How Does Particle Concentration and the Face Velocity Impact the Removal of Sub-100 nm Particles for High-purity Water?
Gary Van Schooneveld
CT Associates, Inc
WL Gore and Associates, Inc.
Particles in ultrapure water (UPW) used during the manufacture of microcircuits on semiconductor wafers can deposit onto the wafer surface thereby causing decreased yield and reliability of the microcircuits. At present particles as small as 30 nm in diameter can cause problems. In the future, even smaller particles are expected to be problematic.
The concentrations of these small particles in UPW are considered to be low (presumably < 1E3/mL > 30 nm). However, even at these low concentrations the particles can decrease device performance with the degree of performance degradation increasing with increasing concentration.
Microporous membrane filters and ultrafilters are often used to remove these small particles from UPW. Ideally, the ability of these filters to remove these particles would be tested under conditions representative of expected operating conditions. However, measuring the concentrations of these particles in UPW is extremely difficult due to their small size and low concentration. Hence, the filters are often tested for particle retention with challenge concentrations much higher than those found in UPW; sometimes as high as 1E10-1E12/mL.
Filters have historically been tested under the assumption that particle capture is largely independent of particle concentration, filter loading, and filter face velocity. However, recent testing has indicated that retention of sub 100 nm particles often decreases substantially with increasing particle concentration, filter loading and face velocity. These effects have been observed with multiple particle types and membrane filter materials.
This paper presents the magnitude of these effects under a variety of testing conditions and discusses the implications for the evaluation of filters used to remove particles from UPW.
Ultrapure Water Journal September/October 2015