- SEMI ME1392 - Guide for Angle Resolved Optical Scatter Measurements on Specular or Diffuse Surfaces
This Standard was technically approved by the Silicon Wafer Global Technical Committee. This edition was approved for publication by the global Audits and Reviews Subcommittee on August 31, 2015. Available at www.semiviews.org and www.semi.org in January 2016; originally published by ASTM International as ASTM E1392-90; previously published November 2009.
The microroughness and contamination due to particulates and films on silicon wafers are interrogated with varying forms of light scattering techniques. The angular distribution of light scattered by semiconductor surfaces is a generalized basis for most scanning surface inspection systems and as such may be used to cross-correlate various tools.
The angular distribution of scatter from optically smooth surfaces, such as polished silicon wafers, can be used to calculate surface parameters or reveal surface characteristics. For example, the total scatter found by integrating the bidirectional reflectance distribution function (BRDF) over the hemisphere can be related to surface roughness. The amount of scatter at a given scatter angle can be associated with a specific surface spatial frequency.
The angular distribution of scatter is a general property of surfaces that may have direct consequences. Scatter from mirrors and other components in an optical system can be the limiting factor in resolution or optical signal to noise level. Scatter can be an important design parameter for telescopes. Scatter measurements are crucial to correct operation of ring laser gyros. Scatter from a painted surface, such as on automobiles, can influence sales appeal.
This Guide explains a procedure for the determination of the amount and angular distribution of optical scatter from an opaque surface. In particular it focuses on measurement of the BRDF, which is a convenient and well accepted means of expressing optical scatter levels for many purposes. Additional data presentation formats described in Related Information 1 have advantages for certain applications. Surface parameters can be calculated from optical scatter data when assumptions are made about model relationships. Some of these extrapolated parameters are described in Related Information 2.
Optical scatter from an opaque surface results from surface topography, surface contamination, and subsurface effects. It is the user’s responsibility to be certain that measured scatter levels are ascribed to the correct mechanism. Scatter from small amounts of contamination can easily dominate the scatter from a smooth surface. Likewise, subsurface effects may play a more important scatter role than typically realized when surfaces are super polished.
This Guide does not provide a method to extrapolate data for one wavelength from data for any other wavelength. Data taken at particular incident and scatter directions are not extrapolated to other directions. In other words, no wavelength or angle scaling is to be inferred from this Guide. Normally the user must make measurements at the wavelengths and angles of interest.
Referenced SEMI Standards
SEMI M20 — Practice for Establishing a Wafer Coordinate System
SEMI M59 — Terminology for Silicon Technology
SEMI MF1048 — Test Method for Measuring Reflective Total Integrated Scatter
SEMI MF1811 — Guide for Estimating the Power Spectral Density Function and Related Finish Parameters from Surface Profile Data