silicon, water, and propylene filters create an explosive event · 2017-05-08 · silicon, water,...

Silicon, Water, and Propylene Filters Create an Explosive

Event

Jamie Rubin

Avago Technologies

970.288.4880

Jamie.Rubin@avagotech.com

September 2015

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?

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

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

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

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?

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

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!”

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

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)

What We Did

� Because we have a sealed drum:

– Do not have access to the contents in the drum to conduct any air monitoring

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!

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!

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?

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

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)

The Thinning Process

� Removal of the Ground Up Silicon:

– Filter media:� Polypropylene bags

� Polypropylene cartridges

– Replace filters when “clogged”

Backgrind/Wastewater Process

Back grinder

Lift

station

in

tunnel

1500 gallon

tank

Bag Filter Cartridge Filter

Discharge

Re-circulated

Tank

Cartridges

Filter bags

cartridges

Filter Bags

� Filter Bags used to remove

silica particles

– First stage.

� Made of polypropylene

New bag

(white)

Used bag

(black)

Filter Cartridges

� Filter Cartridges used to

remove silica particles

– Second stage.

� Made of polypropylene

– 0.5 micron filter

Used cartridge

(black)

New cartridge

(white)

What Would You Do?

� Root Cause Analysis

– Is this a repeatable event?

– Drained filters and monitored them

� Began to heat up again.– Your Suggestions?

What Would You Do?

� Sample using a multigas meter for flammables

� Sample for silane

� Take a grab sample and send to a lab

Initial Investigation

� What we did:

� Sampled for silane

– Hydride gas portable

monitor

– Paper tape system

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.

Paper Tape Monitoring

� General categories

– Hydrides

– Mineral acids

– Oxidizers

– Bases

� Picture of

What is a Hydride Gas?

– Silane - SiH4,

– Arsine - AsH3

– Phosphine - PH3

– Diborane - B2H6

– And others

Initial Investigation

� Each dot represents an

separate sampling

period

� Hydride gas

consistently detected

– BUT which hydride gas?

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?

Basic Multigas Meter

� Oxygen concentration

� Flammable gas/vapor

� Carbon monoxide

� Hydrogen sulfide

Initial Investigation

� Samples using Multi-

Gas meter

– Oxygen normal or

slightly reduced levels

– Flammable gas detected

– CO detected

– H2S not detected

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

Methods/Research

� Testing Performed:

– Hydride ChemCassette monitor

– Direct reading gas instruments

– Additional ChemCassette monitoring

– Detector tube testing

– Analytical Testing

– GC Mass Spectrometer grab sample

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)

TLD-1 hydride

Monitor

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

Backgrind/Wastewater Process

Back grinder

Lift

station

in

tunnel

1500 gallon

tank

Bag Filter Cartridge Filter

Discharge

Re-circulated

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

ChemCassette Monitor

� Oxidizers ChemCassette

– None detected

� Mineral Acids ChemCassette

– None detected

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

Hydrogen confirmation

� Literature search indicated hydrogen production is

possible:

– FORMULA

�2 H20 + Si = SiO2 + 2 H2 (exothermic)

– But hydrogen will not stain hydride Chemcassette

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

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

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

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

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.

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

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?

Phosphine (NIOSH 1003)

� Four Separate Locations

– Additionally, a sample blank was included.

� Sampled according to NIOSH Method 1003.

� No phosphine present.

Arsine (NIOSH 6001)

� Arsine was sampled according to NIOSH Method

6001.

� Arsine is not present.

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

GC Mass Spectrometer

� A sample was collected in a large Mylar bag and

analyzed at Colorado State University.

� Nothing was detected.

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

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

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

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.

What Should NIOSH Do?

� Your Suggestions?

What Should NIOSH Do?

� Determine hydride generation issue (silane?)

� Confirm hydrogen main hazard

� Review and verify preliminary investigation

results

� Identify other health issues

NIOSH HHE

� The day before NIOSH arrived:

– Removed filters and placed in drum

– Covered drum (with plastic) to concentrate off-

gassing constituents

NIOSH HHE Sampling

� Collected samples in

“blood draw” vacuum

tubes

� Monitored for temperature

change

� NIOSH contract analytical

lab used

NIOSH HHE Sampling

� Direct reading AND

personal monitor

– All instruments

detected CO

– Could be

interference from

hydrogen?

NIOSH HHE Sampling

� Temperature rise

documented

� Note moisture on underside

of plastic “shower cap”

– >10o F rise

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.

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

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!

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 …..

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

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.

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

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

Follow-up sampling

� Draeger does not have a

silane detector tube

� HOWEVER: Sensidyne

has a silane detector tube

� Indicates silane is present

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!

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

HHE and Carbon Monoxide

� What should NIOSH do regarding carbon

monoxide results?

� Your Suggestions?

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

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)

What Should HHE State?

� Time to write the HHE report

– Specifically regarding Carbon Monoxide exposure levels.

� Your Suggestions?

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.

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.

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.

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.

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].

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].

Hydrogen and Silane Formation:

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.

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].

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 …

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.

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.

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.

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

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

Mystery Solved?

� Open to recommendations/suggestions

Jamie Rubin

970.288.4880

Jamie.Rubin@avagotech.com