� Note moisture on underside
of plastic “shower cap”
– >10o F rise
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HHE Preliminary Results
� Exothermic reaction occurring
– Temperature in drum slowly rising
� Gases detected: (lab results)
– Hydrogen concentrations gradually increasing
– Elevated carbon monoxide concentrations were noted
– Elevated carbon dioxide concentrations were noted
� Silane (if present) could not be detected with GC-
MS set-up.
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NIOSH Initial Results
� Examples of results from one drum:
– Vacutainer air sample data collected from inside
covered drum.
– Thirteen vacutainer air samples were collected
over approximately 4 hours. The concentrations
ranged from
� H2 = ND to 84,700 ppm
� CO = ND to 19,900 ppm
� CO2 = ND to 16,700 ppm
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HHE Preliminary Results
� Carbon monoxide levels reported by lab are very
high – exceeding IDLH (1200 ppm)
– No employees in the area – including NIOSH
investigators had headaches but should have if results
correct.
– Lab stands by results
– NIOSH needs explanation or HHE report would say
overexposure to CO is occurring.
� REMEMBER: Preliminary investigation CO ruled out
because Draeger tube (2a) showed no CO present!
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Follow-up sampling by site IH
� CO present according to NIOSH samples yet
detector tube readings indicated none detected
� Conducted follow-up sampling.
– ChemCassette hydride sampling
– CO detector tubes (Drager 2a)
� Results
– Same hydride readings noted
– No CO readings from detector tubes BUT …..
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Follow-up sampling
� Interesting detector tube results
� Two of many tubes turned green a few hours after sample was taken
� Drager responded this green delayed reaction may be caused by hydrogen
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Follow-up sampling
� NIOSH requested
additional sampling
– used a Drager 8a detector
tube for CO commonly used
in refining industry where
hydrocarbons are present
(pre-filter absorbs)
� Results indicated CO is
present but in the <100
ppm range.
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Detector Tubes – Variations!
� I’m Curious:
– Tested three types CO
Drager tubes
– Drager 10d
� >3000 ppm
– Drager 8a
� 20-30 ppm
– Drager 2a
� None detected
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CO Detector Tubes
� Detail Sheet on each Detector Tube:
� Reaction Principle
– All three tubes:
– 5 CO + I2O5 I 2 + 5 CO2
– Cross sensitivity
� No mention of hydrogen or silane
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Follow-up sampling
� Draeger does not have a
silane detector tube
� HOWEVER: Sensidyne
has a silane detector tube
� Indicates silane is present
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Silane Conclusion?
� Detector Tube:
– Sensidyne Detector tubes --results indicate silane is likely present
� Chemcassette with hydride tape stains
– Ruled out arsine, phosphine causing stain
– Hydrogen and carbon monoxide do not stain tape
– Quantitative results from Chemcassette compare within +/- 25% to Sensidyne Detector tube readings
Silane is likely present. No analysis has ruled it out!
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Sampling Results HHE phase one and onsite follow up
Analytical Method AsH3 PH3 SIH4 CO H2 H2S
Other
Hydride
Initial Summary NO NO Maybe NO YES NO Maybe
Drager 2a Tubes NO
NIOSH Sampling YES YES
Drager 8a Tubes YES
Drager 15d Tubes YES
Sensidyne Tubes Likely
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HHE and Carbon Monoxide
� What should NIOSH do regarding carbon
monoxide results?
� Your Suggestions?
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NIOSH’s Plan
� Sample again – use different lab
– OSHA’s lab in Salt Lake City
� Use a different analytical procedure than the
procedure the other AIHA certified lab used.
� Use additional CO personal dosimeters
� Still no GC-MS method for silane detection
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HHE Follow-up Sampling
� Analytical (SLC)
stilled show CO
present but much lower
levels
� All CO personal
dosimeters showed
very high CO levels
(but hydrogen
interference noted)
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What Should HHE State?
� Time to write the HHE report
– Specifically regarding Carbon Monoxide exposure levels.
� Your Suggestions?
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Confounding Results
Excerpts for HHE report:
� In our first site visit, we were surprised to measure
high CO and CO2 concentrations inside the covered
drums because there was no apparent source to
generate these gases
� The different measurement and analysis techniques
we used during the second site visit did not clarify
whether CO and CO2 were truly present as our
results continued to show a wide range of
concentrations.
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NIOSH Conclusion for CO
� The vast difference in the gas concentrations
measured by different analytical and measurement
techniques makes it difficult to have confidence in
the results, especially the high CO concentrations.
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NIOSH Conclusion for CO
� In addition, employees working near the process
and conducting tasks such as removing cartridge
tank filters reported no health symptoms such as
headaches and nausea that might indicate high CO
exposures.
� However, there is not sufficient evidence to
suggest the CO concentrations found should be
disregarded.
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What NIOSH Report States:
� Hydrogen was measured at concentrations up to
118,000 parts per million (~12%) inside the covered
drums.
� Silane was measured at concentrations of 20 to 50
parts per million inside a covered drum.
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H2 production possible
� Theoretically, silicon reacts with water to produce
silicon dioxide (silica) and hydrogen, and this
reaction (wet oxidation) can be expected to occur
during the wet grinding process, as shown below
[Doremus and Breed 1976; Stephen and Riley 1989;
Chemguide 2010; Gatech 2010].
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SiH4 production possible
� It is possible that some silicon can react with the
generated hydrogen gas to form silane as shown
above. Experiments involving the reaction of water
vapor with freshly cleaved, etched, or polished
silicon surfaces have shown silane in ppm
concentrations [Lampert et al. 1986].
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Hydrogen and Silane Formation:
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What NIOSH Reported
� Disposing of used filters from the silicon wafer
grinding filtration process in covered drums can
pose a health and safety hazard for employees.
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HHE Conclusion
� Regarding the initial drum incident: On the basis of
a scientific literature review and air sampling
results, we conclude that the covered drum was
pressurized primarily because of H2 formation and
that silane was also formed.
� Silane is a pyrophoric gas. Concentrations greater
than 0.5% in hydrogen may ignite spontaneously in
contact with ambient air [ACGIH 1989].
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HHE Conclusion
� Disposing of used filters from the silicon wafer
grinding filtration process in covered drums may
pose a health and safety hazard to employees.
� This hazard can be minimized by leaving the drums
uncovered …
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Safeguards
� Filters removed – placed in open
drum and left to off-gas– One to two weeks!
� Testing before disposal
– Cover drum with loose plastic
– Wait overnight
– Test with hydride or multigas
monitor
� None detected reading required before
disposal.
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New Process in Place
� No longer remove particulate
– Contacted our POTW – have no problems with small
particle – go right through the system
– Contacted CSU wildlife and asked if harmful to fish etc
and they also indicated small particle pass right through
and should not be harmful.
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Personal Conclusions
� CO sampling is not an exact science!
– Even different AIHA certified labs give widely
varying CO results with grab samples when H2 is
present.
– Multi-gas meters and personal monitors for CO
usually give high readings if H2 also present.
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Personal Conclusions
� Detector tubes for Carbon Monoxide
– Widely ranging results with CO if H2
present (other issues?)
� No good method for silane monitoring
– Paper tape system very sensitive, but not
specific for silane
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Personal Conclusions
� Investigating this incident – a learning experience
� I failed in the “anticipation, recognition, evaluation
and control” process:
– Removing silica particles prior to discharge to POTW