Developing a Fluorinated Greenhouse Gas Stack Testing Method using FTIR

Transcription

1 Developing a Fluorinated Greenhouse Gas Stack Testing Method using FTIR Tina M. Gilliland, P.E., Texas Instruments (TI) Curtis T. Laush, PhD, IMACC 1

2 Background TI is interested in identifying an alternative method to more accurately estimate Greenhouse Gas (GHG) emissions from their semiconductor fabrication operations (fabs) to reduce the cost burden of the final Mandatory Reporting Rule, Subpart I (40CFR98). TI proposes estimating emissions using a facility wide (fab) mass balance approach based on emissions characterization using familiar analytical equipment such as the Fourier Transform InfraRed spectrometer (FTIR). Correlation of production metrics and gas usage is desired to develop facility wide (fab) specific emissions factor by compound. Lower detection limit of GHGs was necessary. IMACC, a company that specializes in designing and manufacture of FTIR monitoring systems for industry and government, understood the challenges of measuring these compounds at sub-ppbv levels and performed experiments in their lab with a modified FTIR to achieve lower detection limits. 2

3 Typical F- Gas Distribution and Exhaust A (A n ) Gas Supply (MFC) B Tool Effluent Measurement C Final Tool Emission Measurement 3

4 Typical Exhaust Layout Abatement Unit Continuous operation of hundreds of GHG using process tools which exhaust to central exhaust systems. Measurements required at many locations vs. stack testing. 4

5 FTIR Stack Testing Method Demonstration Objectives: Primary objective of this pilot study was to measure in the field exhaust stacks at a typical operating semiconductor fab to demonstrate the feasibility of measuring GHGs using approved EPA Methods 301 and 320 at lower detection limits i.e. below ppbv. In previous FTIR stack testing, lower detection limits for F-gases were typically in the 30 to 50 ppbv range. Experienced periods during testing of below detection limits for F-gas Some F-gases not detected IMACC enhanced FTIR to improves detection limits Lab detection limits range from 50 to700 pptv for F-gas Secondary objective was to collect facility (fab) production data and chemical use to determine correlation with the stack emissions to develop a facility (fab) emissions factor for each GHG. Ex: Kg emitted/kg Used; Kg emitted/wafer processed. Challenge is protecting confidential information for fab activity metrics. 5

6 Test Plan and Methods Develop and test system in the laboratory for GHG sensitivities (detection limits) and measurement accuracies. Conduct field studies at 3 semiconductor fabs chosen to demonstrate that stack lower detections limits (sub-ppbv) can be measured and validated. Sample and analyze using approved EPA Validation Method 301 and FTIR Test Method 320. Collect fab operational data during testing Fab production related data GHG usage data GHG abatement status 6

7 Improvements of Enhanced FTIR System over Conventional FTIR High temperature infrared source Narrow bandwidth, high responsivity IR detector Long pathlength FTIR sample cell Interferometer operation at low resolution Drying of extracted stack samples Analysis method optimization Sub-ppbv detection limits for most F-GHG 7

8 Gas Spiking and Sampling Configuration Submicron particulate filter ~100 ft of PFA Teflon tubing Enhanced FTIR Sample Cell Water impinger or stack source 0-5 lpm MFM Sample Pump 0-1 lpm MFC Certified Gas Standard in Cylinder (100 ppb each of SF 6, CF 4, C 2 F 6, C 3 F 8, CHF 3 in UHP nitrogen balance) 8

9 Enhanced FTIR Measurement Protocols Enhanced FTIR analytical method was demonstrated in the field on the first stack, consistent with EPA Method 301 at the lower concentrations in Feb Subsequent stacks tested via EPA Method 301 and 320, which includes QA/QC gas spiking procedures for accuracies and stability of calibrations were conducted in March Test periods were 24 continuous hours per stack. EPA Method 2 performed at each stack for measurement of exhaust velocities (hence, GHG mass emission rates). 9

10 Lab Lower Detection Limits (LDLs) vs. Field Detection Limits (FDL) for GHG determined by Enhanced FTIR Determined by statistical analysis of clean water saturated air samples. Field detection limits (FDLs) typically 2-5 times these values. Compound LDL (pptv) SF6 55 CF4 76 C3F8 546 CHF3 613 C2F6 299 NF3 702 cc4f8 598 CH2F N2O < 1ppm 10

11 Comparison of measured vs. spiked concentrations (FTIR) % Recovery is within Method allowances. 11

12 EPA Method 301 Field Results - FTIR 24 runs: 12 spiked (s) and 12 unspiked (u); Reported in ppb 12

13 EPA Method 301 Field Results - FTIR s-spiked; u-unspiked EPA Method 301 validation successful for SF 6, C 2 F 6 and C 3 F 8 ; biases are very low and statistically insignificant. No correction factors (CF) are needed. Method 301 also successful for CF 4 and CHF 3 (biases statistically insignificant), but not a rigorous validation since native concentrations well above spike level. In effect, native stream dilutions, or negative spikes, were performed and proper analyzer response was observed. 13

14 Method 320 Higher Concentration Spikes % Recovery Gas Cylinder Conc (ppm) MFC Spike Flow, LPM MFM Total Sample Flow, LPM Spike Conc (ppm) Native Conc (ppm) Expected Conc (ppm) Measured Conc (ppm) %Recovery CF C2F C2F Obtained pure CF4 and C2F6 cylinder from the laboratory to spike higher concentrations for Method 301 validation on a different fab stack. Spiked higher concentrations of C2F6 and CF4 in the field to help validate % recovery. 14

15 EPA FTIR Method Validation Status and Recommendations Conducted the first pilot study at a semiconductor source stack for EPA Method 301validation and measured GHGs at sub-ppbv. Subsequent stacks were measured using EPA Method 320. EPA Method 301 validation was successful for lower concentration ppbv with the standards available. May need additional standards at expected concentration range for measuring different fabs and/or multiple stacks. Recommend using standards for the most representative compounds. Stack moisture was measured with the FTIR after compound sampling. Simplifies measurement as it eliminates additional test equipment required for EPA Method 4. 15

16 Next Steps Calculate measured mass emissions and compare to calculated methods. Continue analyzing data collected for correlation of fab activity to develop fab specific emissions factor. Determine best approach for measuring and correlating Semiconductor fabs GHG stack emissions. 16