Prakt. Met. Sonderband 46 (2014) 29
size regime, significant as it would occur, for example, during grinding, the use of contact-pressure free
der to enable a reliable techniques such as electrochemical etching or broad beam ion milling are preferable.
measure the stresses
le. To get an idea of the
man hair with a typical 3 2.1 ELECTROCHEMICAL ETCHING
Using electrochemical etching, comparably large amounts of material can be removed with
respect to the actual sample size, which is on the order of tens of nanometers to tens of
micrometers. This technique is by itself not position selective and precise enough to
fabricate near net shape mechanical testing samples. However, etching of the material of
interest allows to remove unwanted material modifications originating, for example, from a
cutting or grinding process. Rods and wires can be shaped to the form of long needles
[19], or thin lamellas can be prepared in a selective way to prepare single grain boundaries
[17, 20]. Subsequent use of the FIB enables to place single or multiple samples onto the
thin freestanding structure. This approach significantly reduces the amount of material to
be removed by FIB milling and limits the possibility of unwanted material re-deposition.
Moreover, these approaches prevent any unintended contact between the testing tip and
any material surrounding the specimen. They provide a free sight onto the sample during
in situ micromechanical testing, e.g. in the scanning electron microscope [21, 22]. The
required preparation steps to FIB machined miniaturized samples starting from
electrochemically etched needles or wedges will be described below.
2.2 BROAD BEAM ION MILLING
e sample, and a 300 nm A limitation of electrochemical processes is their selectivity. Thus, once multiple materials
or phases are of interest, a homogenous material removal can become very challenging or
even impossible. In such situations, broad beam ion milling techniques can be beneficially
ical top-down approach. applied. We use a Hitachi E-3500 ion milling system that operates with an Ar" beam and
small structures, such as an acceleration voltage up to 6 keV and beam currents of ~100 pA. The diameter of the
ic processes [14]. While beam is in the range of several hundreds of micrometers, following a Gaussian profile. The
mples, they are typically sputter yield is lower compared to the FIB, which is counterbalanced by an increased ion
ties and suffer flexibility current (4-7 orders of magnitude) and an increased beam diameter (-5 orders of
aight forward. Therefore, magnitude). This allows to remove hundred thousands of pm? of material in reasonable
) realized with common time. The sputtered areas have a width and depth of several hundreds of micrometers
stching, broad beam ion [23]. The material of interest, which shall not be affected by the milling process, is
protected by a mask that is placed on top of the region of interest using an optical
microscope. The mask shadows the ion beam and fully prevents material removal. The
accuracy of positioning the mask is not better than a few micrometers. As such, the lateral
selectivity of this technique is limited to this accuracy. Nonetheless, it allows to prepare
cross-sections over large areas, the production of free-standing lamellas with a thickness
. of a few micrometers, or even readymade micro-samples when adding structured masks
oar to the process. Thus, broad beam ion milling serves as a universal tool to pre-shape
oe a > | nS TA ; material systems not accessible by electrochemical etching or too large for FIB milling, and
and sputtering angle, as in some cases it allows even to fabricate net shape micro-samples. This is particularly
al milling current. To "ne useful for materials that cannot be processed with the Ga* beam in the FIB (e.g.
“e bottleneck of sample aluminium, polymers, etc.), as these materials would be altered and even damaged. In the
mount of material to be following, we will describe the above mentioned applications in more detail.
: during material thinning,
