hatfield john
TRANSCRIPT
THE EFFECTS OF LAMINARAIRFLOW IN SEMICONDUCTOR
FABS ON THE VENTILATIONDESIGN FOR EXHAUSTED
ENCLOSURES
Presented By:
John Hatfield, CIHJohn O’Hehir
Global Semiconductor Safety Services1365 Adams Court
Menlo Park, California
at
Semiconductor Safety AssociationAnnual ConferenceArlington, Virginia
April 2000
Exhaust Ventilation in Fabs forTools
• Used for primary control– Spin Coaters– Wet benches
• Used for secondary control– Gas boxes– Diffusion furnaces
Slide 3 of 27
Downdraft Laminar Flowin Fabs
• Primary purpose is particulate control• Not intended as primary protection against
chemical releases• This paper will present results from an
analysis of the effects of downdraft laminarairflow on the ventilation for exhaustedenclosures such as gas boxes
Slide 4 of 27
Downdraft Laminar Flow in Fabs
• Estimates of laminar flowrates vary widely• 10 fpm - 100 fpm
– SEMI S6-93 “Exhaust Ventilation”– “Semiconductor Industrial Hygiene
Handbook" by Michael E. Williams andDavid G. Baldwin
– UBC; 1202
Slide 5 of 27
Study of Effects of Laminar Flow onEffectiveness of Exhausted
Enclosure Ventilation
• Does laminar flow influence theeffectiveness of exhausted enclosureventilation?
• What is the effect?• What is the magnitude of the effect?
Slide 6 of 27
Tracer Gas Testing
• Used for testing exhausted enclosureventilation design using SEMI F15
• Release of known concentrations of testgas inside of an enclosure
• Measure the resulting concentration of testgas outside of the enclosure
• Ratio of outside to inside is measure ofeffectiveness of ventilation design
Slide 7 of 27
Tracer Gas Testing and SEMI S2
• S2 specifies maximum acceptableconcentration outside of exhaustedenclosure in the event of aleak = 25% TLV
• Primary means of verifying that exhaustventilation design meets this criteria forexhausted enclosures like gas boxes
Slide 8 of 27
Tracer Gas Testing and SEMI S2
• Can simulate releases of highly toxicgases (arsine, phosphine, chlorine, etc.)
• Not possible to test actual releasesbecause of hazard
• Example - release of 100% phosphine intogas box (TLV = 0.3 ppm)
Slide 9 of 27
GS3 Tracer Gas Lab
Cleanroom
10 Feet
Local Exhaust Fan (Variable Speed)
Exhaust for EnclosureOutside Air Supply to HEPAs
Enclosure being tested
Slide 10 of 27
Tracer Gas Test Lab
• Provides number of advantages– Provides laminar flow during testing to
simulate fab conditions– Can vary laminar flowrate– Can adjust exhaust ventilation flowrate
for enclosure over wide range– Simulates clean room conditions
Slide11of 27
Study Methodology
• Tracer gas testing performed onexhausted enclosure in Tracer Gas TestLab
• Study to determine the effect of laminarflow on concentration outside of theenclosure
• Based upon SEMI F15 Tracer Gas TestMethodology
Slide 12 of 27
Study Methodology
• Measured concentrations released fromexhausted enclosures during simulatedleak inside of gas box
• Release point at position of gas line filteror fitting
• Simulated release of 100% chlorine gas
Slide 13 of 27
Study Methodology
• Tested under a variety of test parameters– Varying laminar flowrate– Different sized exhausted enclosures– Varying flowrates (Q)– Different sizes of leak openings
Slide 14 of 27
Tracer Gas Test Method
• Used very sensitive gas chromatograph(ppt capability)
• Release inside of enclosure with knownconcentration of Sulfur Hexafluoride at28.5 lpm (based upon SEMI F15)
• Measured concentration at 0, 1, 3 , 5minutes during release
Slide 15 of 27
Results
• Presented as plots of concentrationoutside of enclosure vs. time of release
• Varying laminar flowrates• All other conditions were the same during
test (flowrate, leak opening,size ofenclosure, release rate, etc.
Slide 16 of 27
First Test ConditionsHigher Airflow (Q)
• Exhaust Flowrate (Q) = 84 cfm• Larger enclosure (3 ft x 2 ft x 1ft)• Three levels of Laminar flow: 7 fpm, 44
fpm, 62 fpm• Chlorine release• Large opening (slot opening) along door
Slide 17 of 27
Effect of Laminar Flow on Released Concentrations from Exhausted Enclosure
R2 = 0.9699
R2 = 0.929
R2 = 0.8925
0
5
10
15
20
25
0 1 2 3 4 5 6Time of Release (minutes)
Ch
lori
ne
Co
nce
ntr
atio
n (%
TL
V)
44 fpm laminar flow
62 fpm laminar flow
7 fpm laminar flow
Slide 18 of 27
Test Results Data Analysis
• Each data point is the average of threereplicates
• Each chart shows error brackets basedupon standard deviation of results
• Linear regression is used to explain thevariance in the plotted data
• Each linear regression line indicates thecorrelation coefficient R2
Slide 19 of 27
First Test ResultsHigher Airflow (Q)
• Shows increasing effect of laminar flow onexhaust effectiveness
• Increasing laminar flow results in largerconcentrations outside of enclosure
• Laminar flow appears to create Venturieffect - increases amount of gas releasedat leak point
Slide 20 of 27
First Test ResultsHigher Airflow (Q)
• Effectiveness of exhaust ventilation withlaminar flow decreases as compared witheffectiveness of exhaust ventilation withoutlaminar flow
• Factor of about 4 fold difference between 7fpm and 62 fpm at 5 minutes
Slide 21 of 27
Second Test ConditionsLower Airflow (Q)
• Q = 60 cfm• Larger enclosure (3 ft x 2 ft x 1ft)• Laminar flow - 7 fpm, 62 fpm• Chlorine release• Large opening (slot opening)
Slide 22 of 27
Effect of Laminar Flow on Released Concentrations from Exhausted Enclosures
R2 = 0.7238
R2 = 0.9842
0
50
100
150
200
0 2 4 6Time of Release (minutes)
Ch
lori
ne
Co
nce
ntr
atio
(%
TLV
)
7 fpm laminar flow
62 fpm laminar flow
Slide 23 of 27
Second Test ResultsLower Airflow (Q)
• Shows large difference in effectivenesswith and without laminar flow
• Smaller flowrate is effected more bylaminar flow
• Exhaust would meet S2 criteria withoutlaminar flow
• Exhaust would fail S2 criteria with laminarflow
Slide 24 of 27
Conclusions
Laminar flow decreases the effectiveness ofexhaust ventilation as a control againstchemical gas leaks inside of exhaustedenclosuresa. Larger effect for smaller exhaust flowrates
b. Larger effect for higher laminar flowratesc. Larger effect for larger release openings
d. Larger effect for smaller enclosures
Slide 25 of 27
Conclusions
• Tracer gas testing should be performedunder laminar flow conditions
• Tracer gas test results obtained withoutlaminar flow conditions will underestimatethe amount of exhaust needed to meet S2criteria
Slide 26 of 27
John Hatfield Global Semiconductor Safety Services
1365 Adams Court Menlo Park, CA 94025 (650) 463-4640 [email protected]
John O’HehirGlobal Semiconductor Safety Services
1365 Adams Court Menlo Park, CA, 94025 (650) 463-4640 [email protected]