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 jhatfield@itsqs.com

John O’HehirGlobal Semiconductor Safety Services

1365 Adams Court Menlo Park, CA, 94025 (650) 463-4640 johehir@itsqs.com