Biomedical Engineering Reference
In-Depth Information
Fig. 5.8
Yield of
pulverization as a function of
acceleration voltage
E kV
there is greater resistance. The flow of incident ions decreases with the cosine of the
angle incidence.
The penetration depth of the ions in the material is proportional to the acceler-
ation energy of normal incidence and inversely proportionate to their mass. With
argon ions, it is 10 nm under 6 keV with an incidence angle of 70
◦
, but falls to 1 nm
in low-angle incidence.
The process generates many artifacts as follows:
-
The implantation of ions in the depth of the thin slice, the creation of a thick layer
(some tens of nanometers) of material rendered amorphous (destruction of the
crystalline network) on sample surfaces.
-
A strong sample temperature increase (depending on ion milling parameters and
whether or not the sample can be cooled), which may induce a change in the
stoichiometry, phase transformation, demixing, or decomposition of the material.
-
Possible differential scouring, causing differential component sputter rates
(roughness), resulting from the difference in hardness and atomic weight between
these components, differences in crystallographic orientations of the material, and
the incidence angle of the ion beam (Fig.
5.9)
.
The use of a low voltage, low current, and minimal incidence angle can reduce
damage; however, this drastically increases the thin slice preparation time.
This type of abrasion is used to polish any type of hard and soft, single-phase or
multiphase materials.
4.2 Techniques Involving Ion Abrasion
4.2.1 Ion Beam Thinning and Focused Ion Beam Thinning (FIB)
Ion beam thinning
is performed using equipment under a vacuum on the order of
10
-4
-10
-5
Pa.
Classical ion guns are composed of an ionization chamber and acceleration elec-
trodes (0-15 keV). Focusing the beam enables a precise area of the sample to be
thinned, on the order of a millimeter. Often, sample cratering is caused until a central
hole is formed.
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