Characterizing antireflection coatings on textured monocrystalline silicon with Spectroscopic Ellipsometry

Mono-crystalline silicon is chemically textured and coated with a silicon nitride surface anti-reflection (AR) film to improve light trapping and device efficiency in photovoltaic applications. The thickness and optical properties of the AR coating determine the effective suppression of reflected light. However, optical characterization of films on chemically textured surfaces is challenging due to the low reflectance. We present new measurement geometries and modeling methodology using Spectroscopic Ellipsometry (SE) to determine film thickness and optical properties of thin AR coatings on textured mono-crystalline silicon. Special measurement geometries are used to collect specular reflected light from the etched silicon pyramid facets. Both apexial and lateral measurement geometries are demonstrated, where the latter requires a special sample stage to tilt and rotate the sample to detect the specular reflected light from the pyramid facets. Measurement considerations are discussed including probe-beam incident angle, sample tilt and rotational angles. Effects of pyramid surface-coverage are also discussed in relation to the proposed measurement geometries. Previous modeling attempts using Effective Medium Approximation (EMA) theory showed inconsistent results when comparing apexial and lateral measurements of the same sample surface. In this work, results from a scattering-corrected modeling approach provide improved consistency for a series of SiNx coatings on textured monocrystalline silicon for both lateral and apexial measurement geometries.