Ground and space based optical analysis of materials degradation in low-Earth-orbit

There is strong interest in being able to accurately and sensitively monitor materials degradation in both ground-based and space-based environments. Two optical techniques for sensitive degradation monitoring are reviewed: spectroscopic ellipsometry and photothermal spectroscopy. These techniques complement each other in that ellipsometry is sensitive to atomically thin surface and subsurface changes, and photothermal spectroscopy is sensitive to local defects, pin-holes, subsurface defects, and delamination. Progress in applying these spectroscopies (both ex situ and in situ) to atomic oxygen degradation of space materials is reviewed

Atomic layer deposition of LiF using LiN(SiMe3)2 and SF6 plasma

Lithium fluoride films were prepared by atomic layer deposition (ALD) using a new route in which LiN(SiMe3)2 is used as precursor and SF6 plasma as coreactant. Conformal LiF films were deposited at 150 °C at a growth rate of ~ 0.4 Å/cycle. All deposited films were polycrystalline and slightly lithium-rich with a composition of LiF0.8, independently of the plasma conditions (e.g. exposure time, pressure and power). The levels of H, C, N, O, Si, and S were all < 1 at. %. Spectroscopic ellipsometry measurements were carried out over the wavelength range of 140–2480 nm and showed a refractive index of 1.37 at 633 nm for films deposited using 1 s plasma exposure time. We conclude that short plasma exposures are preferred, since a prolonged exposure time leads to an increase in optical absorption and lower growth per cycle values. Furthermore, mass spectrometry measurements revealed the formation of SiMe3F species during both half-cycles, originating from the reaction between the precursor ligands and fluorine species present either at the surface or in the plasma. Moreover, the SF6 plasma step led to the formation of fluorocarbon species, suggesting that dissociation and recombination in the plasma takes place. Overall, this work demonstrates that SF6 plasma offers a promising alternative to other coreactants for ALD of high purity lithium fluoride

Solid Tellurite Optical Fiber Based on Stack-and-Draw Method for Mid-Infrared Supercontinuum Generation

Broadband, high-power mid-infrared (mid-IR) sources are critical for many applications. Compared to alternatives such as fluorides and chalcogenides, tellurite fibers are more robust and can handle much higher power. Tellurite fibers also have high nonlinearity and a material zero dispersion close to 2 µm, making them ideal for nonlinear processes pumped by Tm-doped silica fiber lasers. In this work, we have demonstrated solid tellurite fibers fabricated by a stack-and-draw process and investigated their potential for broadband mid-IR supercontinuum generation. We have identified that fibers with low dispersion are beneficial and that low residual hydroxyl (OH) is critical for broadband mid-IR supercontinuum generation in tellurite fibers pumped at ~2 µm

Progress in spectroscopic ellipsometry: Applications from vacuum ultraviolet to infrared

Spectroscopic ellipsometry (SE) is a noncontact and nondestructive optical technique for thin film characterization. In the past 10 yr, it has migrated from the research laboratory into the semiconductor, data storage, display, communication, and optical coating industries. The wide acceptance of SE is a result of its flexibility to measure most material types: dielectrics, semiconductors, metals, superconductors, polymers, biological coatings, and even multilayers of these materials. Measurement of anisotropic materials has also made huge strides in recent years. Traditional SE measurements cover the ultraviolet, visible, and near infrared wavelengths. This spectral range is now acquired within seconds with high accuracy due to innovative optical configurations and charge coupled device detection. In addition, commercial SE has expanded into both the vacuum ultraviolet (VUV) and midinfrared (IR). This wide spectral coverage was achieved by utilizing new optical elements and detection systems, along with UV or Fourier transform IR light sources. Modern instrumentation is now available with unprecedented flexibility promoting a new range of possible applications. For example, the VUV spectral region is capable of characterizing lithographic materials for 157 nm photolithography. The VUV also provides increased sensitivity for thin layers (e.g., gate oxides or self-assembled monolayers) and allows investigation of high-energy electronic transitions. The infrared spectral region contains information about semiconductor doping concentration, phonon absorption, and molecular bond vibrational absorptions. In this work, we review the latest progress in SE wavelength coverage. Areas of significant application in both research and industrial fields will be surveyed, with emphasis on wavelength-specific information content

Self-referencing photothermal common-path interferometry to measure absorption of Si3N4 membranes for laser-light sails

Laser-light sails are a spacecraft concept wherein lightweight "sails" are propelled to high speeds by lasers with high intensities. The sails must comprise materials with low optical loss, to minimize the risk of laser damage. Stoichiometric silicon nitride (Si$_3$N$_4$) is a candidate material with low loss in the near infrared, but the precise absorption coefficient has not been characterized in the membrane form-factor needed for sails. We use photothermal common-path interferometry (PCI), a sensitive pump-probe technique, to measure the absorption coefficient of stoichiometric and nonstoichiometric silicon nitride. To calibrate PCI measurements of membranes, we developed a self-referencing technique where a measurement is performed twice: once on a bare membrane, and a second time with a monolayer of graphene deposited on the membrane. The absorption of the sample with graphene can be measured by both PCI and more-conventional spectroscopic techniques, enabling the calibration of the PCI measurement. We find that with an absorption coefficient of (2.09 $\pm$ 0.76) $\times$ 10$^{-2}$ cm$^{-1}$ at 1064 nm, Si$_3$N$_4$ is a suitable laser-sail material for laser intensities as high as ~10 GW/m$^{2}$, which have been proposed for some laser-sail missions, while silicon-rich SiN$_x$ (x~1), with an absorption coefficient of 7.94 $\pm$ 0.50 cm$^{-1}$, is unlikely to survive such high laser intensities