Ellipsometric techniques for determining oxide layer thickness on aluminum.

Ellipsometry is an optical tool used for measuring the thickness of extremely thin films. Ordinarily, useful results can be obtained only for very smooth layers of uniform thickness. This study describes an attempt to employ ellipsometric techniques to rough, non-uniform layers such as the natural oxide layers on ALCLAD, an aircraft aluminum. The method uses the formulas pertinent to ellipsometry on ideal layers, but interprets the layer thickness and the substrate conductivity in a phenomenological fashion. Traditional ellipsometric procedures are followed and the data obtained from a sample are plotted on a coordinate system derived from the parameters for ideal oxide layers on ideal aluminum, with substrate conductivity serving as a phenomenological parameter. Once the data have been plotted, values can be determined for the effective layer thickness and effective substrate conductivity. From features of internal consistency, it appears that the obtained parameters characterize the ALCLAD samples in a useful way.