Focused Ion Beam Processing at the Nano-Scale
Figure 2: (A) Schematic of the membrane FIB sculpting process with back and forward sputtering. (B) Batch patterning of a complete wafer of individual nano-pores.
(C) Picture of a two-inch wafer containing 284 identical devices patterned in a one-shot FIB experiment.
(5-nm calculated resolution [2]), and, owing to the strongly
positioning accuracy in reading (using the Scanning Ion
collimated mode, an efficient rejection of off-axis emitted ions
Microscopy imaging mode) or defining alignment marks and
is achieved, leading to minimum probe current tails. In practice
the possibility of achieving extended automated patterning
our system is capable of delivering a deep sub-10-nm ion probe,
tasks (Figure 2) without operator control are key advantages.
transporting a probe current of several picoamperes within a
FIB Patterning at the Nano-Scale: The Resolution
probe current distribution that follows a pure Gaussian or “bell
Limiting Factors
type” distribution.
The FIB nano-writer architecture. The FIB system
Prior to considering FIB nano-fabrication of structures on
we have developed (Figure 1) is based on a nano-writer
a substrate [2], it is important to keep in mind that this process
architecture concept, that is, a single FIB column positioned
encounters several kinds of limitations that are independent of
over a high-accuracy sample stage and operated with short
the ion optics itself. They can be summarized as follows:
working distances (WD) to achieve a strong geometrical source
Sample characteristics. As FIB machining is a direct
demagnification. In our “single beam” configuration we have
writing process, a first limitation in fabrication of features
“sacrificed” the in-situ Scanning Electron Microscopy (SEM)
originates from the physical characteristics of the target
imaging capability for the following reasons: (a) Many of the
(composition, hardness, electrical conductivity) and its
nm-sized structures we aim at fabricating (surface defects with
geometrical features (surface roughness, homogeneity). Indeed
no SE contrast, sub-surface ion beam mixing, etc.) are difficult
target characteristics have a huge effect both on etching speeds
or even impossible to monitor using a SEM; (b) in-situ electron
and the resulting structure geometry.
bombardment and the resulting energy deposition may be a
Spatial extension of the defects induced by FIB
source for perturbation when attempting to pattern nanometer-
irradiation. These effects may originate primarily from a lack
scaled devices (enhanced defect diffusion in sensitive III-V
of selectivity of the FIB probe. Ultimately they are caused by
semiconductors) or for material reconstruction through surface
the scattering of the implanted ions inside the target material
diffusion (structure clogging); (c) because our FIB optimum
and by the radiation enhanced diffusion (RED) effect, taking
WD is only a few mm, in-situ SEM imaging capability would be
place only during the ion bombardment process.
too limited.
Redeposition of sputtered materials. Scanning an
Thus, our experimental approach does not require an
energetic ion beam over a substrate allows etching patterns of
in-situ SEM imaging capability. In our case, the extreme
arbitrary shape as a result of physical sputtering. This sputter
Figure 3: (A) Scanning electron microscopy image of a nano-wire sculpted within a SiC thin membrane. Minimum width is less than 7 nm. (B) SEM image of a nano-wire
sculpted in a 2-3 nm thick graphene sheet. Note that the nano-wire seems to roll on.
16
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