by Professor Chang-Hwan Choi
As scientific quests and engineering applications reach down to a nanometer
scale, there is a strong need to fabricate three-dimensional (3-D) nanostructures
with regularity and controllability in their pattern, size, and shape. Recently,
a simple and efficient 3-D nanofabrication method that couples the Bosch deep
reactive ion etching (DRIE) process with laser interference lithography has
been reported to create a dense-array (nanoscale pitch) of silicon nanostructures
of varying height and shape over a large sample area with excellent regularity
By regulating etching parameters, the nanoscopic scalloping problem typical
in the Bosch DRIE process was not only controllable but also capable of realizing
sophisticated 3-D sidewall profiles and tip sharpness. These well-defined large-area
nanostructures over a large area with controllable sidewall and tip shapes opened
new application possibilities in areas beyond nanoelectronics, such as microfluidics3,4
and biomaterials5,6. In this article,
we review the new nanofabrication process of using the Bosch DRIE process for
the simple high-aspect-ratio 3-D nanopatterning and its potential applications/benefits.
In silicon-based MEMS (micro-electromechanical systems) fabrication, the Bosch
DRIE has been commonly used to etch microscale deep trenches with vertical sidewalls
due to its high etch selectivity for silicon over various mask materials such
as photoresist, silicon oxide, and silicon nitride layers (e.g., greater than
100:1). However, the Bosch DRIE process has rarely been used to construct nanostructures
because the well-known effect of sidewall rippling, or so-called 'scalloping',
is intolerably prominent on the nanoscale (Fig. 1).
Schematic of cyclic Bosch DRIE process. (a) Opening of etch mask layer
for Bosch DRIE. (b) Isotropic SF6 etch of silicon substrate with anisotropic
bombardment. (c) Isotropic polymer formation with C4F8. (d) SF6 etch
and polymer deposition is repeated for deep trenches. Scallops, whose
peak-to-valley height is over 50 nm in typical DRIE, appear on the walls
due to the isotropic nature of the etch. The nanoscale scalloping effect
can be controlled and utilized for sidewall profile and tip sharpness
control for 3-D nanostructure fabrication by regulating the etching
parameters such as pressure, RF power, gas mixture, and the relative
duration of etching time (step b) versus deposition time (step c).
Recent reports1,2 show that the
nanoscale scalloping effect can be modulated by regulating the etching parameters
and adapted to realize high-aspect-ratio 3-D nanostructures with well-defined
sidewall profiles and tip sharpness. Although several parameters in the Bosch
DRIE, such as pressure, RF power, and gas mixture influence the sidewall profile,
it was determined that the relative duration of etching time (step b in Fig.
1) versus deposition time (step c in Fig. 1) in the cyclic Bosch process was
the most convenient parameter to control the structural three-dimensionality
with good reproducibility in association with the total number of etch cycles.
A similar approach can also be applied for the 3-D nanopatterning of metals
in the anisotropic reactive ion etching technique by exploiting the cyclic etching
and passivation (e.g., oxidation) steps.
In most applications, the nanostructures are not useful unless they cover a
relatively large area and the manufacturing cost is kept within an acceptable
range. While numerous nanopatterning techniques have been explored, most involve
a serial method such as e-beam or scanning probe lithography, covering only
a small area (typically less than 1 mm2).
Parallel X-ray lithography can pattern a large area, but it is too expensive
for most applications. Soft lithography-based fabrication methods, such as nanoimprinting,
replicate patterns in a parallel fashion but need a master mold first manufactured
by e-beam or X-ray lithography. Most non-lithographic methods, such as the use
of nanotemplates of self-assembled nanomaterials or the direct deposition/growth
of nanostructures by chemical methods, lack regularity over a large area.
Currently, interference (or holographic) lithography is considered the most
efficient way to make submicron-scale periodic patterns over a large area with
superior control of pattern regularity. It uses simple and relatively inexpensive
optics to generate uniform interference patterns such as lines and dots on a
substrate without any photomask. In this review, the 3-D nanofabrication results
of the Bosch DRIE process are presented utilizing the photoresist nanopatterns
created by the interference lithography as the etch mask to demonstrate the
large-area 3-D nanopatterning and nanofabrication scheme1,2.
Figure 2 shows an example of high-aspect-ratio 3-D nano-post structures of
varying sidewall profiles and tip sharpness. Regular silicon nanostructures
with less than 10% deviation in size and shape can be obtained over a 4-inch
substrate by using the laser interference lithography followed by the Bosch
The Bosch DRIE process allows the creation of high-aspect-ratio (e.g., greater
than 10) nanostructures with a thin (e.g., ~50 nm thick) photoresist mask layer,
suggesting that this new approach makes the process of regular 3-D nanostructure
fabrication over a large coverage area simple and practical, even for high-aspect-ratio
Figure 2a shows sidewall profiles programmed to be re-entrant. The degree
of the re-entrance was controlled by the first nano-scalloping size of the Bosch
DRIE process. The 3-D nanostructures with such re-entrant sidewall profile are
desirable in several applications, such as T-gates for microwave transistors,
wave modulators for nano-optics, robust omniphobic surfaces, and various nanoelectromechanical
systems (NEMS). With conventional techniques used to create 3-D features, multiple
lithography steps with precise alignment or a single lithography step with multi-layer
resists (or multi-step post processes) would be required. The result suggests
that a cost-effective direct 3-D nanostructure fabrication is possible by controlling
the nano-scalloping effect.
Figure 2b shows the 3-D nanostructures with a reentrant sidewall profile of
repeated concaveness or convexness. The three-dimensional variation of the sidewall
profile can be imposed along the selected sidewall slope by modulating the nano-scalloping
effects, enabling hierarchical or multi-level nanostructures.
Figure 2c also shows that the tip sharpness can further be tailored. For example,
the nanostructure tips of a positively-tapered sidewall profile can conveniently
be sharpened by thermal oxidation and subsequent removal of the oxide. The well-regulated
sharp-tip nanostructures covering a large pattern area, especially the needle-like
nanopost structures, commonly interest such electronic applications as field
emitter structures. This simple but efficient method of sharp-tip nanofabrication
will also facilitate the design and fabrication of high-aspect-ratio scanning
probe tips. These results support that the well-programmed nano-scalloping effect
in Bosch DRIE can be a simple and useful tool for the 3-D nanostructure fabrication.
Among many benefits of the 3-D nanostructures, the densely-populated nanostructures
over a large sample area can open non-electronic application possibilities.
For example, the high-aspect-ratio sharp-tip nanostructures enable the fabrication
of nano-patterned superhydrophobic surfaces of good mechanical robustness
and de-wetting stability, compared with the micro-patterned or irregularly-patterned
(e.g., chemically-formed or polymer-roughened) superhydrophobic surfaces.
Figure 3 shows the well-regulated sharp-tip (~10 nm in tip radius) nanopost
structures of varying heights (50-500 nm). Although the tips are all sharp,
only tall nanopost structures with a small slope angle maintain a de-wetted
state, exhibiting great superhydrophobicity (a contact angle of ~180°).
These nanostructures with regular and dense pitch not only allow one to study
the effect of nanostructure geometries on the superhydrophobic wetting. But
they also make flow applications, such as hydrodynamic drag reduction, more
practical by tolerating highly pressurized flows without losing surface superhydrophobicity3,4.
SEM images of sharp-tip nanopost structures for superhydrophobic surfaces1,2.
Each inset shows the apparent contact angle of a water droplet after
a hydrophobic coating of Teflon (~10 nm thick) on each surface. High-aspect-ratio
nanoposts (e.g., more than 200 nm as shown in b and c) show dramatically
enhanced hydrophobicity (e.g., a contact angle greater than 175°),
while the short nanoposts (e.g., less than 100 nm shown in a) do not
(e.g., a contact angle not more than 130°). As a reference, the
contact angle on Teflon coated on a non-structured flat surface is ~120°.
The well-regulated 3-D nano-topographical properties enable another possibility
for exploration in cell biology. A cell in vivo lives in a 3-D nano-environment,
interacting with the extracelullar matrix materials feabured with nano-tophographical
projections and depressions that vary in composition, size and periodiciry.
It differs from focal and fibrillar adhesions characterized on two-dimensional
substrates in vitro.
Although several cell behaviors over various surface topographies werestudied
with micro- and nanostructured surfaces, the inadequacy to control the surface
3-D topography systematically, especially in the nanoscale, had precluded us
from isolating the effect of three-dimensionality of nanoscale surface features
on cell adhesions. The development of the 3-D nanofabrication technique now
allows the systematically controlled 3-D nanotopography model surfaces for the
in-vitro study of 3-D cell adhesions. Figure 4 shows a recent study of fibroblast
cell interactions with the sharp-tip nanopost and nanograte structures tested
as the regulated 3-D nanotopography models5,6.
The well-defined 3-D nanostructures revealed that cells would use filopodia
for spatial sensing in their movement around the nanoenvironment.
While the nanopost structures worked as 'stepping stones' in the filopodia
movement (Figs. 4a and 4b), the nanogrates functioned as 'guiding tracks' (Figs.
4c and 4d), with the total effect also being dependent on the structural aspect
ratios. More details on associated cell behaviors on 3-D nanotopographies such
as cell proliferation, morphology, and adhesions can be found elsewhere5,6.
Well-defined 3-D nanostructure systems provide a unique opportunity to elucidate
many aspects of the nanobiology of cells, the understanding of which can further
be utilized for cell and tissue engineering applications.
This short review article overviewes a simple but useful method to fabricate
3-D dense-array nanostructures with good regularity of pattern, size, and shape
over a large sample area. The Bosch DRIE process combined with laser interference
lithography not only simplifies the nanofabrication process, but also makes
possible the tailoring of nanostructured 3-D sidewall profiles. The subsequent
simple method of tip sharpening is also discussed. Affordable surfaces with
well-controlled 3-D nanostructures over a large area open new applications in
electronics and beyond through their unique properties originating from their
Most works presented in this article were performed as the PhD thesis work
under the supervision of Prof. Chang-Jin "CJ" Kim at the University
of California at Los Angeles (UCLA). The author thanks Prof. Kim for support
and discussion throughout the works, Prof. Joonwon Kim for initial help in nanofabrication,
Prof. Chih-Ming Ho and Dr. Umberto Ulmanella for microfluidic applications,
and Profs. Benjamin Wu, James Dunn, Ramin Beygui, and Dr. Sepideh Hagvall for
1. C.-H. Choi, C.-J. Kim, "Fabrication of Dense Array
of Tall Nanostructures over a Large Sample Area with Sidewall Profile and Tip
Sharpness Control", Nanotechnology 17, 5326-5333 (2006).
2. C.-H. Choi, C.-J. Kim, "Design, Fabrication, and Applications
of Large-Area Well-Ordered Dense-Array Three-Dimensional Nanostructures",
in Nanostructures in Electronics and Photonics, Ed. Faiz Rahman, Pan Stanford
3. C.-H. Choi, C.-J. Kim, "Large Slip of Aqueous Liquid
Flow over a Nanoengineered Superhydrophobic Surface", Physical Review
Letters 96, 066001 (2006)
4. C.-H. Choi, U. Ulmanella, J. Kim, C.-M. Ho, C.-J.
Kim, "Effective Slip and Friction Reduction in Nanograted Superhydrophobic
Microchannels", Physics of Fluids 18, 087105 (2006)
5. C.-H. Choi, S. H. Hagvall, B. M. Wu, J. C. Y. Dunn, R. E.
Beygui, C.-J. Kim, "Cell Interaction with Three-Dimensional Sharp-Tip
Nanotopography", Biomaterials 28, 1672-1679 (2007).
6. C.-H. Choi, S. H. Hagvall, B. M. Wu, J. C. Y. Dunn, R. E.
Beygui, C.-J. Kim, "Cell Growth as a Sheet on Three-Dimensional Sharp-Tip
Nanostructures", Journal of Biomedical Materials Research 89A, 804-817
Copyright AZoNano.com, Professor Chang-Hwan Choi (Stevens Institute
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