When replacing particle filters as part of a regular maintenance schedule, keep in mind that the manufacturer’s filter rating may not correlate with the actual cleanliness of the filter. The real-world performance of particle filters can depend on filter type, manufacturer and even the length of operation. Older filters accumulate particulate matter over time that decreases the filter’s porosity, which results in greater particle retention at the expense of increased flow resistance. At the very end of the filter’s lifespan, it can start to shed some of this accumulated particulate matter, resulting in increased particle contamination downstream. Newer filters can also take some time to ‘settle in’ after installation before performing at optimal efficiency.
Studying the Lifespan of Filters
In this study, IPA was circulated within a wafer cleaning tool as the Chem 20™ Chemical Particle Counter from Particle Measuring Systems took measurements to check the filter cleanliness. The IPA was pumped out of the rinsing tank, through a particle filter, through a heater, and then back into the tank with the Chem 20 particle coutner connected via a T-connector immediately downstream from the filter. Since IPA is a high-molecular scattering fluid, the Chem 20 particle counter was operating in high-scatter mode for these tests, which means the minimum detectable particle size was raised slightly to 24 nm.
The Filter Cleanliness Testing Results
The original filter, rated at 10 nm, had been installed in the tool for more than one year prior to testing. After the measurement, this filter was replaced with a brand new 5-nm-rated filter. The average particle concentrations before and after replacing the old 10 nm filter with the new 5 nm filter are summarized below. Despite being rated at a lower particle size, the old 10 nm filter was found to perform much better than the new 5 nm filter during testing, with fewer than half the number of particles larger than 24 nm detected downstream.
As we can see, the expected performance of particle filters based on manufacturer specifications can differ from reality. The difference in performance between the two filters may be due to the newer filter not yet working at its optimal filtration efficiency. However, one takeaway is clear: Real-time data provided by the 20 nm Particle Counter allows the user to evaluate the actual performance of their particle filters. For older filters, the Chem 20 particle counter can be used to determine whether the filter is still performing satisfactorily, or if it needs replacement.
Learn more about the Chem-20 Chemical Particle Counter, and look forward to our next blog on Mask-Blank Manufacturing. Until next time, check out the full analysis of all semiconductor applications discussed in this blog series!
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