IRGAS XFlo™: Process Gas FTIR System for Flange Mounted Applications

PDF Version

Figure 1: The IRGAS XFlo ™ system

Figure 2: Response of XFlo ™ to high level gas challenge

Figure 3: Response of XFlo ™ to multiple low level gas challenges

 Introduction

When doing process tool or exhaust abatement tool installations, it is desirable to monitor the exhaust gases as a means of controlling or setting up the tool 1, 2. FTIR, with its rapid analysis speed, high sensitivity, and ease of use, offers one means of monitoring such systems at the time of installation and during the working life of the tool. Often, these applications contain small amounts of analyte relative to a very large flow of exhaust gas. For this reason, it is desirable to retain as much sensitivity as possible in order to measure low levels of impurities that are entering the system. With current “White Cell” designs, the user faces the issue of having to interface the small diameter gas cell ports to the large diameter duct work. This will often restrict the amount of flow that can be processed through the gas cell and slow the exchange rate through the cell. A slower exchange rate means a slower response time which might miss fast events. The alternative is to use a short path single pass gas cell which allows good flow through the gas cell but at the price of sensitivity to small scale events. In response to this need, CIC Photonics has developed the IRGAS XFlo™, a modified 4Runner gas cell configured for in-duct installation. This gas cell provides a wide bore gas inlet and outlet based on a 2 inch MDC ™ flange attached perpendicular to the optical path. This design takes advantage of the excellent optical throughput of the 4Runner gas cell (>45% at 4 meters path length) while also maximizing the gas flow zone. The gas cell volume, including the attached flanges, is 0.8 liters, the smallest gas cell volume on the market for a 4 meter cell.

Evaluation

In order to evaluate this design, we installed an XFlo™ gas cell on a Bomem WorkIR™. The FTIR was operated at a resolution of 4cm-1 and the data was collected with CIC Photonics SPGAS ™ software package. Full spectral scans were collected every three seconds under these conditions. The system was operated with a blower fan which yielded a gas flow through the cell of 108 standard liters per minute(slpm). We then added 0.5 slpm of a 3.5% mixture of CO in nitrogen to the gas stream yielding a net concentration of 162 ppm of CO through the gas cell. The results from this experiment are found in Figure 1 below. Note the rapid response time to the challenge with a 90% of full scale response within 7 seconds as well as the rapid clearing of the gas cell once the analyte has been shut off. A second evaluation was also performed with the gas cell to identify the ability to catch low concentration transient events under very high flow conditions. In this experiment, we injected 1 cc of the 3.5% CO mixture over about 6 seconds into the same 108 slpm gas flow. This yields a final concentration of CO of about 3.2 ppm in the total flow. Note, again, the rapid response to the gas impurity challenge with the response to 90% of full scale in one 3-second scan. In addition, the signal for the analyte is well resolved from the noise level even at these low concentrations of analyte. This demonstrates the ability of the IRGAS XFlo™ system to respond to low level impurity challenges as may occur with transient events.

Conclusion

The IRGAS XFlo™ gas analysis system is based on the combination of the rugged Bomem WorkIR process FTIR with a flanged “White Cell” design. This package offers the user the capability to mount the FTIR system directly into process exhaust flanges. This, in turn, makes the Xflo™ system ideal for monitoring abatement tools, multiple process tools, and other high-flow, low-concentration applications. In addition, the excellent sensitivity characteristics of the XFlo™ system make it suitable for measuring process tool progress in order to determine the presence of undesirable impurities or end-points. For more information on the XFlo™ gas analysis system or any other CIC Photonics product, please do note hesitate to contact us at the address above.

Credits

1 High-Sensitivity, Fast-Response Monitoring of PFC and Abatement Tool Emissions By FTIR Spectroscopy, Perez, J.E. and Meyer, R. T., Semicon West 2003 Technical Symposium.

2 Multiple Gas Impurity Analyses and Verification For Semiconductor Process Tools, Meyer, R.T., Gases and Technology, January/February 2003.