Fast and accurate optical determination of gold-nanofilms thickness

We propose and investigate an alternative optical method to determine the thickness of metallic nanofilms thinner than about 50 nm deposited on glass substrates. The method consists of measuring the internal TM reflectivity as a function of the angle of incidence and fitting the curve with Fresnel formulae for a bilayer system to find the film thickness. Gold nanofilms were fabricated by electron beam evaporation (nominal thicknesses of 20 nm, 30 nm and 50 nm) and characterized by common methods (SEM, AFM, Profilometer). The reflectivity curve of TM (or p) polarized light presents a clear peak around the critical angle with air and then a dip due to the excitation of a surface plasmon-polariton mode at the gold/air interface. Fitting the theoretical model around the reflectivity peak readily gives the film's thickness, and fitting other sections of the curve away from the critical angle with air without altering the fit around the peak, provides the thickness and effective refractive index of the adhesive layer (Cr, Ti). The present approach to gold nanofilm's thickness measurement is robust and expeditious. It is less sensitive to surface roughness, contamination and nanofabrication defects such as nanometer sized holes.