Ellipsometric characterization of magnetron sputtered metal-dielectric-metal (MDM) multilayers for Optical Vernier Scale biosensors with ultra-high sensitivity and large dynamic range

We provide a systematic investigation of the optical properties of a hybrid plasmonic–photonic Optical Vernier Scale (OVS) biosensor using multiple zero-reflection points (ZRPs) with ultra-low limit of detection (LoD) of ~10⁻⁸ refractive index unit (RIU) and large dynamic range of ~6 orders of magnitude in RIU sensing. The synergic collaboration of p- and s-polarized ZRPs results in the OVS operation mode that enables precise optical quantification of biomolecular interaction events based on the dynamic fitting of the sensing medium's refractive index changes. The optical model provides a quantitative description of the biosensor operation in terms of its structure and the parameterized dispersion properties of the constitutive silver (Ag) and aluminum nitride (AlN) layers, in the spectral range from 320 to 1690 nm, as well as the ability to follow the dynamic RIU changes in real-time during sensor functionalization and operation. The AlN thin films were first deposited on glass substrates by reactive magnetron sputtering from Al target under different Ar/N₂ gas ratios at room temperature. The optical constants, thickness, and surface roughness of the AlN thin films were then investigated using spectroscopic ellipsometry. It was found that the reactive gas flow ratio can control the optical properties of the AlN films. When the reactive gas ratio was adjusted from 30 % to 70 %, the refractive index (n) of the AlN thin films varied from 1.75 to 1.84 at a wavelength of 633 nm, the extinction coefficient (k) of the AlN samples varied from 0.005 to 0.05 at a wavelength of 233 nm, and the band gap $E_g$, defined where the extinction coefficient k < 0.001, varies between 0 and 5.03 eV. Finally, the application of the AlN film in a metal-dielectric-metal (MDM) multilayer was investigated, showing unique spectral characteristics by total internal reflection ellipsometry measurements and used for the bio-detection of rabbit IgG with a LoD below 10 ng/mL. The demonstrated stability and reliability of our fitting method, coupled with the excellent sensitivity of the MDM multilayer structure, make this system highly competitive in the field of biosensing.