Optimized Molecular Contamination Monitoring for Lithography
A new approach to monitoring molecular contamination in lithography is presented. Recent technical advances have made it feasible to perform continuous real-time monitoring with significant advances in sensitivity and stability while minimizing sample tubing effects. These improvements are realized by using a small, low-cost monitor that is dedicated to monitoring a single location. A dedicated, point-of-use monitor offers the following advantages over a conventional multipoint sampling system: continuous monitoring, no missed contamination events, sample tubing lengths reduced from 20 – 30 meters to 2 – 3 meters, and 5 – 10x better sensitivity. Improvements in sensitivity and stability are realized through a dedicated monitor approach to molecular contamination monitoring. Because the monitor is continuously sampling the same environment, sample averaging can be used in a highly effective manner to reduce the detection limit. This is particularly useful in chemically filtered environments where the concentrations are usually low and stable. An automated monitoring software package can simultaneously plot individual one minute data points and a long-term running average. The one minute samples are used to immediately detect the onset of a contamination event while the long term running average is used to monitor background contamination at the lowest levels.
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