Measuring sub-50nm particle retention of UPW filters.
Don Grant, Gary Van Schooneveld – CT Associates, Inc., Eden Prairie, MN
The critical features size of state-of-the-art semiconductor devices is on the order of 50 nm and expected to decrease to ~ 20 nm by 2015. Particles on the order of ½ the feature size in process liquids used during device manufacturing can reduce device manufacturing yield and finished device reliability. Microfilters and ultrafilters with pore size ratings below 50 nm are often used to control particle concentrations in theses liquids. However, the ability of the filters to remove particles is typically measured using optical particle counters with a minimum detection limit of 50 nm or larger.
This paper describes a new technique that allows measurement of removal of particles as small as 5 nm in diameter from liquids. In this technique filters are challenged with particles ranging from 5 to 100 nm in diameter. Total concentrations ranging from 107/mL to 1010/mL ? 10nm are used. Filter inlet and outlet concentrations are measured using a Liquid Nanoparticle Sizer, a recently-developed technique. In this technique, a very fine mist of particle-laden water is created, the water in the mist droplets is evaporated and sizes and numbers of the remaining particles are measuring using a scanning mobility particle sizer. This technique allows very accurate resolution of particle size with 64 size channels per decade of size (e.g. between 10 and 100 nm).
Examples of retention of 30 nm polystyrene latex (PSL), gold, and silica particles by a commercially available 30 nm filter cartridges are included in the paper. The filters were found to have high retention efficiency for the PSL and gold particles, and significantly lower retention of the silica particles. These results indicate that adsorption plays a significant role in the retention of PSL and gold particles, while sieving is the dominant capture mechanism for silica particles. In addition, silica particles represent “real world” particles since silica is commonly found in UPW. Hence, silica particles are the preferred choice of particle types for determining particle retention of UPW systems.
CTA publication #102: 2011 CMP and Ultrapure Users Conference