Lithography-free fabrication of silicon nanowire and nanohole arrays by metal-assisted chemical etching
© Liu et al.; licensee Springer. 2013
Received: 23 January 2013
Accepted: 16 March 2013
Published: 4 April 2013
We demonstrated a novel, simple, and low-cost method to fabricate silicon nanowire (SiNW) arrays and silicon nanohole (SiNH) arrays based on thin silver (Ag) film dewetting process combined with metal-assisted chemical etching. Ag mesh with holes and semispherical Ag nanoparticles can be prepared by simple thermal annealing of Ag thin film on a silicon substrate. Both the diameter and the distribution of mesh holes as well as the nanoparticles can be manipulated by the film thickness and the annealing temperature. The silicon underneath Ag coverage was etched off with the catalysis of metal in an aqueous solution containing HF and an oxidant, which form silicon nanostructures (either SiNW or SiNH arrays). The morphologies of the corresponding etched SiNW and SiNH arrays matched well with that of Ag holes and nanoparticles. This novel method allows lithography-free fabrication of the SiNW and SiNH arrays with control of the size and distribution.
KeywordsMetal-assisted chemical etching Silicon nanowire arrays Silicon nanohole arrays Silver thin film dewetting
Silicon nanostructures such as silicon nanowire (SiNW), nanocone, or nanohole (SiNH) arrays have attracted intensive attention due to their unique optical, electrical, and thermal properties for promising applications in the fields of solar cells [1–6], field-effect transistors , as well as chemical and biological sensors [8, 9]. Besides their intrinsic characteristics inherited from bulk silicon, the morphologies and distribution of the nanostructures play a dominant role on their properties. As for both the basic studies and applications of SiNW arrays, precise control of the diameter, the length, the density, and the surface are of vital importance. To achieve large-area vertically aligned SiNW arrays with high uniformity, it is very popular to apply metal-assisted chemical etching (MaCE) as a low-cost etching method [6, 10–12]. In this method, a thin noble metal film with arrays of holes is formed on a silicon substrate and then the silicon underneath the metal is etched off with the catalysis of metal in an aqueous solution containing HF and an oxidant, leaving behind arrays of SiNW whose distribution and diameter are determined by the metal film. To prepare a metal film with good ordered arrays of nanoholes, nanosphere lithography [2, 13, 14], interference lithography [15, 16], block copolymers , or anodic aluminum oxide [18–20] has been extensively adopted. Though SiNW arrays with well-controlled diameter, length, and density have been achieved, complicated processing steps are involved prior to MaCE. The fabrication of SiNH array structure also faces the same issues. In addition, specific techniques such as deep ultraviolet lithography are also required in order to achieve high-quality periodic SiNH arrays [4, 21]. In this work, we present a facile method to fabricate SiNW arrays as well as SiNH arrays based on metal film dewetting process, which dramatically simplifies the fabrication process by avoiding complicated lithography patterning process. The patterned silver (Ag) structure can be tuned by varying the thickness of the Ag film and annealing temperature on the silicon substrate. With the control of the annealing process, metal film with arrays of holes or nanoparticles can be generated on the substrate. The silicon underneath the silver is etched off, thus SiNW or SiNH arrays can be achieved by MaCE with the catalysis of the metal. The as-fabricated Si nanostructures match well with the self-patterned metal structure.
Results and discussion
Dewetting process of Ag films
Dewetting process of thin film on a solid substrate has been well investigated in the past decades [22–25]. Solid films are usually metastable or unstable in the as-deposited state, and they will spontaneously dewet or agglomerate to form islands when heated to certain temperatures at which the mobility of the constituent atoms is sufficiently high. Dewetting occurs at the holes preexisting during the deposition process (as in this case), at film edges, or at newly formed holes, which is overall a hole nucleation and growth phenomena. Whatever their source is, a process that leads to hole formation in a film is a prerequisite for dewetting where the holes could potentially serve as nucleation sites or as nuclei themselves . The most common origin for the heterogeneous nucleation is grain boundary grooving which may occur from the free surface of the film and the film/substrate interface. Hole formation would be most likely when the grain boundary grooves grow sufficiently large. The formation and growth of these holes takes an incubation time for dewetting that depends on film thickness. Hole formation can also occur by grain sinking that results from a diffusional flow when a lower tensile grain loses material to a higher tensile one . Whether the initial holes are developed by grain grooving, grain sinking, or just deposition process, the overall dewetting process is determined by the growth of the holes. As the holes grow, the development of rims slows down the rate of edge retraction by reducing the strain energy of the system. At the early stage, small circular holes grow immediately until neighboring holes meet and form common rims of networks, and new holes may still continue to form throughout the dewetting process. The networks finally become unstable and break up into stable islands with minimum local energy state via the Rayleigh instability.
Meantime, for a given film thickness (e.g., 16 nm), as the annealing temperature increases gradually, the morphologies of the film transfer from compact film to mesh one with circular or quadrate holes (Figure 3b) and finally to isolated Ag semispherical nanoparticles (Figure 3d). If the film is thin enough (e.g., 5 nm), only isolated island can be achieved even at a very low annealing temperature, which may originate from the initial uncontinuous feature during the deposition process. If the film is too thick (e.g., 41 nm), no obvious hole can be observed even for annealing temperature as high as 300°C. The dependence of morphologies on the film thickness displays a similar behavior. To a certain degree, the same morphology can be achieved with different combinations of film thickness and annealing temperature.
Fabrication of SiNW arrays utilizing Ag meshes
Fabrication of SiNH arrays utilizing Ag nanoparticles
We demonstrate a simple and low-cost method based on the metal dewetting process combined with Ag-assisted chemical etching to fabricate SiNW and SiNH arrays. Both Ag mesh with holes and Ag nanoparticles can be formed without a lithography step. The morphologies are controlled by the Ag film dewetting behavior via thermal annealing. By adjusting the film thickness and annealing temperature, the size and distribution of the holes and nanoparticles can be manipulated. The morphologies of the as-fabricated SiNW and SiNH arrays match well with the holes and nanoparticles. The density, diameter, and distance between adjacent nanowire (nanohole) can be facilely tuned by controlling the respective Ag mesh (nanoparticle). In addition, the length (depth) of nanowire can be adjusted by the etching time. As a result, this is a simple, mask-free, and cost-effective method to fabricate wafer-sized silicon nanostructures.
This work was supported by the National Natural Science Foundation of China (61176057, 91123005, 60976050, 61211130358), the National Basic Research Program of China (973 Program) (2012CB932402), the Natural Science Foundation of Jiangsu Province (BK2010003), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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