EELS Measurement of the Local Electronic Structure of Copper Atoms Segregated to Aluminum Grain Boundaries

Author(s):  
S.J. Splinter ◽  
J. Bruley ◽  
P.E. Batson ◽  
D.A. Smith ◽  
R. Rosenberg

It has long been known that the addition of Cu to Al interconnects improves the resistance to electromigration failure. It is generally accepted that this improvement is the result of Cu segregation to Al grain boundaries. The exact mechanism by which segregated Cu increases service lifetime is not understood, although it has been suggested that the formation of thin layers of θ-CuA12 (or some metastable substoichiometric precursor, θ’ or θ”) at the boundaries may be necessary. This paper reports measurements of the local electronic structure of Cu atoms segregated to Al grain boundaries using spatially resolved EELS in a UHV STEM. It is shown that segregated Cu exists in a chemical environment similar to that of Cu atoms in bulk θ-phase precipitates.Films of 100 nm thickness and nominal composition Al-2.5wt%Cu were deposited by sputtering from alloy targets onto NaCl substrates. The samples were solution heat treated at 748K for 30 min and aged at 523K for 4 h to promote equilibrium grain boundary segregation. EELS measurements were made using a Gatan 666 PEELS spectrometer interfaced to a VG HB501 STEM operating at 100 keV. The probe size was estimated to be 1 nm FWHM. Grain boundaries with the narrowest projected width were chosen for analysis. EDX measurements of Cu segregation were made using a VG HB603 STEM.

1998 ◽  
Vol 13 (9) ◽  
pp. 2389-2395 ◽  
Author(s):  
D. L. Carroll ◽  
P. M. Ajayan ◽  
S. Curran

The recent application of tunneling probes in electronic structure studies of carbon nanotubes has proven both powerful and challenging. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), local electronic properties in ordered aggregates of carbon nanotubes (multiwalled nanotubes and ropes of single walled nanotubes) have been probed. In this report, we present evidence for interlayer (concentric tube) interactions in multiwalled tubes and tube-tube interactions in singlewalled nanotube ropes. The spatially resolved, local electronic structure, as determined by the local density of electronic states, is shown to clearly reflect tube-tube interactions in both of these aggregate forms.


1997 ◽  
Vol 482 ◽  
Author(s):  
M. K. H. Natusch ◽  
G. A. Botton ◽  
R. F. Broom ◽  
P. D. Brown ◽  
D. M. Tricker ◽  
...  

AbstractThe optical properties and their modification by crystal defects of wurtzite GaN are investigated using spatially resolved electron energy-loss spectroscopy (EELS) in a dedicated ultra-high vacuum field emission gun scanning transmission electron microscope. The calculated density of states of the bulk crystal reproduces well the features of the measured spectra. The profound effect of a prismatic stacking fault on the local electronic structure is shown by the spatial variation of the optical properties derived from low-loss spectra. It is found that a defect state at the fault appears to bind 1.5 electrons per atom.


1993 ◽  
Vol 319 ◽  
Author(s):  
D.A. Muller ◽  
P.E. Batson ◽  
S. Subramanian ◽  
S. L. Sass ◽  
J. Silcox

AbstractWe have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24 using electron energy loss spectroscopy (EELS), x-ray fluorescence (EDX) and annular dark field (ADF) imaging in a UHV STEM. A detailed study of a high angle grain boundary in nickel rich Ni3Al doped with 1000 ppm boron shows nickel enrichment occurring in a 5Å wide region. Boron segregation to the boundary is observed with EELS and is seen to vary along the boundary, coinciding with ADF contrast changes in the surrounding grains that may be due to local strain fields. Spatially resolved EELS of the Ni L2,3 core edge, which is sensitive to changes in the hole density in the nickel d band, shows boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. Boundary regions without boron have an electronic structure similar to that of the undoped grain boundaries where the Fermi level lies deeper in the nickel d band. In addition to studying boron segregation, EELS provides a unique opportunity to examine the changes in bonding that control the local properties of the material.


Author(s):  
Roger H. French

The spatial variation of the electronic structure at interfaces is critical to both interatomic bonding at atomically abrupt interfaces such as grain boundaries and also to the development of van der Waals (vdW) attraction forces at partially wetted interfaces. This interfacial electronic structure, as represented by the interband transition strength , can be determined by Kramers Kronig (KK) analysis of either vacuum ultraviolet (VUV) optical reflectance spectra or spatially resolved valence electron energy loss (SR-VEEL) spectra. Quantitative analysis of SR-VEELS requires accurate spectral line shapes coupled with single scattering deconvolution, convergence correction, and KK analysis. Both the energy loss functions (Fig. 1) and the interband transitions (Fig. 2) determined for α-Al2O3 using SR-VEELS compare well with the VUV results. In addition the use of the spectral line scan method, whereby typically 200 SR-VEEL spectra are acquired along a scan line of 20 nm, helps overcome many uncertainties in the data acquisition and analysis.


1994 ◽  
Vol 364 ◽  
Author(s):  
D. A. Muller ◽  
S. Subramanian ◽  
S. L. Sass ◽  
J. Silcox ◽  
P. E. Batson

AbstractOne of the fundamental questions concerning Ni3Al is why doping with boron improves the room temperature ductility of the polycrystalline material. Boron is thought to prevent environmental embrittlement and increase the cohesive strength of grain boundaries since it changes the fracture mode from intergranular to transgranular. This change in cohesive energy must be reflected in the bonding changes at the grain boundary which can be probed using spatially resolved electron energy loss spectroscopy (EELS). We have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24 using EELS, EDX and ADF imaging in a UHV STEM. Ni-enrichment is seen in a 0.5–1 nm wide region at large angle grain boundaries, both in the absence and presence of B. EELS shows that B segregation can vary along the interface. The Ni L2, 3 core edge fine structure which is sensitive to the filling of the Ni d-band, shows only the boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. These results demonstrate that boron segregation increases the cohesive energy and hence improves the fracture resistance of the grain boundary, by making the bonding at boundaries similar to that in the bulk. The measured changes in d band filling may also affect the local solubility of hydrogen.


1990 ◽  
Vol 182 ◽  
Author(s):  
S. Pizzini ◽  
M. Acciarri

AbstractThe conductivity of single grain boundaries of electronic grade, Bridgman grown polycrystalline silicon samples was measured using the dc polarization technique with the aim of detecting any influence on the carrier transport regime resulting from the segregation of oxygen and carbon, which, by themselves, should behave as electrically inactive impurities. To this scope as grown samples and samples heat treated at 1123 and 1223 K, differing in their initial oxygen and carbon content, were used and the conductivity measured in the 298-100 K range.The results indicate that for both the carbon rich and for the oxygen rich samples the conductivity across the grain boundaries is of the thermally activated type and that it could be discussed in terms of carriers which are thermally emitted over the barrier and cross the barrier in the limit of the short mean free path.Samples which do not present a net excess of oxygen or carbon, apparently behave as single crystal specimens, instead.The deconvolution of the I-V curves is then used for obtaining the densitl of the interface states responsible of the set-up of the grain boundaries barrier, whose shape supports a completely new hypothesis about the configuration of the impurity cloud at the grain boundaries.


2001 ◽  
Vol 7 (S2) ◽  
pp. 194-195
Author(s):  
David A. Muller

There is an intimate connection between the electronic structure of a material and its physical properties. to change one, is to change the other. Some of the most striking illustrations of this relationship can be found at grain boundaries in metals and their alloys. Here, the most important changes in cohesion can be described by changes in the local density of states (LDOS), which in turn can be measured using EELS [1]. The first demonstration that EELS could be used to connect the electronic and mechanical properties of a material was in revealing the role that boron has in restoring a bulk-like bonding to grain boundaries in Ni3Al [2,3]. Boron was known to change the fracture mode in Ni3Al from intergranular to transgranular, possibly by enhancing grain boundary cohesion.What interested me in this project, when John Silcox first suggested it as a thesis topic, was the potential of using the EELS fine structure to measure materials properties directly.


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