New optical depth-profiling technique by use of the multiple-frequency approach with single ATR FT-IR spectrum: Theoretical development

A new depth-profiling technique by attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy by using the multiple-frequency approach has been developed. The technique utilizes spectral intensities at various frequencies of a single spectrum for the depth-profiling calculation. Any spectrum at an incident angle greater than the critical angle with an incident beam of known degree of polarization may be used. A multiple-reflection attachment can be used to obtain a spectrum with a high signal-to-noise (S/N) ratio and to perform real-time depth-profiling analysis. Neither an estimated profile nor any information about the profile is required for the calculation. The new technique consists of three calculation steps, including estimation of the volume-fraction profile by linear equations of absorptance, linear least-squares fitting of absorptance, and nonlinear fitting of reflectance. Simulated spectra with added white noise are used to verify the applicability of the newly developed technique. The accuracy and spatial resolution of the technique have been verified by various types of profiles and degrees of polarization.