This guide was technically approved by the Global Silicon Wafer Committee and is the direct responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by the North American Regional Standards Committee on December 10, 2004. Initially available at www.semi.org January 2005; to be published March 2005. Original edition published by ASTM International as ASTM E 1392-90. Last previous edition SEMI ME 1392-96 (Reapproved 2002).
1.1 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.
1.2 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.
1.3 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.
2.1 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.1,2 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.
2.2 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 superpolished.
2.3 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.
2.4 This guide applies only to BRDF measurements on opaque samples. It does not apply to scatter from translucent or transparent materials. There are subtle complications, which affect measurement of translucent or transparent materials that are best addressed in separate standards (see, for example, ASTM Practice E 167 and ASTM Guide E 179).
2.5 The wavelengths for which this guide applies include the ultraviolet, visible, and infrared regions. Difficulty in obtaining appropriate sources, detectors, and low scatter optics complicate its practical application at wavelengths less than about 0.25 m. Diffraction effects that start to become important for wavelengths greater than 15 m complicate its practical application at longer wavelengths. Diffraction effects can be properly dealt with in scatter measurements,3 but they are not discussed in this practice.
2.6 Any experimental parameter is a possible variable. Parameters that remain constant during a measurement sequence are reported as header information for the tabular data set. Related Information 3 gives a suggested reporting format that is adaptable to varying any of the sample or system parameters.
2.7 This guide applies to flat or curved samples of arbitrary shape. However, only a flat, circular sample is addressed in the discussion and examples. It is the user’s responsibility to define an appropriate sample coordinate system to specify the measurement location on the sample surface for samples that are not flat.
2.8 The apparatus and measurement procedure are generic, so that specific instruments are neither excluded nor implied in the use of this guide.
NOTICE: This standard does not purport to address safety issues, if any, associated with its use. It is the responsibility of the users of this standard to establish appropriate safety and health practices and determine the applicability of regulatory or other limitations prior to use.