Oxide thickness measurements by infrared ellipsometry

Current interest' in anodized aluminum surfaces as substrates for adhesive bonding has created a need to measure the properties of the oxide layer in a non-contacting manner. Visible light ellipsometry is a very sensitive non-contacting technique for measuring the thickness of very thin films on smooth surfaces whose optical constants are known. However, the method is limited to film thicknesses which are generally less than 2000 Å and validity is lost when there is appreciable scattering caused by the roughness of the substrate and the structure of the oxide itself. These objections become much less severe if the operating wavelength is in the infrared region. Such an infrared ellipsometer has been developed to measure the thickness of oxides produced by anodization of aluminum with production-finished surfaces. The instrument operates with a 10.6 μm beam from a low power CO₂ laser and uses a 6328 Å beam from a He-Ne laser for alignment and location of the measurement region. The oxides were formed on unclad 7075 aluminum by anodization in an ammonium pentaborate solution at constant current to termination voltages of from 25 to 275 V in 25 V increments. The measured ellipsometric quantities Δ and ψ were used to compute the corresponding metal oxide film thicknesses using a complex refractive index N = 2.39?i41.36 for the substrate. The results for a film refractive index of 1.50 were in close agreement with those measured with a scanning electron microscope. Elemental concentration profiles for each surface were made by Auger electron spectroscopy.