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 and uniformity1,2.
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).
Figure 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 DRIE.
Figure 2. Scanning electron microscope (SEM) images of 3-D nanostructures of various sidewall profiles and tip sharpness created on silicon substrates1,2. Well-regulated nano-periodic structures with superior control of the structural three-dimensionality can be conveniently created on a large sample area (up to 4"x4" substrate) by combining the Bosch DRIE process with a laser interference lithography. The laser interference lithography can define a uniform array of photoresist nanopatterns (line, pillar, or holes), where a pattern periodicity is determined by the laser wavelength and the angle between two interfering beams. The nanostructures shown in the figures are tall pillar structures (~500 nm in height) in a square array of ~200 nm in periodicity.
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 nanostructures.
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.
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 nanoscale geometry.
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 cell studies.
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 Publishing (2008)
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 (2009).
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