Biomedical Engineering Reference
In-Depth Information
For XPS the material to be examined is irradiated with aluminum or magnesium X-rays.
Monochromatic aluminum K
α
X-rays are normally produced by diffracting and focusing a beam of
non-monochromatic X-rays off of a thin disk of crystalline quartz. Such X-rays have a wavelength
of 8.3386 Å, corresponding photon energy of 1486.7 eV, and provide a typical energy resolution of
0.25 eV. Non-monochromatic magnesium X-rays have a wavelength of 9.89 Å, corresponding pho-
ton energy of 1253 eV, and a typical energy resolution of 0.90 eV. The kinetic energy of the emitted
electrons is recorded. This kinetic energy of the ejected electrons is directly related to the element-
specific atomic binding energy of the liberated. A plot of these energies against the corresponding
number of electron counts provides the spectrum which indicates the qualitative and quantitative ele-
mental composition. At these higher energies, XPS only analyzes to a depth of 10 nm into the surface.
Electrons emitted at greater depths are recaptured or trapped in various excited states within the mate-
rial. Spectral profiles as up to 1 μm deep can however be obtained by continuous spectral recording
during ion etching or from consecutive ion etching and XPS measurement steps.
XPS is usually performed in UHV and typically provides resolutions down to 1000 ppm. With
optimum settings and long recording times, resolutions down to 100 ppm can be achieved. Non-
monochromatic X-ray sources can produce a significant amount of heat (up to 200°C) on the surface
of the sample as the anode producing the X-rays is typically only a few from the sample. This level
of heat when combined with high energy Bremsstrahlung X-rays can degrade the surface. Organic
chemicals are therefore not routinely analyzed by non-monochromatic X-ray sources.
18.3.2.1 XPS Case Study
The effect of aluminum addition to titanium nitride (TiN) matrix on the structural, mechanical, and
corrosion resistance properties of titanium-aluminum-nitride was studied
[61]
. Si wafer, AISI 316L
stainless steel, and low carbon steel substrates were coated with Ti
1
x
Al
x
N (where
x
was 0, 0.5, and 1)
films by a direct current magnetron sputtering process. Layers were sputtered in pure Argon with a sub-
strate temperature at 400°C, power of 250 W, and a sputtering time of 120 min. The XPS survey spec-
tra of the normally un-etched surface of the Ti
0.5
Al
0.5
N film on steel exhibited the characteristic Ti2p,
Al2p, N1s, and O1s peaks at the corresponding binding energies of 454.5, 74.3, 399.6, and 532.1 eV,
respectively. From high resolution XPS measurements of the normal surface of the films, the spin orbit
doublet Ti2p1/2 and Ti2p3/2 peaks at binding energies 463.1 and 460.3 eV, respectively, were found
in the Ti spectra. The Ti2p3/2 peaks included three components whose peaks centered at 460.3 (I),
456.9 (II), and 454.5 eV (III). These components were associated with TiO
2
, TiO
x
N
y
, and TiN phases,
respectively. The chemical bonding in the coating could be understood further by analyzing N1s pho-
toelectron spectra. The N1s spectrum of the unsputtered specimen exhibited a bump at the higher bind-
ing energy side. The spectrum were deconvoluted into the components using curve fitting; one part
was associated with the NeO bonding state at 396.65 eV and the other with the NeN bond at 399.6 eV,
which indicated some adsorbed nitrogen in the surface of the film. Based on these analytical results, it
could be concluded that the TiAlN films were composed of the AlN and TiN phases mainly, on which a
thin TiAlON surface layer was formed upon exposure of the films to the air after deposition.
18.3.3
Secondary Ion Mass Spectroscopy
Secondary ion mass spectroscopy (SIMS) is a destructive analytical technique in which material is
removed from a surface by ion beam sputtering, and the resultant positive and negative ions are mass
