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Optical Scattering From Surfaces

We study how material properties, surface topography, and contaminants affect the distribution of light scattered from surfaces, with an aim toward

  • Developing standard measurement methods and standard artifacts for use in industry, and
  • Providing a basis for interpreting scattered light distributions so that industry can optimize their use of optical scatter methods.
Applications include evaluation of highly polished optical surfaces, bulk optical materials, surface residues, and diffuse scattering materials. Optical scattering is also used to assess uniformity of periodic structures such as found on compact disks, patterned photoresists, and deposited lines on semiconductors. Experiments are underway to correlate the optical scatter from silicon wafers with properties such as surface microroughness, particulate contamination, and subsurface defects in order to facilitate optical scattering measurements in assembly line applications. Different sources of scattered light are expected to have unique signatures in the scattered light distribution.

polarization of light scattered by a silicon surface

Light Scattering Ellipsometry:

The polarization of scattered light can often indicate the source of that scattered light. Using Light Scattering Ellipsometry, whereby the polarization of light scattered into directions out of the plane of incidence is measured for a fixed incident polarization, scattering from microroughness, subsurface defects, and particulate contamination can be distinguished. Experimental measurements and theoretical modeling have been carried out to demonstrate this effect in a variety of systems:

  • Roughness of a single material (silicon, glass, steel, and titanium nitride)
  • Subsurface defects (fused silica, glass ceramic, and subsurface defects in silicon)
  • Roughness of a dielectric layer (SiO2 and polymer films on silicon)
  • Particles above a single interface (polystyrene, copper, and gold spheres on silicon)
  • Particles above a thin film (polystyrene spheres on polystyrene films on silicon)
  • Special-effect pigmented coatings (metallic and pearlescent flakes)
  • Overlay structures

Placing the technique on a firm metrological basis, so that it is quantitatively accurate, is a high priority of the program. Polarized light scattering in the Stokes-Mueller representation is also studied.

Model Software:

SCATMECH: Polarized Light Scattering C++ Class Library -- A C++ object class library has been developed to distribute models for polarized light scattering from surfaces. It is the intent of this library to allow researchers in the light-scattering community to fully utilize the models described in the publications found below. Included in the library are also a number of classes that may be useful to anyone working with polarized light. The library is constructed so that it can easily be expanded to include new models.

MIST: Modeled Integrated Scatter Tool -- The MIST program has been developed to provide users with a general application to model an integrated scattering system. The program performs an integration of the bidirectional reflectance distribution function (BRDF) over solid angles specified by the user and allows the dependence of these integrals on model parameters to be investigated. The models are provided by the SCATMECH library of scattering codes.

Resources:

A laser-based goniometric optical scatter instrument (GOSI) is available for measuring the bidirectional reflectance distribution function (BRDF), its polarization counterpart (Mueller matrix BRDF), or other light scattering ellipsometry parameters, from a variety of samples or surfaces. Another instrument, the Scanning Optical Scatter Instrument, is being developed to yield the scattering distribution in multiple directions at once, with partial polarimetric capabilities. These facilities are housed in clean environments to maintain sample integrity. See Bidirectional Optical Scattering Facility for details. Other instruments exist within the division under Spectrophotometry.

Optical Scattering Instrument Characterization:

Integrated light scatter instruments can be characterized with respect to their ability to measure microroughness on different length scales. A methodology and computer program has been developed which allows instrument manufacturers to determine the transfer functions for their instruments. See Spatial Frequency Response Function.

Characterization of light scattering methodologies, such as determining instrument signature functions, play an important role in our work. For example, the BRDF that an instrument measures for a perfectly flat and defectless surface is dominated by the Rayleigh scatter in the air within the field of view of the instrument. This Rayleigh-equivalent polarized BRDF has been calculated and experimentally verified.

Standard Reference Material Development:

Possible candidates for low-level BRDF calibration standards are always being considered. Such a standard could consist of a set of artifacts having varying degrees of BRDF levels in the range of 10-2 sr-1 to 10-6 sr-1. These standards must be relatively insensitive to viewing and incident angles within specified ranges and be relatively durable. For further information, please contact Thomas A. Germer.

References:

   Polarized Light Scattering from Particles:

Scattering by slightly non-spherical particles on surfaces (Preprint 58 kB), T.A. Germer, in Seventh Conference on Electromagnetic and Light Scattering by Nonspherical Particles: Theory, Measurements, and Applications, in press, (2003).

Polarized light scattering by dielectric and metallic spheres on oxidized silicon wafers (Preprint 205 kB), J.H. Kim, S.H. Ehrman, G.W. Mulholland, and T.A. Germer, submitted to Appl. Opt. (2003).

Modeling, Measurement, and Standards for Wafer Inspection (Preprint 67 kB), G.W. Mulholland, T.A. Germer, and J.C. Stover, in Government Microcircuit Applications and Critical Technology Conference 2003, in press (2003).

Polarized light scattering by dielectric and metallic spheres on silicon wafers (Preprint 205 kB), J.H. Kim, S.H. Ehrman, G.W. Mulholland, and T.A. Germer, Appl. Opt. 41 (25), 5405-5412 (2002).

Measurement of the 100 nm NIST SRM® 1963 by laser surface light scattering (Preprint 253 kB), T.A. Germer, G.W. Mulholland, J.H. Kim, and S.H. Ehrman, in Advanced Characterization Techniques for Optical, Semiconductor, and Data Storage Components, A. Duparré and B. Singh, Eds., Proc. SPIE 4779, 60-71 (2002).

Light scattering by slightly non-spherical particles on surfaces (Preprint 43 kB), T.A. Germer, Opt. Lett. 27 (13), 1159-1161 (2002).

Polarized light scattering from metallic particles on silicon wafers, (Preprint 253 kB) J.H. Kim, S.H. Ehrman, G.W. Mulholland, and T.A. Germer, in Optical Metrology Roadmap for the Semiconductor, Optical, and Data Storage Industries, A. Duparré and B. Singh, Eds., Proc. SPIE 4449, 281-290 (2001).

Polarization of light scattered by spheres on a dielectric film, (Preprint 545 kB) L. Sung, G. W. Mulholland, and T. A. Germer, in Rough Surface Scattering and Contamination, P.-T. Chen, Z.-H. Gu, and A. A. Maradudin, Eds. Proc. SPIE 3784, 304-313 (1999).

Polarization of light scattered by spheres on silicon wafers, (Preprint 426 kB) L. Sung, G.W. Mulholland and T.A. Germer, Opt. Lett. 24, 866-868 (1999).

   Polarized Light Scattering from Dielectric Layers:
Characterizing surface roughness of thin films by polarized light scattering (Preprint 326 kB), T.A. Germer and M.J. Fasolka, in Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies, A. Duparré and B. Singh, Eds., Proc. SPIE 5188, in press (2003).

Polarized light scattering by microroughness and small defects in dielectric layers, (Preprint 224 kB) T.A. Germer, J. Opt. Soc. Am. A, 18(6), 1279-1288 (2001).

Measurement of Roughness of Two Interfaces of a Dielectric Film by Scattering Ellipsometry, (Preprint 55 kB) T.A. Germer, Phys. Rev. Lett. 85(2), 349-352 (2000).

   Polarized Light Scattering from Roughness and Subsurface Defects:
Large angle in-plane light scattering from rough surfaces: comment, (Preprint 49 kB) T.A. Germer, Appl. Opt., 40(31), 5708-5710 (2001).

Polarized light scattering measurements of polished and etched steel surfaces, (Preprint 119 kB) T.A. Germer, T. Rinder, and H. Rothe, in Scattering and Surface Roughness III, Z.-H. Gu, and A. A. Maradudin, Eds., Proc. SPIE 4100, 148-155 (2000).

Polarization of light scattered by microrough surfaces and subsurface defects, (Preprint 1023 kB) T. A. Germer and C. C. Asmail, J. Opt. Soc. Am. A, 16, 1326-1332 (1999).

Angular dependence and polarization of out-of-plane optical scattering from particulate contamination, subsurface defects, and surface microroughness, (Preprint 1.1 MB) T. A. Germer, Appl. Opt. 36, 8798-8805 (1997).

Polarization of out-of-plane scattering from microrough silicon, (Preprint 316 kB) T. A. Germer, C. C. Asmail, and B. W. Scheer, Opt. Lett. 22, 1284-1286 (1997).

   Appearance Research:
Ray model of light scattering by flake pigments or rough surfaces beneath smooth transparent coatings, (Preprint 129 kB) T.A. Germer and E. Marx, submitted to Applied Optics.

Polarized light diffusely scattered under smooth and rough surfaces (Preprint 202 kB), T.A. Germer, in Polarization Science and Remote Sensing, J. A. Shaw and J. S. Tyo, Eds., Proc. SPIE 5158, in press (2003).

Modeling the appearance of special effect pigment coatings, in Surface Scattering and Diffraction for Advanced Metrology, T.A. Germer, and M.E. Nadal, Z.-H. Gu, and A.A. Maradudin, Eds., Proc. SPIE 4447, 77-86 (2001). (Preprint 129 kB).
   Instrumentation:
Multidetector Hemispherical Polarized Light Scattering Instrument, (Preprint 1.2 MB) T.A. Germer, in Rough Surface Scattering and Contamination, P.-T. Chen, Z.-H. Gu, and A. A. Maradudin, Editors, 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. 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. R. P. Breault, Proc. SPIE 2260, 52-61 (1994).

   Instrument Characterization:
Proposed methodology for characterization of microroughness-induced optical scatter instrumentation, (Reprint 349 kB) T. A. Germer, and C. C. Asmail, in Flatness, Roughness, and Discrete Defect Characterization for Computer Disks, Wafers, and Flat Panel Displays, ed. J. C. Stover, Proc. SPIE 2862, 12-17, (1996).

"Rayleigh scattering limits for low-level bidirectional reflectance distribution function measurements," C. Asmail, J. Hsia, A. Parr, and J. Hoeft, Appl. Opt. 33, 6084-6091 (1994).

   Standard Reference Material Development:
Measurement of the 100 nm NIST SRM® 1963 by laser surface light scattering (Preprint 253 kB), T.A. Germer, G.W. Mulholland, J.H. Kim, and S.H. Ehrman, in Advanced Characterization Techniques for Optical, Semiconductor, and Data Storage Components, A. Duparré and B. Singh, Eds., Proc. SPIE 4779, 60-71 (2002).

"Status of Bidirectional Reflectance Distribution Function (BRDF) Calibration Standards Development," C. Asmail, J. Fuller, and R. Parks, in Quality and Reliability for Optical Systems, ed. J. W. Bilbro and R. E. Parks, Proc. SPIE 1993, 44-53 (1993).
   Overlay Metrology:
Measurement of lithographic overlay by light scattering ellipsometry (Preprint 214 kB), T.A. Germer, in Surface Scattering and Diffraction for Advanced Metrology II, Z.-H. Gu, and A.A. Maradudin, Eds., Proc. SPIE 4780, 72-79 (2002).

For technical information or questions, call:
Thomas A. Germer
Phone: (301) 975-2876
Fax: (301) 869-5700
Email: mailto:tgermer@nist.gov

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Online: September 1997   -   Last updated: June 2004