An analytical method for the measurement of trace level acidic and basic AMC using liquid-free sample traps

摘要

The measurement of parts-per-trillion (ppt) level acidic and basic airborne molecular contamination (AMC) is essential for process protection and yield control in semiconductor photo-lithography and adjacent applications. Real-time monitoring solutions are highly desired, as they provide instantaneous and continuous measurement. However, even the most advanced monitors cannot achieve detection limits in the low parts-per-trillion (ppt) range and many restrictions apply for the detection of acidic AMC. High cost of ownership is another disadvantage. Discontinuous sampling with sample traps is capable of achieving ppt-level measurement, but the currently accepted methods use sample traps filled with de-ionized water (impingers) to capture soluble acidic and basic AMC. Several inherent disadvantages of these methods result in inconsistent data and increased detection limits. Some proprietary solid state solutions have been reported, but involve complex preparation, have high background signals and require 24-72 hour sample duration, or they are protected trade secrets that are not available as an industry standard. To eliminate these disadvantages, we developed a liquid-free sample trap that allows parts-per-quadrillion level (ppq) measurement of acidic and basic AMC within one work shift, typically a 4-6 hour sample period. The traps can easily be manufactured and prepared in small lab operations, are sealed and protected from the outside and operator handling in the field, have months of shelf life and show high capture efficiencies while minimizing reactions and artifacts. Capacity results for the liquid-free base trap using ammonia (NH_3) as a test gas yielded more than 200 ppb-h at 100% capture efficiency without any moisture (simulating sampling of CDA or N_2) and 350 ppb-h at 40% RH. The capacity results for sulfur dioxide (SO_2) were highly dependent on moisture content of the sample gas and yielded 5 ppb-h at 90% capture efficiency and 0% RH, but increased exponentially to more than 1200 ppb-h at 40% RH. Performance testing indicates that the liquid-free trap provides both more precise and more accurate results for NH_3, SO_2 and HF in comparison to standard impinger in lab testing, with a relative standard deviation not exceeding 8% and capture efficiency greater than 95% for all three compounds. Acetic acid was the only compound that shows slightly decreased performance but still maintained a precision and accuracy comparable to the other compounds tested. In-field validation deployment to external and internal customers in parallel with standard wet impingers resulted in less than 10% difference between the traps, providing the necessary evidence that liquid-free traps are suitable for replacement of and better than wet impingers.