August 2004

BFRL Monthly Highlights

August 2004

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PAST Highlights

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Surface Light Scattering Used for Size Determination of 100 nm Polystyrene Spheres

Light scattering is used by the semiconductor, optical, and data storage industries to inspect materials for surface quality. Light scattering instruments often require calibration with well-characterized scattering artifacts, such as surfaces with roughness or deposited particles. Small uncertainties in the scattering artifact standard can lead to unacceptable errors in the interpretation of the scattered light.

In the semiconductor industry, tolerable contamination levels often are specified for starting wafers. Accurate particle-size standards are required so that wafer vendors and integrated circuit manufacturers can agree on the quality of materials being delivered.

PL’s Optical Technology Division, in collaboration with BFRL and the University of Maryland, has performed a light-scattering measurement of the 100 nm polystyrene sphere standard (SRM 1963) after deposition of the spheres onto a silicon wafer. The measurement was carried out using 441.6 nm, p-polarized light. The measurement yielded a value of 99.7 nm with an expanded uncertainty (95 percent confidence limit) of 1.7 nm. The uncertainty is dominated by the reproducibility of the measurement and is believed to result from the presence of a small number of doublet spheres on the sample.

The value obtained by surface light scattering agrees well with the certified value of 100.7 nm ± 1.0 nm, which was determined by differential mobility analysis of an aerosol.

While the certified uncertainty of SRM 1963 was lower than that obtained by light scattering, there is a growing need for smaller size standards, and the light scattering method should be substantially more accurate if the number of doublet particles is reduced. In fact, preliminary measurements of particles in the 50 nm to 70 nm range, where the particles were aerosolized by electrospray with fewer doublets, suggest that the reproducibility of the measurement has been reduced by an order of magnitude. These measurements suggest that accurate size standards in this range should be certified by light scattering.

CONTACT:
Thomas Germer (PL)
 

Persily Receives ASHRAE Standards Award

Andrew Persily of the Indoor Air Quality and Ventilation Group in the Building Environment Division received the Standards Achievement Award from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) at their annual meeting in June 2004. This award, which recognizes exceptional service in the area of standards leadership and technical contributions, is based on Persily’s participation in the revision of ASHRAE’s ventilation and indoor air quality standard as a member of the committee from 1991-1999 and chair from 1999-2003. During his tenure as chair, Persily managed the revision process through many contentious issues involving a diverse group of interested parties, maintaining an open process throughout. As a result, ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, will be republished later this year in a revised form that updates the technical content and contains exclusively mandatory and enforceable requirements suitable for adoption into building codes.

Contact:
Andrew Persily (BFRL),
301-975-6418

 

 

 

 

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