Remotely measuring a thin dielectric coating on a metallic cylinder

Abstract

A laser optical metrology system is described that remotely measures at high rate the presence and thickness of a thin-film lubricant on metallic cylinders with diameters on the order of 0.5 mm. Applications include remote measurement of hypodermic needle dielectric coating thickness in a clean room environment. High accuracy computer simulation of the electric field scattered from a coated cylinder by an incident laser beam is demonstrated using the condition numbers of the matrices defined by the boundary value matching equations derived from the eigenfunction expansion of the exact solution to Maxwell's equations. Dielectric coatings from 1 μm to 50 μm are seen to be readily observed and accurately measured using a remotely placed CMOS array. Distinctive signatures are shown for film thicknesses in the range from 0 to 10 μm, and an appropriate location for CMOS detector placement is determined from the scattering patterns.