Two methods for monitoring acid/base airborne molecular contamination (AMC) in 193nm lithography exposure tools, tracks, and process bays have historically been used for cleanroom and process contamination control. Ion Mobility Spectrometry (IMS) is a predominant technique used by semiconductor manufacturers for real-time AMC detection allowing for identification of subtle trends in AMC levels over time or alarming during high concentration events. Manual impinger sampling with ion chromatography (IC) analysis is a second technique used which pre-concentrates the air sample to allow for low detection limits and allows for some increased speciation of ions. The use of one or both of these techniques has a significant impact on the effectiveness of AMC detection and control, therefore, the equipment and strategy used within an AMC monitoring program can provide large competitive advantages that are not easily or quickly duplicated by other companies.
Monitoring and controlling Airborne Molecular Contamination (AMC) has become essential in deep ultraviolet (DUV)
photolithography for both optimizing yields and protecting tool optics. A variety of technologies have been employed
for both real-time and grab-sample monitoring. Real-time monitoring has the advantage of quickly identifying "spikes"
and upset conditions, while 2 - 24 hour plus grab sampling allows for extremely low detection limits by concentrating
the mass of the target contaminant over a period of time. Employing a combination of both monitoring techniques
affords the highest degree of control, lowest detection limits, and the most detailed data possible in terms of speciation.
As happens with many technologies, there can be concern regarding the accuracy and agreement between real-time and
grab-sample methods. This study utilizes side by side comparisons of two different real-time monitors operating in
parallel with both liquid impingers and dry sorbent tubes to measure NIST traceable gas standards as well as real world
samples. By measuring in parallel, a truly valid comparison is made between methods while verifying the results against
a certified standard. The final outcome for this investigation is that a dry sorbent tube grab-sample technique produced
results that agreed in terms of accuracy with NIST traceable standards as well as the two real-time techniques Ion
Mobility Spectrometry (IMS) and Pulsed Fluorescence Detection (PFD) while a traditional liquid impinger technique
showed discrepancies.
The cleanliness of optical components is a critical process parameter in microlithography. Molecular contamination is often the culprit for contaminated optical elements. The increasing use of purge gases and/or gas phase chemical filtration in photolithography tools are methods to help reduce or eliminate airborne molecular contamination (AMC), and is evidence that this form of contamination is problematic. And unfortunately, AMC becomes even more challenging in the transition from 248nm to 193nm and 157nm radiation wavelengths. Because of this, quantitative monitoring techniques and strategies play a crucial role in identifying and resolving molecular contamination that affects optical components. This paper, and the examples within, describe work performed using surface acoustic wave (SAW) technology to monitor and quantify in real-time and with high sensitivity AMC as it adsorbs, desorbs, or reacts with critical optical components. A high frequency oscillating quartz crystal is coated with SiO2 to mimic critical optical component surfaces. As gas phase molecular contamination interacts with the sensor surface, the frequency of oscillation is altered. Contamination information is then extracted from the change in output frequency, including mass, rate, and trending data and correlated to process, equipment, or facility logs in order to determine the contamination source.
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