Liquid phase deposition is a method used for the nonelectrochemical production of polycrystalline ceramic films at low temperatures, most commonly silicon dioxide films. Herein, we report that silica spheres are organized in a hexagonal close-packed array using a patterned substrate. On this monolayer of silica spheres, we could fabricate new nanostructures in which deposition and etching compete through a modified LPD reaction. In the early stage, silica spheres began to undergo etching, and then, silica bridges between the silica spheres appeared by the local deposition reaction. Finally, the silica spheres and bridges disappeared completely. We propose the mechanism for the formation of nanostructure.
Silicon dioxide (SiO2) films are widely useful in various fields. These films act as interlayer dielectrics and gate oxides in transistors and in the fabrication of integrated circuits and ultralarge-scale integration (ULSI) technologies [
Recently, among various techniques, the liquid phase deposition (LPD) has been focused on as a useful method for the deposition of oxide films at low temperatures in aqueous solutions. LPD can overcome the abovementioned drawbacks of conventional methods because it does not require the use of vacuum systems and sensitive reagents. LPD progresses easily at room temperature, demands low production costs, and has minimal environmental impact. LPD is especially useful in coating not only flat substrates but also nonplanar substrates [
Up until now, research regarding LPD has investigated the formation and kinetics of films on planar substrates. In 1998, Nagayama et al. reported that SiO2 films could be deposited on glass in an H2SiF6 solution supersaturated with silica, after H3BO3 was added to the H2SiF6 solution which was saturated with SiO2 [
In this study, we assembled silica spheres with hexagonally close-packing by using a patterned substrate. Then, we performed a modified LPD reaction on the nanostructure, which meant the silica sphere array, without additives but by changing the saturation of hydrofluorosilicic acid (H2SiF6). As a result, we fabricated a new nanostructure showing local deposition and etching reaction at the same time and proposed the mechanism of formation of the new nanostructure.
Silica spheres were synthesized using the Stöber method, which involves the hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol in the presence of ammonia (NH3) as a catalyst [
The silicon wafer with a 300 nm thick SiO2 layer was coated with poly(methyl methacrylate) (PMMA) C2 by spin coating. The patterned Si substrates were immersed in piranha solution for 30 min and washed with deionized (DI) water. To prepare the poly(dimethylsiloxane) (PDMS) stamp, PDMS was poured onto the cleaned, patterned Si substrates and baked at 70°C for a few hours. Then, a small amount of the silica spheres powder was placed on the PDMS stamp and rubbed repeatedly in the same direction using a PDMS slab. After rubbing, the randomly aggregated upper layers of silica spheres were removed from the bottom layer with hexagonally close-packing (HCP) by using a fresh sticky PDMS slab for a few seconds on top of the silica sphere array and subsequently removing the PDMS slab. Finally, this monolayer of the HCP silica spheres on the PDMS stamp was transferred to the precoated silicon wafer with a 300 nm thick SiO2 layer with PMMA [
As mentioned in the previous section, HCP silica spheres were heated at 200°C for 2 min to soften PMMA, which immobilized the silica spheres on PMMA layer. On the other hand, the liquid phase deposition solution was prepared as follows. First, 110 mL of hydrofluorosilicic acid (H2SiF6, 35%) and 2 g of fumed silica (SiO2) powder were mixed and stirred at 400 rpm overnight. After stirring, this solution was filtered using a vacuum filtration, and DI water was added to the filtered solution at a ratio of 1 : 2 to allow for supersaturation with silicic acid. Immediately after preparing the solution, the samples were immersed in the solution for various times from 5 to 30 min. After the reaction, the sample was rinsed with DI water and dried with nitrogen blowing.
We synthesized silica spheres with various sizes by controlling the concentrations of the reactants (Figure
SEM images of silica spheres synthesized by the Stöber method. The average diameter of the silica spheres is 450 nm (a), 590 nm (b), and 810 nm (c), denoted by S-450, S-590, and S-810, respectively. The distribution of the diameters of silica spheres is shown in (d). Scale bars are 1
Schematic diagram of the general procedure to prepare perfect hexagonally close-packed arrays of silica spheres.
We conducted the LPD of silica on the hexagonally close-packed monolayer of silica spheres. The general LPD reaction, hydrofluorosilicic acid supersaturated with silicon dioxide is used to make the LPD solution. In this study, we experimented with LPD solutions consisting of unsaturated hydrofluorosilicic acid with 2 g of fumed silica powder, as the source of silicon dioxide. When supersaturated hydrofluorosilicic acid is filtered, undissolved silica substances are commonly sieved from the solution. However, our solution was not supersaturated because of insufficient silicon dioxide, and hence, there were no filtered substances. The experiment progressed using this LPD solution, and the results according to time are shown in Figure
SEM images of liquid phase deposition according to the deposition time: (a) 0 min, (b) 5 min, (c) 10 min, and (d) 30 min. The dashed circle represents the bridge between silica spheres. Scale bars are 1
The changes in silica sphere size during the LPD reaction with time are shown in Figure
(a) Graph of the reaction rate of liquid phase deposition with respect to time. EDS analysis of the center of a silica sphere (b) and a bridge (c).
In our previous work, the sphere-bridge network structure was formed by etching using a neutral solution of 1 : 30 (v/v) mixture of 49% HF acid and 40% NH4F, and a mechanism for the formation of SB-NW structures was proposed. This work demonstrates an increased ammonium hexafluorosilicate (AHFS) concentration in the formed droplets at the interstitial sites of three adjacent silica spheres, and it indicates where the three materials, that is, AHFS, silica sphere, and aqueous silicic acid, are in contact, that is, the bridges formed between the silica spheres through the precipitation of silicic acids. As opposed to our previous work, in the present case, we used an LPD solution containing 2 g of fumed silica instead of a near neutral solution. In our LPD solution, the following reaction occurred [
Schematic illustration depicting the mechanism of formation of nanostructure. (a) Hexagonally close-packed spheres in an aqueous solution. (b) Decrease in the size of silica spheres through homogeneous etching. (c) High concentration of silicic acid (blue) between the silica spheres. (d) Deposition of silica bridges between silica spheres. (e) Reduction of the nanostructure size due to etching. (f) Disappearance of the bridges.
In this work, we fabricated an array of perfect hexagonally close-packed silica spheres over a large area of a patterned substrate. Then, we experimented with a modified LPD reaction on this nanostructured substrate and demonstrated a new nanostructure that certified simultaneous deposition and etching, that is, between these two processes; then, we proposed the mechanism for the formation of this nanostructure. This nanostructure could be useful in the biointerface field as a substrate that influences biological systems such as cell and bacteria. Because biological behaviors are dependent on the external environment, the unique surface topography of the nanostructure could trigger new biological responses such as cellular adhesion, development, differentiation, proliferation, and so on.
This work was supported by the SRC Research Center for Women’s Diseases of Sookmyung Women’s University (2010).