Seung-Ki Chae’s research while affiliated with University of Minnesota, Duluth and other places

Light scattering particle counters are widely used for aerosol research. They are also important tools for monitoring airborne particles in the semiconductor and pharmaceutical industries. For the latter application, it is important to know the influence of particle material properties on the counter response, particularly the effect of particle refractive index on the lower detection limit of the counter. In this paper, the effect of particle refractive index on the lower detection limit of aerosol particle counters has been studied using the Mie theory. Counting efficiencies have also been measured to verify the theoretical results. The measurements were made with PSL (polystyrene latex), silicon, silicon nitride, and silicon dioxide particles. Two commercially available aerosol counters and a condensation nucleus counter were used in the study. The theoretical study show that both the real and the imaginary parts of the particle refractive index affect the lower detection limit of a light scattering particle counter. For transparent particles, an increase in the particle refractive index causes a decrease in the lower detection limit. And the absorptive component in the refractive index of the particle causes a further drop in the lower detection limit for the specific counters studied. Experimental measurements show good agreement with the theoretical results. Among the test particles used, silicon had the largest refractive index, followed by silicon nitride, PSL, and silicon dioxide. The lower detection limit of the counters studied also shows a corresponding decreasing trend with silicon dioxide giving the highest lower detection limit, followed by PSL, silicon nitride, and silicon as the refractive index of the particle is increased and the lower detection limit of the counter is decreased. The difference between the theoretical and experimental lower detection limits was found to be less than 10% in most cases.

Aerosol particle counters based on light-scattering have a broad range of applications including cleanroom monitoring, air pollution research, and pharmaceutical studies. Each application may deal with particle materials having various refractive indices. The effect of the particle refractive index on the lower detection limit of aerosol particle counters was investigated using the Mie theory. Counting efficiency measurements were made to verify the theoretical results. Measurements were performed with PSL (polystyrene latex), silicon, silicon nitride, and silicon dioxide particles. Two commercial aerosol counters and a condensation nucleus counter were used in the study. The theoretical study showed that both the real and the imaginary parts of the particle refractive index play an important role in the lower detection limit of an aerosol counter. For transparent particles, as the absolute difference between the particle and medium refractive index increases, the lower detection limit of a counter is decreased. Light-absorbing particles generally showed a smaller lower detection limit than transparent particles. Experimental measurements agree well with the theoretical results. Among the test particles used, silicon had the largest refractive index, followed by silicon nitride, PSL, and silicon dioxide. The lower detection limit of the counters studied showed a decreasing trend with an increasing real part of the refractive index as the particle material is changed from silicon dioxide to PSL, silicon nitride, and silicon. The difference between the theoretically calculated and experimentally determined lower detection limits were found to be less than 10 percent.

The performance characteristics of a Tencor Surfscan 4000 wafer surface scanner have been evaluated using ideal polystyrene latex (PSL) spheres and irregularly shaped real-world particles of Si and SiO2. The particles were uniform in size and were deposited on bare silicon wafers and used as standard calibration wafers to study the scanner response. Particles in the 0.1 to 1.0 μm diam range were used. The sizing accuracy, counting efficiency, lower detection limit, and count repeatability of the scanner were studied systematically. The full Maxwell's electromagnetic equations also have been solved numerically on a supercomputer to obtain the light-scattering cross section of the particles on bare silicon wafers. The calculation compares favorably with the experimental results. Because of the high refractive index of silicon, the wafer surface scanner detects Si particles to a considerably smaller size than PSL. The data suggest that Si particles as small as 0.1 μm have been detected by the Surfscan 4000 with 90% counting efficiency even though the nominal lower limit of the instrument is 0.3 μm based on PSL calibration.

A light-scattering experiment on particles was performed to determine radiative transport properties of the suspension. Extinction coefficient, scattering coefficient, and phase function are the important properties. A generalized light-scattering experimental setup was designed and constructed to perform systematic research on light scattering by particles. By measuring the transmitted energy and angular scattered-energy distribution around a particle suspension, complete information on radiative transport properties could be obtained. This study focused on the performance of the experimental setup with purely scattering particle suspensions. Transparent polymer monospheres with diameters of 0.091, 0.546, 1.001, and 7.04 μm were used in separate suspensions of water for this study. The radiative transport properties obtained experimentally are compared with results from Mie theory. The single-scattering condition was examined and maintained during this experiment. The number concentrations of all but the smallest particles were independently measured using an optical liquid-borne particle counter.

The complex refractive indices of commercial composite spherical particles (black dyed [Dp = 1.041 μm] and red dyed [D p = 0.999 μm]) were determined by two different methods. The first method measured angular scattered-energy distributions to determine experimental phase functions that were compared with Mie phase functions. The second method examined several mixture rules to obtain the effective refractive indices of dyed polymer monospheres. The refractive indices of black and red dyes were measured independently for the mixture rule study. The effect of different refractive indices on the particle sizing accuracy was investigated using an optical liquid-borne particle counter and a wafer surface scanner.

The response of a wafer surface scanner is usually determined by means of calibration wafers carrying polystyrene latex (PSL) spheres. The responses of a scanner to real-world contaminant particles with a nonspherical particle shape and a different refractive index are generally not well-understood. In this paper, uniform-sized particles of Al, Al2 O3 Si, SiO2, Si3 N4 and Zn have been deposited onto bare silicon wafers for use as calibration standards to study the response of wafer surface scanners to nonspherical, real-world particles. The particles used were irregular in shape and covered a size range of 0.2 μm to 1.0 μm. The morphological characteristics of the particles have been examined by a scanning electron microscope (SEM). The experimental response of a commercially available wafer surface scanner (Tencor Surfscan-4000) to these uniform-sized, nonspherical particles has been determined. The experimentally determined response of the wafer surface scanner has been compared with theoretical calculations based on Mie scattering and Fresnel surface reflections.