Nanocrystalline Low‐Refractive Magnesium Fluoride Films Deposited by Reactive Magnetron Sputtering: Optical and Structural Properties1. Issue 11 (2nd July 2015)

• Record Type: Journal Article
• Authors: Mertin, Stefan; Marot, Laurent; Sandu, Cosmin S.; Steiner, Roland; Scartezzini, Jean‐Louis; Muralt, Paul
• Is Part Of: Advanced engineering materials. Volume 17:Issue 11(2015:Nov.)
• Page Start: 1652
• Page End: 1659
• Publication Date: 2015-07-02
• Subjects: Materials -- Periodicals; 620.11
• DOI: 10.1002/adem.201500129
• Languages: English
• ISSNs: 1438-1656

Abstract

Abstract : 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. Abstract : A novel reactive sputtering process for highly transparent MgF2 films is studied. These nanocrystalline films exhibit a very ‐ low refractive index of 1.382 at 550 nm and a near‐zero absorption in the whole solar spectral range. A quaternary film of this nanocrystalline MgF2 on glass reduces light reflection by 3%.