98
D. W. Eastman et al.
polishing papers ranging from 600 to 1200 grit. This process resulted in precise
foil thicknesses, removal of surface defects, and a near-mirror finish, which also
made it easier to employ digital image correlation (DIC). Once a foil of the correct
thickness was achieved, microtensile samples were machined from it using one of
the three techniques. For samples with thicknesses and widths of about 20 μm, FIB
machining was employed as the final machining tool, whereas wire EDM was used
for samples with thicknesses and widths of 200 μm or larger, and femtosecond laser
machining was employed for intermediate samples with thicknesses and widths of
50–100 μm.
3.1 Focused Ion Beam Machining
The focused ion beam (FIB) has become a widely used tool in microscale sample
fabrication due to its precise milling capabilities. FIB milling occurs by sputtering,
accelerating, and focusing charged ions, typically Ga. The interaction of these
ions with atoms in the sample results in ablation at a relatively slow rate, but the
nature of this process allows for delicate and precise machining and procedures,
such as the extraction of TEM samples using FIB lift out. One of the first major
applications of FIB machining for microscale test samples was its use in milling
micropillars for compression testing [58]. This methodology, which has inspired
many subsequent microscale machining efforts, showed the viability of the FIB as a
microscale machining tool and opened the opportunity for the precise machining of
samples at the single digit micron scale. FIB-induced damage of microscale samples
associated with Ga implantation and amorphization has been reported and shown to
vary with material. While this effect is commonly observed, especially in lower
atomic mass metals, it has been suggested that it only plays a major role at the
outer layer of the sample and with careful control of the beam size and current can
be limited to 100 nm or less in most materials [59]. While this level of damage
will not play a significant role in microtensile samples with dimensions that are
orders of magnitude greater than this, caution is still warranted when FIB milling
microsamples.
High throughput use of an FIB as a machining technique for complex geometries
and three-dimensional samples, such as micropillars, relies on the ability to
automate the machining process. One way in which this can be done is through the
use of a fiducial marker and image processing to realign the sample for machining
between each cut. For micropillars, a lathe milling process can be used as the sample
is rotated around the fiducial marker. A similar process, which will be discussed later
in this section, can be used for machining more complex microtensile geometries
even though most of the milling is done in a two-dimensional plane [23].
Two of the main parameters that affect the quality of an FIB cut are the beam
current, which affects the material removal rate, and the beam focus, which affects
the shape of the beam. For bulk milling, a higher beam current is used to increase the
rate of material removal, and therefore the focus and shape of the beam become less
D. W. Eastman et al.
polishing papers ranging from 600 to 1200 grit. This process resulted in precise
foil thicknesses, removal of surface defects, and a near-mirror finish, which also
made it easier to employ digital image correlation (DIC). Once a foil of the correct
thickness was achieved, microtensile samples were machined from it using one of
the three techniques. For samples with thicknesses and widths of about 20 μm, FIB
machining was employed as the final machining tool, whereas wire EDM was used
for samples with thicknesses and widths of 200 μm or larger, and femtosecond laser
machining was employed for intermediate samples with thicknesses and widths of
50–100 μm.
3.1 Focused Ion Beam Machining
The focused ion beam (FIB) has become a widely used tool in microscale sample
fabrication due to its precise milling capabilities. FIB milling occurs by sputtering,
accelerating, and focusing charged ions, typically Ga. The interaction of these
ions with atoms in the sample results in ablation at a relatively slow rate, but the
nature of this process allows for delicate and precise machining and procedures,
such as the extraction of TEM samples using FIB lift out. One of the first major
applications of FIB machining for microscale test samples was its use in milling
micropillars for compression testing [58]. This methodology, which has inspired
many subsequent microscale machining efforts, showed the viability of the FIB as a
microscale machining tool and opened the opportunity for the precise machining of
samples at the single digit micron scale. FIB-induced damage of microscale samples
associated with Ga implantation and amorphization has been reported and shown to
vary with material. While this effect is commonly observed, especially in lower
atomic mass metals, it has been suggested that it only plays a major role at the
outer layer of the sample and with careful control of the beam size and current can
be limited to 100 nm or less in most materials [59]. While this level of damage
will not play a significant role in microtensile samples with dimensions that are
orders of magnitude greater than this, caution is still warranted when FIB milling
microsamples.
High throughput use of an FIB as a machining technique for complex geometries
and three-dimensional samples, such as micropillars, relies on the ability to
automate the machining process. One way in which this can be done is through the
use of a fiducial marker and image processing to realign the sample for machining
between each cut. For micropillars, a lathe milling process can be used as the sample
is rotated around the fiducial marker. A similar process, which will be discussed later
in this section, can be used for machining more complex microtensile geometries
even though most of the milling is done in a two-dimensional plane [23].
Two of the main parameters that affect the quality of an FIB cut are the beam
current, which affects the material removal rate, and the beam focus, which affects
the shape of the beam. For bulk milling, a higher beam current is used to increase the
rate of material removal, and therefore the focus and shape of the beam become less