Collections > Master's Papers > Gillings School of Public Health > Light Scattered by Particles on Reflective Surfaces

This report describes an analytical approach to light scattering from particles resting on reflective surfaces. The phenomenon of light scattering from particle-deposited surfaces depends on the optical characteristics of the particle, the surface, and the coated film individually and the combined optical effects among these three. Mie theory was used to compute scattering from the particle while ray tracing was used to estimate the influence of the surface. Four paths have been analyzed: (1) back scattering from a particle on which incident light impinges directly, (2) forward scattering from a particle of incident light reflected by a surface, (3) forward scattering from a particle, which then reflects from a surface to reach the detector, and (4) back scattering of reflected light which then reflects again from a surface to reach the detector. A "shadow factor" concept was developed and applied for numerical determination of the amount of light reflected from the surface for each path. Calculations were performed by a computer program using an IBM PC. The results of this study suggest a theory to quantitatively determine the light scattering from particles on reflective surfaces. Non-monotonic response for submicron particles was found. Particle location on the wafer is crucial and the presence of surface coatings always enhances response because of the shadow factor. A dominance index was developed to describe the relative magnitude of each path. Path Bl (reflect-scatter) was proved the most important, having a dominance index 95% at an incident angle of about 16° for the oxide thickness greater than 3 pm and 91% at an incident angle of about 21° for bare surfaces.