BYU Physics and Astronomy

We use laser high-order harmonics and a polarization-ratio-reflectance technique to determine the optical constants of copper and oxidized copper in the wavelength range 10-35 nm. This measurement resolves previously conflicting data sets, where disagreement on optical constants of copper in the extreme ultraviolet most likely arises from inadvertent oxidation of samples before measurement. (C) 2009 Optical Society of America

Optics in the extreme ultraviolet (XUV) have important applications in microelectronics, microscopy, space physics, and in imaging plasmas. Because of the short wavelengths involved in these applications, it is critical to account for interfacial roughness to accurately predict the reflection and absorption of XUV optics. This paper examines two possible effects of roughness on optical absorption, non-specular reflection and enhanced transmission and compares these to measured experimental data on a rough Y2O3 thin film.

In the VUV spectrum we see a significant decrease in reflection due to organic contamination on the surface of mirrors. To study VUV mirrors it is requisite to have calibration standards. Such standards are useless as calibration tools if the surface has organic contamination. For our standard, we use a thermally oxidized silicon wafer with a 27 nm oxide overlayer. We found that silicon wafer samples capped with native oxide acquire 0.1 to 0.2 nm of organic contamination within two hours of being cleaned with stored in closed, but nonvacuum, conditions. After a week there is an additional 0.2 to 0,5 run deposition after which no further significant deposition is measured Lip to 90 days. We place the samples in air within one cm of a xenon excimer lamp that radiates 7.2 eV photons which remove half of the remaining contamination every minute. Five minutes exposure is sufficient to clean both fresh and stored samples. Data are determined using spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). Additionally this paper addresses the need to ensure that these characterization tools are not a source of organic contamination. We determined that the antechamber of our XPS was contaminating samples at a rate of 0.6 nm/30 min as they waited for transfer to the analysis chamber. This contamination was virtually eliminated by attaching an oxygen radical source (ORS) device (Evactron (R) C De-Contaminator RF Plasma Cleaning System) directly to the antechamber. (C) 2008 Elsevier B.V. All rights reserved.

We studied the cleaning of the native oxide surfaces of silicon wafers using variable-angle spectroscopic (multiwavelength) ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). We focused on removing surface contamination, while preserving the oxide layer and minimizing surface roughness. Five minute under a xenon excimer lamp in air was adequate to render “carbon free” (<0.05nm overlayer) oxide surfaces previously cleaned with detergents and/or solvents. We further investigated different ways of storing samples and how quickly carbon-containing contamination returns. With nonvacuum storage conditions, two hours after being cleaned an overlayer of 0.1 to 0.2nm reappeared on the surface as measured by SE. After a week in closed storage conditions an additional 0.2 to 0.4nm was deposited. Between the time span of a week and 90 days, we saw no further significant adventitious overlayer deposition. XPS indicates that the overcoat is most probably hydrocarbon/organic with significant oxygen content. We observed that (impure) vacuum storage can contaminate samples more than air. We traced instrumental hydrocarbon contamination to our XPS antechamber and show how attaching a commercial low-pressure oxygen radical source removes the bulk of the contamination.

When considering the optical performance of thin films in the Extreme Ultraviolet (EUV), developing an accurate physical description of a thin film coating is necessary to be able to successfully model optical performance. With the short wavelengths of the EUV, film interfaces and sample roughness warrant special attention and care. The surfaces of thin film samples are routinely measured by Atomic Force Microscopy, from which roughness can be determined. However, characterizing the quality of interfaces below the surface is much more challenging. In a recent study of scandium oxide thin films, High Resolution Transmission Electron Microscopy and Annular Dark Field Scanning Transmission Electron Microscopy (ADF STEM) were used to study the cross section of the samples. ADF STEM data analyzed along a path into the volume of the sample (normal to the interfaces) reveals information of sample density versus depth. This density-depth profile reflects the presence of subsurface film interfaces in the volume of the sample. Additionally, information from the ADF STEM profile can be used to gauge the roughness of the subsurface interfaces, which is used to refine the sample description during modeling. We believe this is the first use of ADF STEM in this capacity. This characterization technique may provide key insight to subsurface interface quality, which is particularly important when optimizing the performance of multilayer coatings in the EUV.