An Application of Laser Speckle Techniques to Surface Characterization

A laser based non-conducting method is described for determining the thickness of organic residues or polar liquids on metallic surfaces. There exist measurement systems operating with fluorescence labelled analytes detecting film thicknesses ranging from 100 to 1000 nm. Other systems use terahertz transmission spectroscopy to determine layer thicknesses by utilizing the attenuation of strong spectral absorption lines to give alternate sensitivity. \newline A laser based system allows for large stand-off distances so this principle could lend itself towards possible industrial applications especially in harsh environments. The measuring effect utilizes changes in the structure of so-called laser speckles due to any dielectric coating on metallic surfaces, as viewed at by a video camera. For known and sufficiently constant experimental parameters (temperature, laser wavelength, viewing geometry, and statistical properties of the metallic scatterer) film thicknesses down to $\mu$m-levels can be measured with low variances, provided that the dielectric and viscous properties of the substances used for the coating are known and constant. The shape of the power spectral density of so-called subjective speckles on the one hand is dependent on surface roughness as viewed at by the camera and on the other hand on properties of any transparent thin film surface coating. We present preliminary results of a laser speckle based film thickness measurement system. Furthermore it turned out that the very same system is able to track the generation of increased roughness of surfaces in various dry-etching processes, we present preliminary results in this regard, too.