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Eliminating carbon contamination on oxidized Si surfaces using a VUV excimer lamp

BYU Authors: Elisabeth (Liz) Strein and David Allred, published in Thin Solid Films

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.

Eliminating adventitious carbon contamination on SiO2 using a UV excimer lamp

BYU Authors: E. Strein, D. Allred, and R. S. Turley, published in Society of Vacuum Coaters, 2008 Technical Conference, (April 2008, Chicago, IL), 448-442 (2008).

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.

Nanoscale Characterization of Thin Film Coatings Using Annular Dark Field Scanning Transmission Electron Microscopy

BYU Authors: Guillermo Acosta, Richard Vanfleet, David D. Allred, and R. S. Turley, published in Society of Vacuum Coaters, 2008 Technical Conference, (April 2008, Chicago, IL), 448-442 (2008).

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.

Extreme-ultraviolet polarimeter utilizing laser-generated high-order harmonics

BYU Authors: Nicole Brimhall, Matthew Turner, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross, published in Rev. Sci. Instrum.

We describe an extreme-ultraviolet (EUV) polarimeter that employs laser-generated high-order harmonics as the light source. The polarimeter is designed to characterize materials and thin films for use with EUV light. Laser high harmonics are highly directional with easily rotatable linear polarization, not typically available with other EUV sources. The harmonics have good wavelength coverage, potentially spanning the entire EUV from a few to a hundred nanometers. Our instrument is configured to measure reflectances from 14 to 30 nm and has similar to 180 spectral resolution (lambda/Delta lambda). The reflection from a sample surface can be measured over a continuous range of incident angles (5 degrees-75 degrees). A secondary 14 cm gas cell attenuates the harmonics in a controlled way to keep signals within the linear dynamic range of the detector, comprised of a microchannel plate coupled to a phosphorous screen and charge coupled device camera. The harmonics are produced using similar to 10 mJ, similar to 35 fs, and similar to 800 nm laser pulses with a repetition rate of 10 Hz. Per-shot energy monitoring of the laser discriminates against fluctuations. The polarimeter reflectance data agree well with data obtained at the Advanced Light Source Synchrotron (Beamline 6.3.2).

Determining the refractive index in the extreme ultraviolet using Kramers-Kronig on thin-film scandium oxide transmission data

BYU Authors: Jacqualine Jackson Butterfield and David D. Allred, published in J. Utah Acad. Sci.

Whereas the real part of the refractive index is dependent on both transmittance and reflectance, the imaginary part can be determined from transmittance data alone. It is possible to use Kramers-Kronig analysis to calculate the real part if the imaginary part is known over a sufficiently broad range. We show that the delta calculated from reflection and transmission data without taking into account roughness may underestimate the real part of the refractive index of the scandium oxide samples we are studying by up to 40% near 270 eV.

Thin-film iron-catalyzed "beads on a string" carbon nanotubes

BYU Authors: Jonathan Goodsell, Jon Brame, and David D. Allred, published in J. Utah Acad. Sci.

We report on the iron-catalyzed, CVD growth of carbon nanotubes on a variety of substrates and their subsequent analysis. We discuss the use of “indirect evaporation” to prepare the required iron catalyst. The optimum temperature for growth of nanotubes mats was between 925 and 950 degrees C. These were shown by transmission electron microscopy to contain single-walled carbon nanotubes (SWCNTs). We examined the results of the deposition in scanning electron microscopy and saw what appeared to be bright spots randomly arrayed along the carbon nanotube fibers. We termed this pattern ‘beads on a string’. We present evidence that the bright features are not associated with the nanotubes directly but are iron/carbon clusters that happen to lie on the surface of the substrate near where the tubes fell during or after deposition. We have shown that the deposition procedure is fairly robust and growth results have been reproduced using Goddard Space Flight Center (GSFC)-deposited catalyst and CVD facilities at Brigham Young University (BYU).

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