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Thin Solid Films 517 (2008) 1214 – 1218
www.elsevier.com/locate/tsf
Multi-component nanostructure design by atomic shadowing
C.M. Zhou, H.F. Li, D. Gall ⁎
Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY, 12180, USA
Available online 4 June 2008
Abstract
This paper presents a method for multi-component three dimensional nanostructuring during physical vapor deposition, termed simultaneous
opposite glancing angle deposition (SO-GLAD), which exploits atomic shadowing effects to control both the shape and the composition of
nanorods parallel (in-plane) and perpendicular to the rod-axis. Periodic arrays of ~ 200-nm-wide Si–Ta two-component rods are grown by SOGLAD onto monolayers of self-assembled close-packed 260-nm-diameter silica spheres. Simultaneous deposition of Si and Ta from opposite sides
on a stationary substrate yields nanorods consisting of laterally separated Si and Ta components, since atomic shadowing causes deposition only
onto the sphere-sides that are exposed to the respective deposition fluxes. Sequential deposition with a stationary and rotating substrate results in
zigzag two-component nanosprings and vertical multilayer nanorods, respectively. A checkerboard arrangement is achieved by simultaneous
deposition with intermittent 180° substrate rotations.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Atomic shadowing; Glancing angle deposition (GLAD); Nanorods; Silicon; Tantalum
1. Introduction
Multi-component nanostructures such as nanolayered and
coaxial nanowires [1,2] exhibit great potential as opto-electronic
and mechanical nanodevice building blocks, exploiting dissimilar
materials interfaces at the nanoscale. Chemical methods, including
the vapor–liquid–solid growth, have been successfully employed
to synthesize nanolayered and branched heterostructures [1–3],
while physical vapor deposition techniques are used to create nmscale structures along the growth direction perpendicular to the
substrate by subsequent deposition of multiple materials [4,5].
Other approaches yield nanocomposite thin films with nanograins,
rods, or platelets embedded in a homogeneous second phase [6,7].
However, lateral (in-plane) nanostructuring remains a great
challenge despite various successes in nanopatterning research.
Glancing angle deposition (GLAD) [8] exploits atomic selfshadowing effects to create complex 3D nanostructures by
continuously adjusting the polar and azimuthal angles between
the substrate surface normal and the incident deposition flux.
GLAD nanostructures have been engineered into a wide range of
shapes including nanopillars [9–13], zigzags [14,15], nanospirals
⁎ Corresponding author.
E-mail address: [email protected] (D. Gall).
0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2008.05.049
[16–18], nanotubes [19], and branched nanocolumns [20–22],
with potential applications as photonic crystals [16–18], sensors
[15,23], catalyst supports [24,25], magnetic storage media
[26,27], radiation resistant coatings [28], and field emitters
[29,30]. Most reported GLAD nanostructures are built from a
single material. Exceptions include the off-axis co-evaporation or
co-sputtering of Co and Cr [31,32], Fe and Cu [33], Bi and Te
[34], and Pt and metal-oxides [35], as well as recent reports on
multilayer GLAD nanostructures consisting of different materials
that are stacked vertically along the growth direction [36–38].
In this paper, we demonstrate that both lateral and vertical
stacking of components within nanostructures is possible by
simultaneous and sequential physical vapor deposition from two
opposite oblique angles. This technique, referred to as Simultaneous Opposite Glancing Angle Deposition (SO-GLAD) [39],
exploits atomic shadowing effects to cause site-selective
deposition. Regular arrays of Si and Ta two-component
nanostructures are grown by SO-GLAD onto self-assembled
close-packed monolayers of 260-nm-diameter silica spheres. The
nanostructures develop into nanorods with laterally separated
components or zigzag two-component nanosprings during
simultaneous or sequential deposition onto a stationary substrate,
respectively. Sequential deposition with a rotating substrate
results in vertical multilayer nanorods while simultaneous
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C.M. Zhou et al. / Thin Solid Films 517 (2008) 1214–1218
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the magnetrons prevents cross contamination of the targets, and a
collimating plate, 3 mm above the substrate surface, inhibits nondirectional deposition flux from impinging onto the substrate and
also determines the average deposition angle α = 84°. The
patterned Si substrates were attached to a Mo block, introduced
through a loadlock, and placed onto a rotatable sample stage.
Depositions were done in 99.999% pure Ar at 3 mTorr (0.39 Pa) at
a substrate temperature of 40 ± 20 °C. A constant power of 300 W
was applied to both Ta and Si sources, yielding a column growth
rate for simultaneous deposition of ~7 nm/min. The morphologies
and microstructures were investigated by scanning electron
microscopy (SEM, JEOL JSM-6335 Field Emission, operated
at 5 kV with an emission current of 12 μA) and transmission
electron microscopy (TEM, JEOL JEM-2010, operated at
200 kV). TEM samples were prepared by scratching nanorods,
including the attached silica spheres, off the surface onto ethanol
wetted Cu TEM grids with a lacy carbon coating membrane.
3. Results and discussion
Fig. 1. Chamber schematic for simultaneous opposite glancing angle deposition.
deposition with intermittent rotation yields checkerboard nanorods. These results demonstrate the potential of SO-GLAD for
creating uniquely shaped multi-component nanostructures built
from a wide range of materials systems that are joined in a single
deposition process.
2. Experimental
The Si substrates were patterned by self-assembly of 260-nmdiameter silica nanospheres that form close-packed monolayers
during drying of a colloidal suspension on tilted hydrophilic
surfaces in a temperature and humidity controlled environment,
as described in detail in Ref. [40].
Nanostructure arrays were grown onto the nanosphere patterns
in a load-locked ultrahigh vacuum (UHV) stainless steel dc
magnetron sputter deposition system [36]. Fig. 1 shows the
chamber schematic which is designed for SO-GLAD. The 5-cmdiameter Ta (99.95%) and Si (99.95%) targets were positioned at
7.5 cm from the substrate, with the plane of the substrate surface
perpendicularly intercepting both target centers. A shield between
The schematic in Fig. 2(a) illustrates the growth of twocomponent nanorods with laterally separated components by the
SO-GLAD technique: Si and Ta are simultaneously deposited
from oblique deposition angles α. Atomic shadowing causes
selective deposition only on the side of the initial nanosphere or
the growing rod surface that is exposed to the respective
deposition flux. This leads to columnar growth where each
column consists of the two materials which are laterally
separated. All columns are identical with respect to the polar
axis, that is, Si is for all columns on the left and Ta is on the right.
Fig. 2(b) is a typical plan-view SEM micrograph from an array of
well-separated Si–Ta nanorods grown on a patterned substrate
by simultaneous opposite glancing angle sputter deposition. The
nanorods exhibit a regular hexagonal array, replicating the closepacked pattern of the silica nanospheres, which indicates that
each nanosphere acts as a nucleation site for one Si–Ta nanorod.
The rods are 220 ± 40 nm wide and have a measured inter-rod
(center-to-center) separation of 280 ± 20 nm. This is close to the
nominal diameter D = 260 nm of the nanospheres. The array
morphology is rather regular, indicating that column competition
is relatively weak or negligible. This is attributed to the relatively
large 60-nm-wide average gap between neighboring rods, which
Fig. 2. (a) Schematic, (b) plan-view SEM micrograph, and (c) cross-sectional TEM micrograph of a nanorod array grown on silica spheres, where the two rodcomponents (Si and Ta) are laterally stacked.
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C.M. Zhou et al. / Thin Solid Films 517 (2008) 1214–1218
limits long-range inter-rod shadowing interactions. Previous
studies from nanorod arrays grown at high temperatures or with
smaller interspacings have reported considerable size variations
associated with a competitive growth mode that ultimately lead to
column extinction [41,42].
Fig. 2(c) is a cross-sectional transmission electron micrograph from the same array of Si–Ta two-component nanorods as
shown in Fig. 2(b). The rods are 480 nm tall and 225 ± 20 nm
wide, which is in good agreement with the value obtained from
the plan-view SEM image. The orientation of this cross-sectional
sample is along the deposition axis, such that for the orientation of
this micrograph, the Si deposition source is on the left and the Ta
source on the right. The Si and the SiO2 spheres appear bright,
while the Ta is dark, as labeled in the image. This strong
compositional contrast is due to the much stronger electron
scattering in the crystalline high-Z Ta (Z = 73) than in the
amorphous Si (Z = 14) and SiO2. The left half-rods (Si) have a
measured average width of 100 ± 20 nm, while the right sides (Ta)
are 125 ± 16 nm wide. This indicates a slightly larger deposition
rate for Ta which, in turn, also results in a slight tilt of the rods
towards the Ta source, as observed in Fig. 2(c). All rods exhibit
relatively sharp vertical interfaces that separate the Si and Ta
sides. A detailed discussion of theoretical and measured interface
sharpness for such systems is reported in Ref. [39]. During
deposition, Si (Ta) atoms impinge from the left (right) and only
deposit onto the left (right) side of the growing nanorods, which is
exactly the envisioned SO-GLAD process which yields laterally
stacked two-component nanorods, as illustrated in Fig. 2(a).
Fig. 3 illustrates the potential of the SO-GLAD technique for
controllably arranging the components within the nanorods both
vertically and laterally by modifying both the deposition sequence
(sequentially or simultaneously) and the substrate rotation
(stationary, continuous, or intermittent). In addition to the nanorod
arrays with laterally stacked Si–Ta components shown in Fig. 2,
the schematics in Fig. 3(a–c) show vertically stacked rods, twocomponent nanosprings, and checkerboard nanorods, respectively. Nanorods consisting of vertically stacked components, as
illustrated in Fig. 3(a), are obtained by alternate deposition from
different sources with continuous substrate rotation around the
polar axis. The width of each component-layer is controlled by the
particular deposition time and can be varied externally. The
micrograph in Fig. 3(d) shows, as an example, a Si–Ta
nanolayered rod consisting of alternating 100 and 150-nm-thick
Si and Ta layers. The continuous substrate rotation results in an
effectively circular symmetric deposition flux, causing the rods to
grow perpendicular to the substrate surface. The growth front has
an approximately spherical shape, which causes the interfaces
between the stacked layers to also exhibit a curved rather than a
flat morphology.
Zigzag shaped two-component nanosprings, as shown in
Fig. 3(b), are obtained when sequentially utilizing oppositely
positioned deposition sources while keeping the substrate
Fig. 3. Schematics of (a) nanorods with vertically stacked components, (b) two-component zigzag nanosprings, and (c) checkerboard nanorods. (d–f) TEM
micrographs from Si–Ta two-component nanostructures grown on silica spheres, corresponding to the schematics in (a–c), respectively.
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C.M. Zhou et al. / Thin Solid Films 517 (2008) 1214–1218
stationary. GLAD on stationary substrates typically yields
slanted rods that tilt towards the deposition flux with a tilt angle
that increases with α, as studied by various researchers [43,44].
When the deposition source is switched (e.g. from right to left),
the nanorod continues to grow with a tilt in the opposite
direction, yielding ultimately zigzag nanosprings. Fig. 3(e) is a
TEM micrograph of a Si–Ta two-component nanospring. Its
first arm, appearing dark in the micrograph, is a 120-nm-wide
and 360-nm-tall Ta rod, followed by a second brighter 360-nmtall Si arm which forms an approximately right angle with the Ta
rod. The micrograph shows clearly that a noticeable fraction of
the Si deposition flux reaches the Ta rod on the left side rather
than the rounded top, causing the first arm to broaden and
develop into a two-component rod similar to what is observed in
Fig. 2. Correspondingly, the third deposition sequence with Ta
impinging from the right side, leads to the right surface of the Si
arm being covered by a Ta layer which exhibits an increasing
thickness for increasing height. We attribute this deposition on
the side-surfaces to incomplete geometric shadowing which is
most prominent right after each deposition-source switch. The
degree of shadowing is also affected by the orientation of the
pattern with respect to the deposition flux direction. That is, if
the close-packed direction of the hexagonal pattern is
misaligned with the deposition flux, the inter-rod shadowing
is controlled by the 2nd or 3rd nearest neighbors, which yield
less shadowing than the 1st nearest neighbors and allows the
deposition flux to reach the side of the growing nanostructure,
as observed in Fig. 3(e).
Fig. 3(c) is a schematic of a nanorod array where two components are stacked in a checkerboard arrangement. This structure
is obtained by simultaneous deposition from opposite sides, using
a stationary substrate that is intermittently rotated by 180°, such
that, from the perspective of the substrate, the left and right
deposition sources exchange their positions. The Si–Ta checkerboard nanorod shown in Fig. 3(f) was fabricated using four SOGLAD growth sequences between which the substrate was rotated
by 180°. The resulting nanorod consists of ~100 × 100 × 100 nm3
Si and Ta blocks that are stacked both horizontally and vertically.
The interfaces between the Si and Ta sections are relatively abrupt.
They are curved, similarly to those in Fig. 3(d), reflecting the
spherically shaped growth front. In addition, the micrograph also
indicates that a fraction of the deposition flux impinges on the
nanorod sides. For example, the right surface of the first section
(with Ta on the right), is covered with a ~25-nm-wide Si layer that
was deposited during the second growth sequence. In summary,
the micrographs in Fig. 3(d–f) demonstrate the diverse 3D nanostructure shaping that can be achieved by SO-GLAD, which relies
only on geometrical shadowing effects. Therefore, the presented
technique is applicable to a wide range of materials systems and
provides a powerful tool for nanomanufacturing of complex
multi-component structures in a single deposition process.
4. Conclusions
Periodic arrays of Si–Ta nanorods with both laterally and
vertically stacked Si and Ta components were grown by
simultaneous opposite glancing angle deposition (SO-GLAD)
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onto silica nanosphere monolayer patterns. TEM characterizations show that each nanorod consists of Si and Ta components
with relatively sharp horizontal and vertical interfaces. This is
attributed to atomic shadowing which causes site-selective
growth during line-of-sight deposition from the oppositely
positioned sputtering sources. The deposition sequence, as
determined by the substrate rotation and the simultaneous or
sequential material deposition, allows to control the component
arrangement within the nanorods, as demonstrated by the
growth of nanorods with laterally separated components,
straight multilayer nanorods, zigzag shaped two-component
nanosprings, and checkerboard nanorods.
Acknowledgments
This research was supported by the National Science
Foundation, under grant No. CMMI-0727413. We also acknowledge funding from the Donors of the American Chemical
Society Petroleum Research Fund under grant No. 44226-G10.
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