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The nanostencil (Fig. 1) is a tool for the fabrication and in-situ
characterization of ultraclean nanostructures for fundamental
surface science research [1,2]. With this nanostencil technique,
the material is evaporated under ultrahigh vacuum (UHV) conditions
through a shadow mask directly onto the sample, thereby avoiding
the use of photoresists, chemical processing steps, and exposure
to air. This allows the fabrication of high-quality structures
that are atomically clean, as thin as a few atomic layers, and
even epitaxial. Thermal stresses and chemical contamination arising
from resist processing are absent. A further advantage of the nanostencil
is the easy variability of the materials, i.e., any material prone
to evaporation in UHV can be employed, without the need to modify
processing steps or masks.
For in-situ characterization by means of scanning probe microscopy
(SPM), the carousel (see Fig. 1) is turned to bring an AFM
cantilever or STM tip in the position of the fabricated structure
(i.e. the former position of the mask).
Different modes of stenciling can be distinguished, namely, static,
multistep, and dynamic.
Static mode
In the static mode, structures down to a few tens of nanometers
can be fabricated because minimal blurring of the structures is
achieved by putting the mask in contact with the sample during
evaporation. Figure 2 shows a non-contact atomic force microscopy
(nc-AFM) measurement of a test pattern (Cu on SiO2) fabricated
in the static mode. 5-nm-thick copper lines with a width of 40 nm
are achieved.
Multistep mode
An important advancement is the multistep mode, in which several static evaporation
steps are carried out sequentially, allowing the fabrication of multi-mask
and multi-material structures. An example is shown in Fig. 3, which displays
a pattern for 4-point electrical transport measurements on a nanowire (Cu on
silicon oxide). The complete pattern is fabricated using three different masks
for (A) contact pads, (B) connection lines, and (C) nanowire, respectively.
Between evaporations, the masks are exchanged and translated using the carousel
and the linear slider stage, respectively.
Dynamic mode
In the dynamic mode the sample is translated with respect to the mask during
evaporation. This provides three major benefits over static stencil
lithography, namely, the possibilities to (i) fabricate arbitrary two-dimensional
geometries (i.e., a wider range of structures), (ii) vary the material thickness
(i.e., three-dimensional structures), and (iii) stencil different structures
using the same mask (flexibility and adaptability). Figure 4 shows a magnetic
nanostructure for the investigation of domain walls in confined geometries
(7 nm Fe on silicon oxide) fabricated in the dynamic mode using the mask shown
in the inset (silicon nitride membrane structured using a focused ion beam).
During evaporation the sample is translated 500 nm in x-direction
at constant speed. The structure shows a height modulation of 10%.
Scanning probe microscopy
With the nanostencil tool the fabricated structures are characterized in situ
using scanning probe microscopy techniques, i.e. STM (scanning tunnelling microscopy),
contact-/ non-contact AFM (atomic force microscopy, see Figs. 2-4) and MFM
(magnetic force microscopy). Figure 5 demonstrates submolecular resolution
for the nanostencil in nc-AFM. Moreover, scanning probe manipulation techniques
will allow further processing of the structures on the atomic/molecular scale.
References
[1] P. Zahl, M. Bammerlin, G. Meyer, and R. R. Schlittler, Rev.
Sci. Instrum. 76, 023707 (2005).
[2] L. Gross, R. R. Schlittler, G. Meyer, A. Vanhaverbeke, and
R. Allenspach, Appl. Phys. Lett. 90, 093121 (2007).
Funding
Swiss National Center of Competence in Research (NCCR) “Nanoscale
Science“ and EU project NaPa.
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"Nanostencil"
