Biomedical Engineering Reference
In-Depth Information
analyzed in a mass spectrometer
[62]
. The technique is element specific and is capable of detecting
all elements as well as isotopes and molecular species. Of all the beam techniques, it is the most sen-
sitive with detection limits for some elements in the 10
14
-10
15
cm
3
range if there is very little back-
ground interference signal. Lateral resolution is typically 100 μm but can be as small as 0.5 μm with
depth resolution of 5-10 nm
[34]
.
SIMS works by removing material from a sample by sputtering using an ion beam. The mass/
charge ratio of the removed ions is analyzed, detected as a mass spectrum, as a count, or displayed
on a fluorescent screen
[34]
.
Static
SIMS employs very low primary ion density of around 1 nA cm
2
and low primary ion energy (0.5-2 keV) so that a nearly undisturbed monolayer of the surface can be
analyzed.
Dynamic
SIMS involves primary ion currents greater than 1 μA cm
2
and usually more than
one monolayer is removed during the analysis
[46]
. Dynamic SIMS can produce depth profiles, and
quantitative depth profiling is unquestionably the major strength of SIMS. SIMS lends itself to inves-
tigations of grain boundary diffusion or diffusion across interfaces. It is a powerful tool for studying
the transport processes in ceramics in the temperature range where diffusion distances are too small
to be analyzed by serial mechanical sectioning.
18.3.3.1 SIMS Case Studies
In the work of Gao et al.
[63]
, the effect of CO
2
laser and fluoride treatments to inhibit root caries was
investigated. Time of Flight-SIMS (ToFSIMS® IV, ION-TOF GmbH, Münster, Germany) was carried
out for elemental analysis on the cut surfaces of the treated sections. A significant laser-enhanced fluo-
ride uptake, characterized by the ToF-SIMS analysis was revealed. Regions of 50
50 μm were ana-
lyzed beneath the root surface and revealed 37% and 400% increments in loosely and firmly bound
fluorides. In other work by Balter and Reynard
[64]
, the use of SIMS to aid the study the distribution
of Sr at low concentration in bones was shown. This work showed that Sr administration at low dose
reduces bone resorption and increases bone formation, leading to an increase of the bone mass.
18.3.4
Auger Electron Spectroscopy
When an electron or ion is incident on a semiconductor, it may transfer enough energy to an inner-
shell electron to eject it from its parent atom. The atom is in an excited state, and, to lower its energy,
an electron from a less tightly bound shell may fill the hole while simultaneously emitting a third
electron from the atom. This ejected atom is known as an Auger electron
[46]
. Its energy is related
specifically to the electron energy levels involved in the process and, therefore, is characteristic of the
atom concerned. Since the Auger process is a three-electron process, neither hydrogen nor helium can
be detected since both have less than three electrons. AES has two distinct advantages over energy
dispersive X-ray spectroscopy (EDX) analysis.
It is a far more surface-sensitive technique. Escape depths range from less than a nanometer to
a few nanometers. In EDX it can be difficult to analyze small particles on a substrate, because the
electron beam passes through the particles and spreads out in the substrate below it
[62]
. There is the
potential for chemical-state information in Auger spectroscopy, e.g., the oxidation state of silicon at a
Si-SiO
2
interface may be ascertained
[46]
. EDX lacks the same chemical-state information capability.
AES has found applications in measuring semiconductor composition, oxide film composition,
silicides, metallization, particle analysis, and the effects of surface cleaning. AES measurements
are made in a high vacuum environment (10
12
-10
10
torr) to retard the formation of hydrocarbon
