Light scattering by metallic particles on silicon wafers. (General Developments).

Light scattering is used by semiconductor manufacturers to inspect silicon wafers for particulates, defects, and surface roughness. Since a particle's size determines its potential to cause device failure, it is critical to be able to accurately measure particle size. Polystyrene latex (PSL) spheres are often used to calibrate wafer inspection systems, but these model particles do not behave like real-world particles. For example, they scatter much less light than metallic particles do. A. system, calibrated using PSL spheres, might report that a harmless metallic particle is much larger than it really is, causing more wafers to be rejected than necessary. Validated theories for light scattering are necessary to avoid such problems.

NIST, in collaboration with the University of Maryland, has developed a method for generating uniformly sized, pure copper particles and depositing them on silicon wafers. We performed measurements of the light scattered by these particles and found very good agreement with theoretical calculations. The code for these calculations has been made publicly available through the SCATMECH library of scattering codes (physics.nist.gov/scatmech). It can serve as a benchmark for testing other scattering calculations and can be used to help design improved inspection systems.

One hitch has been found, though. When we extended the theory to allow non-spherical particles, we found that the scattering by metallic particles is extremely sensitive to the shape of the particle. We demonstrated that a 1 nm dent on a 60 nm diameter aluminum sphere at the point where it contacts a silicon wafer can change light scattering by 30 %. This sensitivity to shape will make the accurate sizing of particles on surfaces even more challenging than originally thought.

CONTACT: Thomas Germer, [phone]; [e-mail].