Nanolayer Reactions in Aluminum-Metal Interfaces
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NANOLAYER REACTIONS IN ALUMINUM-METAL INTERFACES E. V. BARRERA, M. W. RUCKMAN,
AND S. M. HEALD
Division of Materials Science, Department National Laboratory, Upton, NY 11973
of Applied
Science,
Brookhaven
ABSTRACT Surface extended x-ray absorption fine structure (SEXAFS)
measurements
on the nanometer level were made for AI/M interfaces where M was Cu or Ni. The samples were studied immediately after deposition and after heat treatments. Significant differences in interface reactions were observed depending on deposition direction (Cu on Al or Al on Cu) and the amount of mixing was also related to whether M was Cu or Ni. The SEXAFS measurements revealed that there were no detectable amounts of C or 0 present. One percent Zn was observed to be in the Al layers. The results obtained from the as-deposited interfaces correlate well with data obtained from buried interfaces of like element combinations.
INTRODUCTION This research gives preliminary results from studies of Al-Cu and Al-Ni interfaces which demonstrates the usefulness of surface extended x-ray absorption fine structure (SEXAFS) to study interface reactions. Previous research has shown that burie4 interfaces of Al on Cu deposited at room temperature react to form 30 A of predominately CuA1 2 . Further reaction occurred after heating to 12(f C for 5 min., as observed by Heald et al. [1-31 using glancing angle EXAFS and x-ray reflectivity. Research, using x-ray photoemission, showed that for Al-Cu and Al-Ni, the interfaces reacted at room temperature whether the films were Al/metal or metal/Al [4,51. The before mentioned results are for short-range reactions where the sensitivity of the combined EXAFS and x-ray reflectivity technique is 1i0 A [2] and the of the x-ray photoemission studies have monolayer sensitivity on surfaces. To investigate the Al-metal reaction at a buried interface, where the probe sensitivity is as sensitive as the photoemission, surface extended x-ray absorption fine structure (SEXAFS) is used. SEXAFS is a technique which generally uses total or partial electron yield detection methods to monitor the x-ray absorption [6]. Simply put, an atom excited by x-rays with energy greater than its absorption edge, releases a photoelectron which undergoes scattering by the surrounding atoms. The modification of the final state of the photoelectron produces oscillations in the extended absorption spectra with increasing energy. This fine structure provides short-range atomic structure information from the vantage point of the excited atoms. Since SEXAFS is normally conducted in an ultrahigh vacuum system, detection can be made of partial monolayers on suifaces and shallow burtedinterfaces. Approximate probing depths are -500 A for total yield and -30 A for partial yield.
EXPERIMENTAL PROCEDURE Thin polycrystalline films of Al-metal were deposited on a thin tantalum foil at room temperature. The samples were either metal on Al bilayers or Al-metal-Al trila;ers where the metal was either Cu or Ni. The Al thicknesses were 120-240 A. Thicknesse
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