THE EFFECTS OF LAMINAR AIRFLOW IN SEMICONDUCTOR FABS ON THE VENTILATION DESIGN FOR EXHAUSTED ENCLOSURES

Transcription

1 THE EFFECTS OF LAMINAR AIRFLOW IN SEMICONDUCTOR FABS ON THE VENTILATION DESIGN FOR EXHAUSTED ENCLOSURES

2 Presented By: John Hatfield, CIH John O Hehir Global Semiconductor Safety Services 1365 Adams Court Menlo Park, California at Semiconductor Safety Association Annual Conference Arlington, Virginia April 2000

3 Exhaust Ventilation in Fabs for Tools Used for primary control Spin Coaters Wet benches Used for secondary control Gas boxes Diffusion furnaces Slide 3 of 27

4 Downdraft Laminar Flow in 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 laminar airflow on the ventilation for exhausted enclosures such as gas boxes Slide 4 of 27

5 Downdraft Laminar Flow in Fabs Estimates of laminar flowrates vary widely 10 fpm fpm SEMI S6-93 Exhaust Ventilation Semiconductor Industrial Hygiene Handbook" by Michael E. Williams and David G. Baldwin UBC; 1202 Slide 5 of 27

6 Study of Effects of Laminar Flow on Effectiveness of Exhausted Enclosure Ventilation Does laminar flow influence the effectiveness of exhausted enclosure ventilation? What is the effect? What is the magnitude of the effect? Slide 6 of 27

7 Tracer Gas Testing Used for testing exhausted enclosure ventilation design using SEMI F15 Release of known concentrations of test gas inside of an enclosure Measure the resulting concentration of test gas outside of the enclosure Ratio of outside to inside is measure of effectiveness of ventilation design Slide 7 of 27

8 Tracer Gas Testing and SEMI S2 S2 specifies maximum acceptable concentration outside of exhausted enclosure in the event of a leak = 25% TLV Primary means of verifying that exhaust ventilation design meets this criteria for exhausted enclosures like gas boxes Slide 8 of 27

9 Tracer Gas Testing and SEMI S2 Can simulate releases of highly toxic gases (arsine, phosphine, chlorine, etc.) Not possible to test actual releases because of hazard Example - release of 100% phosphine into gas box (TLV = 0.3 ppm) Slide 9 of 27

10 GS 3 Tracer Gas Lab Local Exhaust Fan (Variable Speed) Outside Air Supply to HEPAs Exhaust for Enclosure 10 Feet Cleanroom Enclosure being tested Slide 10 of 27

11 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

12 Study Methodology Tracer gas testing performed on exhausted enclosure in Tracer Gas Test Lab Study to determine the effect of laminar flow on concentration outside of the enclosure Based upon SEMI F15 Tracer Gas Test Methodology Slide 12 of 27

13 Study Methodology Measured concentrations released from exhausted enclosures during simulated leak inside of gas box Release point at position of gas line filter or fitting Simulated release of 100% chlorine gas Slide 13 of 27

14 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

15 Tracer Gas Test Method Used very sensitive gas chromatograph (ppt capability) Release inside of enclosure with known concentration of Sulfur Hexafluoride at 28.5 lpm (based upon SEMI F15) Measured concentration at 0, 1, 3, 5 minutes during release Slide 15 of 27

16 Results Presented as plots of concentration outside of enclosure vs. time of release Varying laminar flowrates All other conditions were the same during test (flowrate, leak opening,size of enclosure, release rate, etc. Slide 16 of 27

17 First Test Conditions Higher 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

18 25 Effect of Laminar Flow on Released Concentrations from Exhausted Enclosure Chlorine Concentration (% TLV) R 2 = R 2 = R 2 = fpm laminar flow 62 fpm laminar flow 7 fpm laminar flow Time of Release (minutes) Slide 18 of 27

19 Test Results Data Analysis Each data point is the average of three replicates Each chart shows error brackets based upon standard deviation of results Linear regression is used to explain the variance in the plotted data Each linear regression line indicates the correlation coefficient R 2 Slide 19 of 27

20 First Test Results Higher Airflow (Q) Shows increasing effect of laminar flow on exhaust effectiveness Increasing laminar flow results in larger concentrations outside of enclosure Laminar flow appears to create Venturi effect - increases amount of gas released at leak point Slide 20 of 27

21 First Test Results Higher Airflow (Q) Effectiveness of exhaust ventilation with laminar flow decreases as compared with effectiveness of exhaust ventilation without laminar flow Factor of about 4 fold difference between 7 fpm and 62 fpm at 5 minutes Slide 21 of 27

22 Second Test Conditions Lower 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

23 Chlorine Concentratio (% TLV) Effect of Laminar Flow on Released Concentrations from Exhausted Enclosures R 2 = R 2 = Time of Release (minutes) 7 fpm laminar flow 62 fpm laminar flow Slide 23 of 27

24 Second Test Results Lower Airflow (Q) Shows large difference in effectiveness with and without laminar flow Smaller flowrate is effected more by laminar flow Exhaust would meet S2 criteria without laminar flow Exhaust would fail S2 criteria with laminar flow Slide 24 of 27

25 Conclusions Laminar flow decreases the effectiveness of exhaust ventilation as a control against chemical gas leaks inside of exhausted enclosures a. Larger effect for smaller exhaust flowrates b. Larger effect for higher laminar flowrates c. Larger effect for larger release openings d. Larger effect for smaller enclosures Slide 25 of 27

26 Conclusions Tracer gas testing should be performed under laminar flow conditions Tracer gas test results obtained without laminar flow conditions will underestimate the amount of exhaust needed to meet S2 criteria Slide 26 of 27

27 John Hatfield Global Semiconductor Safety Services 1365 Adams Court Menlo Park, CA (650) John O Hehir Global Semiconductor Safety Services 1365 Adams Court Menlo Park, CA, (650) johehir@itsqs.com