May 2002

BFRL Monthly Highlights

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Light Scattering by Metallic Particles on Silicon Wafers

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.

PL’s Optical Technology Division, in collaboration with BFRL and 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 percent. This sensitivity to shape will make the accurate sizing of particles on surfaces even more challenging than originally thought.

Contact: 
Thomas Germer, 301-975-2876

 

 

Sprinklered versus Non-Sprinklered Dormitory Room Fires

BFRL has compiled a nine-minute video of fires in two college dormitory rooms, one equipped with sprinklers and the other without sprinklers. Small fires were ignited in a wastebasket between the desk and bed in each of the two fully-furnished dormitory rooms. In the sprinklered room, the fire grew until sprinkler activation which occurred in less than two minutes. For the unsprinklered room, the fire continued to grow until it flashed-over the room in less than five and a half minutes. The video also includes pre-fire and post-fire photographs which can be used to compare the damage caused by each fire.

Contact: 
Nelson Bryner, 301-975-6868
Fire Research Division

 

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