Nano-coated long-period gratings for detection of sub-nanometric changes in thin-film thickness

This work presents an application of a long-period grating (LPG) for high-precision detection of changes in the thickness of thin films deposited on its surface. LPGs highly sensitive to the external refractive index and reference silicon (Si) wafers were nano-coated with a thin overlay of aluminum oxide (Al2O3) using the atomic layer deposition (ALD) method. Then both LPGs and wafers were exposed to two different concentrations (10 and 100 mM) of sodium hydroxide NaOH, a well-known etchant of Al2O3. Transmission of the LPG was measured at different stages of the experiment for a fixed external refractive index (air or water) and the results were compared to reduction in Al2O3 thickness on reference Si wafers. It was found that the sensitivity to changes in the overlay thickness highly depends on the working conditions of the LPG, specifically the vicinity of the dispersion turning point and mode transition. When both the phenomena are present, the sensitivity reaches over 20 nm in resonance wavelength shift per single nm of the overlay. Compared to standard spectroscopic ellipsometry measurements typically used to determine the optical properties and thickness of thin film on flat surfaces, the measurements taken with this specially coated LPG offer higher (sub-nm) accuracy and the capability of real-time thickness monitoring.