Quantifying Light Scatter

Abstract

<i>But, soft! What light through yonder window breaks?</i> - Shakespeare This chapter discusses the origins of light scatter and the various scatter sources that are commonly observed, and it defines how scattered light is quantified. Except for the following brief overview, the book is largely restricted to the measurement and analysis of scatter caused by surface, bulk, and contaminant imperfections, as opposed to scatter from individual molecules, aerosols, and resonance effects, such as Raman scattering. Scatter from optically smooth components is treated as diffraction in many cases. For the special case of clean, optically smooth, reflective surfaces, there is a well-defined relationship between the scatter distribution pattern and surface roughness statistics, and scatter measurements can be manipulated to characterize the surface. In many cases, insight may be gained into possible improvements in surface-finish techniques. A simple example of this technique is given in this chapter and treated in more depth later. In later chapters it will be seen that in some cases, discrete surface features can be identified as pits or particles from their scatter patterns, and estimates can be made of their diameters from scatter measurements. Scatter from windows, caused by both bulk and surface imperfections, is also introduced here and examined in more detail later. Although the mechanisms of bulk and particulate scatter do not lend themselves to the quantitative analysis used for surface scatter, they are still strong indicators of component quality, and measurement of the resulting scatter patterns is a viable source of metrology. Scatter measurement is proving to be a useful inspection technique for many applications outside the optics industry. It is proving to be particularly useful in the semiconductor industry in applications varying from the study of polishing to inspection during device manufacturing. Similar applications are found in the computer disk and flat panel display industries. Measured scatter can be used to monitor changes in surface appearance in a variety of products. It can be used to detect and map component defects in a variety of materials, including painted surfaces, paper, metallic coatings, and medical implants such as artificial joints and intraocular lenses. Bulk and surface scatter can be separated through the use of special measurement techniques, so it is possible to determine whether or not surface polishing or a better material is required to reduce component scatter.