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Bidirectional Optical Scattering Facility

Goniometric Optical Scatter Instrument

Goniometric Optical Scatter Instrument

The bidirectional reflectance distribution function (BRDF) quantifies the angular distribution of light scattered from a surface as a function of wavelength, polarization state, and angle of incidence. The bidirectional characterization of optical scatter from surfaces is a useful metrological tool in evaluating elements contained within optical systems that require the minimization of scattered light for high image contrast or radiant energy density control. It is also effective for the characterization of materials and surfaces in control and inspection processes in manufacturing settings. The Goniometric Optical Scatter Instrument (GOSI) at NIST employs ultraviolet and visible laser light sources and highly sensitive detection schemes. This instrument is well-suited for studying low-scatter surfaces such as silicon wafers and optically smooth mirrors. Other applications include assessment of the uniformity of periodic structures such as gratings, patterned photoresists, or built-up patterns on semiconductor surfaces.

Features of GOSI:

  • Currently available wavelengths: 633 nm (HeNe), 532 nm (Nd:YAG), 488 nm (Ar), 442 nm (HeCd), and 325 nm (HeCd).
  • The instrument has a dynamic range of about 15 orders of magnitude (in units of sr-1), and a noise level of 10-9 sr-1 determined by Rayleigh scattering in the air surrounding the sample.
  • The scatter direction can be measured with angular resolutions of approximately 0.7 deg, 0.1 deg, and 0.02 deg.
  • The polarization state of the incident light and the scattered light can be independently selected.
  • Measurements of scattering into directions out of the plane of incidence can be readily performed.
  • Illumination diameter can be varied from about 20 µm to about 10 mm.
  • Sample sizes ranging between 1 mm and 200 mm can be accommodated.
  • GOSI is housed in a class 10 clean area to maintain the integrity of samples.
Multidetector Hemispherical Polarized Optical Scattering Instrument

Multidetector Hemispherical Polarized Optical Scattering Instrument

A second instrument, the Multidetector Hemispherical Polarized Optical Scattering Instrument (MHPOSI), is under development. Optimized for measuring the light scattering properties of localized defects, it employs multiple detection systems to capture the BRDF over the most of the scattering hemisphere at once. Its detection systems are polarization sensitive, enabling a partial mapping of the polarimetric properties of the samples. Applications of this instrument include the inspection of optical quality surfaces such as optics, silicon wafers, and magnetic disks, and the analysis of specific types of defects on these surfaces.

Features of MHPOSI:

  • Argon-ion laser illumination of the sample (488 nm) at fixed incident angle and adjustable polarization.
  • Twenty-eight fixed detection systems covering most of the scattering hemisphere,each sensitive to only p-polarization.
  • Raster scanning of the sample over an area 200 mm x 200 mm.
  • Proven capability for detecting 100 nm diameter particles.
  • Housed in class 10 clean environment.
  • Sample holders available for wafers having diameters between 25 mm and 200 mm. Samples holders can be constructed for special samples, including transmissive samples.

Both GOSI and MHPOSI are available for collaborative use by NIST and outside scientists in areas of mutual interest on a time-available basis.

See also Optical Scattering Metrology, and Spectrophotometry.

Representative Publications:

" Multidetector Hemispherical Polarized Light Scattering Instrument," (Preprint 1.2 MB)
T.A. Germer,
in Rough Surface Scattering and Contamination, ed. by P.T. Chen, Z.-H. Gu, and A.A. Maradudin, Proc. SPIE 3784, 296-303 (1999).

" Goniometric optical scatter instrument for out-of-plane ellipsometry measurements," (Preprint 994 kB)
T.A. Germer and C.C. Asmail,
Rev. Sci. Instrum. 70, 3688-3695 (1999).

"A goniometric optical scatter instrument for bidirectional reflectance distribution function measurements with out-of-plane and polarimetry capabilities," (Preprint 546 kB)
T.A. Germer and C.C. Asmail,
in Scattering and Surface Roughness, ed. by Z.-H. Gu and A.A. Maradudin, Proc. SPIE 3141, 220-231, (1997).

"Instrumentation at the National Institute of Standards and Technology for bidirectional reflectance distribution function (BRDF) measurements,"
C.C. Asmail, C.L. Cromer, J.E. Proctor, and J.J. Hsia,
in Stray Radiation in Optical Systems III, ed. by R.P. Breault, Proc. SPIE 2260, 52-61 (1994).

For technical information or questions, please contact:

Thomas A. Germer
Phone: (301) 975-2876
Fax: (301) 869-5700
Email: thomas.germer@nist.gov
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 Online: July 1997   -   Last updated: October 2005