Characterization of MgF2 thin films using optical tunneling photoacoustic spectroscopy

Abstract

Thin films continue to show great promise for improving devices in optical applications, such as improved chemical sensors based on surface plasmon resonance devices (Gardon et al. (2008) [1] and Brolo et al. (2004) [2]). While ellipsometry and reflectometry are standard characterization techniques for determining thickness and refractive index of thin films, these techniques tend to require highly reflective or polished films and rely on empirical equations like the Cauchy, Briot, Hartmann, Conrady, and Sellmeier empirical dispersion equations. While these empirical equations may be accurate in some wavelength ranges and for non-conductive materials, the researcher must identify which equation is appropriate for the film being tested and wavelength range desired. To improve ease of testing, we have created a new technique, Optical Tunneling Photoacoustic Spectroscopy (OTPAS), that uses light-induced ultrasound to measure the amount of optical tunneling that has occurred with frustrated total internal reflection through a thin film. The OTPAS system allows a researcher to obtain thickness and refractive index estimates of transparent films without polishing or knowledge of empirical equations prior to the experiment. Scans of 200 nm thick MgF2 films were used to compare ellipsometry with the OTPAS technique. An example of our results shows mean refractive index and thickness estimates of 1.395±0.011 and 220±19 nm versus 1.392±0.002 and 195.2±1.8 nm at 532 nm for ellipsometry, which suggests a general agreement between the two techniques. We present OTPAS to be used for the determination of optical properties of transparent single layer films in cases where empirical equations cannot be used or in cases of low optical reflection.