Thin film subsurface environments: advanced X-ray spectroscopies and a novel Bayesian inference modeling algorithm
Date
2015
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
New condensed matter metrologies are being used to probe ever smaller length scales. In support of the diverse field of materials research synchrotron based spectroscopies provide sub-micron spatial resolutions and a breadth of photon wavelengths for scientific studies. For electronic materials the thinnest layers in a complementary metal-oxide-semiconductor (CMOS) device have been reduced to just a few nanometers. This raises concerns for layer uniformity, complete surface coverage, and interfacial quality. Deposition processes like chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been shown to deposit the needed high-quality films for the requisite thicknesses. However, new materials beget new chemistries and, unfortunately, unwanted side-reactions and by-products. CVD/ALD tools and chemical precursors provided by our collaborators at Air Liquide utilized these new chemistries and films were deposited for which novel spectroscopic characterization methods were used. The second portion of the thesis focuses on fading and decomposing paint pigments in iconic artworks. Efforts have been directed towards understanding the micro-environments causing degradation. Hard X-ray photoelectron spectroscopy (HAXPES) and variable kinetic energy X-ray photoelectron spectroscopy (VKE-XPS) are advanced XPS techniques capable of elucidating both chemical environments and electronic band structures in sub-surface regions of electronic materials. HAXPES has been used to study the electronic band structure in a typical CMOS structure; it will be shown that unexpected band alignments are associated with the presence of electronic charges near a buried interface. Additionally, a computational modeling algorithm, Bayes-Sim, was developed to reconstruct compositional depth profiles (CDP) using VKE-XPS data sets; a subset algorithm also reconstructs CDP from angle-resolved XPS data. Reconstructed CDP produced by Bayes-Sim were most strongly correlated to the real chemical distributions in our CMOS. Chemical speciation and degradation mechanisms in paint pigment materials from iconic artwork has been investigated using high energy X-rays by the techniques as X-ray absorption near-edge spectroscopy (XANES) and X-ray fluorescence (XRF). Unique features in X-ray absorption spectra using XANES help identify the chemical species present in the paint sample by comparing similar spectral features from known reference standards. By carefully choosing X-ray energies that excite a subset of the present chemical species the distribution of constituents can be determined. This work furthers the understanding of supposed photo-oxidation degradation mechanisms and potential preservation efforts to maintain the painting integrity for cadmium yellows used in Henri Matisse's Le Bonheur de vivre and arsenic sulfides from an 18th century still life painting from Adriaen Coorte.