Nanocrystalline Low-Refractive Magnesium Fluoride Films Deposited by Reactive Magnetron Sputtering: Optical and Structural Properties

In this work, we study MgF2 thin-film synthesis by reactive pulsed DC magnetron sputtering from a metallic magnesium target in a gas mixture of argon, oxygen, and carbon tetrafluoride (CF4). Nanocrystalline films on silicon and glass substrates with excellent properties for optical application are achieved. The plasma discharge is analyzed with a differentially pumped mass spectrometer before and during the deposition process. Without breaking the vacuum, monochromatic photoelectron spectroscopy (XPS) is performed for in situ determination of the atomic C and O concentration. Film microstructure, topography, and thickness are investigated by electron microscopy (SEM and TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The optical constants n and k are determined by spectroscopic ellipsometry and spectrophotometry: a consistent parametric fit of the ellipsometric angles and spectral transmittance and reflectance based on three Lorentz oscillators to determine n and k is achieved for a wide spectral range (300–2 300 nm). At 550 nm, a refractive index of 1.382 and near-zero absorption is obtained, which is in excellent agreement with n = 1.383 of polycrystalline MgF2. The measured light reflection at 760 nm is reduced by 3% for a quarter-wave nanocrystalline MgF2 coating on glass compared to the uncoated glass substrate.