Triangle pore arrays fabricated on Si (111) substrate by sphere lithography combined with metal-assisted chemical etching and anisotropic chemical etching
© Asoh et al.; licensee Springer. 2012
Received: 15 May 2012
Accepted: 29 June 2012
Published: 19 July 2012
The morphological change of silicon macropore arrays formed by metal-assisted chemical etching using shape-controlled Au thin film arrays was investigated during anisotropic chemical etching in tetramethylammonium hydroxide (TMAH) aqueous solution. After the deposition of Au as the etching catalyst on (111) silicon through a honeycomb mask prepared by sphere lithography, the specimens were etched in a mixed solution of HF and H2O2 at room temperature, resulting in the formation of ordered macropores in silicon along the  direction, which is not achievable by conventional chemical etching without a catalyst. In the anisotropic etching in TMAH, the macropores changed from being circular to being hexagonal and finally to being triangular, owing to the difference in etching rate between the crystal planes.
KeywordsSilicon Metal-assisted chemical etching Colloidal crystal templating Sphere lithography Anisotropic chemical etching Triangle pore arrays
A number of promising approaches to nanofabrication, which have potential application to high-throughput and low-cost production, are applied to generate patterns on various substrates . The two-dimensional (2D) nano-/micropatterning of solid substrates using self-assembled colloidal particles as a mask, which is often referred to as colloidal lithography or nanosphere lithography, has also attracted considerable attention as a key fabrication method owing to its relative simplicity and low cost. We previously reported the fabrication of ordered silicon microstructures such as silicon convex arrays and silicon nanopore patterns with a regular periodicity on the order of micrometers by combining colloidal crystal templating and site-selective metal-assisted chemical etching using patterned noble-metal particles as catalysts . Because the metal-assisted chemical etching proposed by Li and Bohn in 2000  is a very simple and efficient process that uses no external bias, the number of studies of 2D or three-dimensional (3D) patterning based on metal-assisted chemical etching using a shape-controlled metal catalyst is increasing yearly [4–9]. If a metal catalytic layer with an ordered pore arrangement is applied, the silicon substrate is etched into an array of silicon nanowires [5, 8]. Using nanosphere-lithography-based metal-assisted chemical etching, it has been demonstrated by Huang et al. that silicon nanowires with an aspect ratio larger than 30 could be obtained . Regarding 3D patterning, Hildreth et al. demonstrated that the fabrication of 3D silicon nanostructures such as sloping channels, cycloids, and spirals could be achieved by metal-assisted chemical etching using various shape-controlled catalysts (e.g., nanorods, nanodonuts, nanodiscs, nanolines, squares, grids, and star-shaped catalysts) . Using 2D catalyst templates with multiple thicknesses, the fabrication of more complex 3D nanostructures was also achieved by a folding process that combines rotational and translational motion .
On the other hand, we have fabricated an ordered arrangement of macropores with diameters above 1 μm in silicon using a circular noble-metal thin film as the catalyst [10, 11]. However, the polystyrene honeycomb mask used for metal deposition, which was prepared using binary colloidal crystals composed of large silica spheres and small polystyrene spheres , had a relatively coarse framework, resulting in imperfect templating into the silicon substrate. Therefore, to fabricate more precise macroporous silicon using a noble-metal thin film as the catalyst, it is essential to prepare the metal deposition mask with a dense framework and periodic openings. In this work, we used metal-assisted chemical etching through a dense photoresist mask with a 2D hexagonal array of openings, which was prepared by sphere photolithography, to fabricate high-quality macroporous silicon. In addition, we aimed to fabricate triangle pore arrays on a Si (111) substrate using only a circular catalyst by controlling the morphology of the pore shape of etched silicon during anisotropic chemical etching.
Au thin films were deposited on silicon substrates by ion sputtering (E-1010, Hitachi High-Tech, Minato-ku, Tokyo, Japan) through a honeycomb mask with a 3-μm periodicity. The sputtering was carried out for 1 min at a discharge current of 15 mA in vacuum with a pressure below 10 Pa. The deposition rate of Au was 10 nm min−1. The deposited Au layer was estimated to be a 10-nm-thick continuous circular thin film composed of clusters of Au nanoparticles. After sputtering, the specimens with locally deposited Au films were etched for 1 min in a mixed solution of 15 mol dm−3 HF and 1 mol dm−3 H2O2 at room temperature under ambient light. This etchant composition corresponds to ρ = [HF]/([HF] + [H2O2) = 93.8 using the unified notation proposed by Chartier et al. . According to their report, when etching is carried out using a high proportion of HF, i.e., ρ > 70%, straight or curved cylindrical pores are formed with a diameter matching the size of the metal nanoparticles embedded at their bottom (in their case, Ag nanoparticles).
To modulate the morphology of the silicon macropores, pre-etched silicon specimens were immersed for a period of either 10, 20, 30, or 40 min in 25% tetramethylammonium hydroxide (TMAH) aqueous solution. After removing the mask by immersing the specimens in toluene for the polystyrene mask and in acetone for the photoresist mask, the morphology of the obtained microstructured silicon was evaluated by scanning electron microscopy (SEM) (JEOL 6701 F, JEOL Ltd., Akishima, Tokyo, Japan).
Results and discussion
Two types of mask with periodic circular opening arrays
On the other hand, the photoresist honeycomb mask, which was prepared by sphere photolithography, had flat and dense walls, as shown in Figure 2c,d. The openings in the resist layer were arranged hexagonally, corresponding to the 2D hexagonal array of silica spheres used as the initial mask to provide information on reactive sites during exposure . The diameter of the openings and the thickness of the resist layer were approximately 1 μm and 850 nm, respectively. The hexagonal contour shape was attributed to interference among adjacent spheres during exposure.
Formation of macroporous silicon using metal-assisted chemical etching
Using a resist mask, an ordered array of macropores with a uniform diameter of approximately 1 μm was formed, as shown in the low-magnification view in Figure 3c. The basic chemical etching behaviors were almost the same regardless of the morphology of the mask. The diameter of the pores was almost in agreement with that of the bottom part of the openings in the resist mask. The shape of the pores was sharply defined. Obviously, chemical etching proceeded only in the Au-coated main area on the silicon surface. From the top view shown in Figure 3d, it was confirmed that there were few clear defects at the interspace among the three main pores on the silicon surface, unlike the surface shown in Figure 3a. These results indicate that the adhesion and coverage between the upper mask and the silicon substrate were markedly improved. The usability of the resist mask was also demonstrated for anodic etching in our previous studies [13, 17].
Figure 3e shows a cross-sectional image of the macropores. This image indicates that the pores were straight and that the pore depth was approximately 4 μm for metal-assisted chemical etching of 1 min. The etching rate of 4 μm min−1 was lower than that described in Figure 3b. The differences in etching rate are considered to be dependent on the differences in size and/or thickness of the catalyst layer caused by sputtering through the two different types of mask.
Morphological change of pore shape of etched silicon during post-catalytic etching
To investigate the effect of the crystal plane on the morphology of macroporous silicon during post-catalytic etching, metal-assisted chemical etching and a subsequent chemical etching in TMAH were conducted for (100) silicon, under the same conditions as those in Figure 5b. Figure 5e shows a SEM image of the top surface of (100) silicon after chemical etching in 25 wt.% TMAH for 20 min. The outermost shape of pores was a square, not a circle, and a hexagon or a triangle as in the case of (111) silicon. Even under the same etching conditions, the diameter of the pores was smaller than that of the macropores shown in Figure 5b. This result indicates that the difference in the crystal plane directly affects not only the morphology of etched silicon, but also the etching rate in the 2D surface.
We described the fabrication of ordered macropore arrays in silicon by metal-assisted chemical etching through a dense photoresist mask with a 2D hexagonal array of openings, which was prepared by sphere photolithography. Using the resist honeycomb mask, an ordered array of macropores with a uniform diameter could be fabricated over the entire area of the specimen. The adhesion and coverage of the resist mask to the underlying substrate were significantly higher than those of the polystyrene honeycomb mask prepared using binary colloidal crystals. Furthermore, the shape of the obtained macropores could also be modulated by anisotropic chemical etching in TMAH. Using this technique combined with metal-assisted chemical etching and a subsequent anisotropic chemical etching, the 2D patterning of a silicon surface with well-defined morphology and orientation was achieved. Controlled silicon structures with an ordered periodicity have potential use in not only optoelectronic devices, but also chemical sensors and biofunctional devices that require ordered cavities and high surface-to-volume ratios. We think that this approach provides a beneficial scheme for generating novel patterns for possible applications.
HA is an associate professor, KF is a graduate student, and SO is a professor at the Department of Applied Chemistry, Kogakuin University.
scanning electron microscopy
This work was partially financially supported by a Grant-in-Aid for Scientific Research (A) no. 20241026 from the Japan Society for the Promotion of Science. We also acknowledge the Strategic Research Foundation Grant-aided Project for Private Universities matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology of Japan and Tokyo Ohka Foundation for the Promotion of Science and Technology.
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