Simulation and deposition of near-IR anti-reflection layers for silicon substrates

Anti-reflection (AR) layers for Si were investigated for potential application in optical communications in the wavelength range of 1270-1330 nm. The optical simulation module of the Essential Macleod program was used to find the optimal thickness of single-layer and double-layer structures using Al2O3 and TiO2. Al2O3 was found to be a better AR single-layer because of a lower reflectance. Less than 1% reflectance was simulated using double-layer structures for both stack sequences Si/TiO2/Al2O3 and Si/Al2O3/TiO2. For experimental work, atomic layer deposition (ALD) of Al2O3 and TiO2 was employed to fabricate two different stacks. Reflectance measurements were conducted and 1.9% and 1.7% maximum reflectance was recorded in the wavelength range 1270-1330 nm. This reflectance establishes the possibility that the two stacks can be used as effective AR layers for Si lenses designed for optical communications. Resistance against humidity was tested for the two structures and only the Si/Al2O3/TiO2 structure was impermeable. Analyses using Fourier transform infrared spectroscopy and atomic force microscopy revealed that ALD-Al2O3 is easily hydroxylated while ALD-TiO2 acts as a good humidity barrier.