Nondestructive multiparameter quantification of nanoporous films by broadband SPR/PWR spectroscopy

Nondestructive measurement of film thickness (<i>d</i>) and porosity (<i>P</i>) is crucial for the rational design, controllable synthesis and accurate characterization of multifunctional nanoporous thin films (NPTFs). In this work, the <i>d</i> and <i>P</i> values of dielectric NPTFs were measured simultaneously by using a new algorithm to process the experimental data obtained by broadband surface plasmon resonance or plasmon waveguide resonance (SPR/PWR) spectroscopy. The NPTF under test is coated onto the pristine gold surface of an SPR sensor chip, which enhances the interaction of light with the NPTF and makes the resonance wavelength highly sensitive to both <i>d</i> and <i>P</i> of the NPTF. The measurements are implemented in three steps: (1) to measure two resonance spectra of a NPTF at different angles of incidence or different surrounding media or different polarizations; (2) to create two <i>d versus P</i> curves by fitting each measured spectrum with a combination of Fresnel and Brueggemann equations; (3) to obtain <i>d</i> and <i>P</i> values of the NPTF by solving the intersection of the two <i>d</i>-<i>P</i> curves. It is worth emphasizing that if the NPTF is a multimode waveguide layer, its thickness and porosity can be simply derived by fitting the two widely separated resonance peaks in the measured single PWR spectrum. Six NPTFs, including four TiO₂ films and two SiO₂ films, were characterized according to the above steps. The measured results show good agreement with those obtained by other existing techniques, verifying that the method proposed in this work is accurate and reliable.