Detection of particles deposited on both smooth and patterned silicon surfaces is examined in terms of maximizing the signal-to-background ratio. Components of the signal and background are examined separately. To understand the origin of the collected signal, laser light scattering of individual polystyrene spheres deposited on a polished silicon surface is studied. It is shown that for sphere sizes of 1 to 20 μm, scattering is an interference phenomenon of light from two sources: one is the reflection of the laser light on the particle, the second is the reflection on the silicon surface after two refractions. The effect of illumination under different angles is estimated and compared with experiment on spheres as small as 100 nm in diameter. The background scatter produced by patterned wafers is also examined. Techniques for its reduction, such as optimization of the azimuthal illumination angle, and in the case of high-density dynamic random access memory (DRAM) wafers, by Fourier filtering, are presented. In regard to the latter, the effects of a selective filtering of this diffraction pattern are examined vis-à-vis the resultant perturbation of the relatively weak transforms of the actual particulates. It was found that even some of the most rudimentary of spatial filters can be used to achieve noteworthy improvements in detection sensitivity. Achievements are reported of up to 5x improvement in SNR for intradie measurements and up to 10-fold improvement in total particle count.
Charly D. Allemand,
Joseph J. Danko,
"Heuristic approach to particle detection on virgin and patterned silicon wafers," Optical Engineering 34(2), (1 February 1995). http://dx.doi.org/10.1117/12.188604