Conventional molecular contamination monitoring systems employ a multi-point air sampling system connected to an analyzer. This monitoring approach has evolved into normal practice, driven by the need to monitor a large number of locations at the lowest reasonable cost. However, significant issues arise in today’s 193 nm semiconductor fabrication environment, and the restrictive monitoring techniques currently in use begin to show their weaknesses. The biggest drawbacks of the conventional monitoring approach are:
- Small time period of actual and recorded measurements per day at each location
- Effects of long sample tubing lengths
- Analyzer response and clear down times when adjacent sample point concentrations differ greatly
- Minimal ability to perform sample averaging to increase sensitivity.
For example, a sixty point sampling system with a 10 minute sample cycle (9 minutes purge, one minute sample) will take 600 minutes or 10 hours to sample all locations. Each location will be monitored for 1 minute every 10 hours, leaving each location unmonitored for the remaining 9 hours and 59 minutes. Certainly quality control cannot be maintained with this monitoring methodology.
A dedicated, point-of-use monitor offers the following advantages over a conventional multipoint sampling system:
- Continuous monitoring
- No missed contamination events
- Sample tubing length reductions from 20 - 30 meters to 2 - 3 meters. Reducing the sample tubing lengths minimizes interactions between contamination molecules and the tubing surface.
- Sensitivity improvements of 5 - 10x over existing technology.
Improvements in sensitivity and stability are realized through the 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. In one application, for example, the limit of detection for one minute samples is 120 ppt and the limit of detection for a 60 minute rolling average is 8.2 ppt. 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.
Download the full paper here to learn more about:
- Ion mobility spectrometry (IMS) monitoring systems
- The comparison between continuous and non-continuous monitoring
- Important considerations when evaluating a monitoring system
- Future challenges of new analyzer systems with improved detection limits and measurement stability