npsm 새물리 New Physics : Sae Mulli

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New Phys.: Sae Mulli 2021; 71: 1027-1030

Published online December 31, 2021 https://doi.org/10.3938/NPSM.71.1027

Copyright © New Physics: Sae Mulli.

Effect of Metal Structure on Silicon Nanowire Fabrication Using Metal-assisted Etching

Jungkil KIM*

Department of Physics, Jeju National University, Jeju 63243, Korea

Correspondence to:jungkil@jejunu.ac.kr

Received: August 12, 2021; Revised: October 21, 2021; Accepted: November 15, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Metal-assisted etching (MaE) is one of the most promising etching methods to fabricate silicon nanowires (Si NWs). Here, the effect of noble-metal structure on Si NW fabrication using MaE is investigated. First, two kinds of patterning method are used to fabricate hole arrays in the Au mesh as a catalyst. One is the preparation of the Au mesh by using an anodic aluminum oxide (AAO) disk, and the other is based on the patterning of polystyrene (PS) beads. The Au mesh prepared by using AAO has hole arrays with a smooth edge structure while the other Au mesh patterned by using PS beads shows a rough edge structure of hole arrays. Si NWs are fabricated by using MaE with each different Au mesh. As a result, Si NWs fabricated by using MaE based the AAO disk as a catalyst were found to show a smooth surface that corresponds to the hole structure of the Au mesh. On the other hand, the surface of Si NWs fabricated by using MaE based on PS beads is quite rough with vertically striped structures.

Keywords: Silicon nanowire, Surface structure, Metal-assisted etching

실리콘 나노선을 제작하기 위해 가장 많이 사용하는 방법 중 하나인 metal-assisted etching에서 촉매로 작용하는 귀금속의 구조가 제작된 나노선의 표면 구조에 미치는 영향을 알아봤다. 먼저 실리콘 기판을 나노선 모양으로 식각하기 위하여 구멍 어레이 구조를 포함한 금 박막을 두가지 방법으로 제작하였다. 두가지 제작법으로는 크게 양극 산화 알루미늄 (Anodic aluminum oxide, AAO)을 패터닝 기판으로 사용하는 방법과, 폴리스티렌 (Polystyrene, PS)를 이용한 패터닝 방법을 택했다. AAO를 이용하여 제작한 금 촉매의 구멍은 모양이 원형에 가까우며 경계면의 거칠기가 매우 낮았다. 반면, PS를 이용하여 제작한 금 촉매의 구멍은 경계면에 거칠기가 존재하였다. 각각의 방법으로 준비한 금 박막을 촉매로 사용하여 metal-assisted etching으로 제작한 나노선을 제작하였다. AAO를 기반으로 하는 실리콘 나노선의 표면은 매우 매끈한 구조를 보였으며 표면 거칠기가 거의 존재하지 않음을 확인할 수 있었다. 반면 PS를 기반으로 하는 실리콘 나노선의 표면은 수직 줄무늬와 함께 상대적으로 높은 거칠기를 보였다.

Keywords: 실리콘 나노선, 표면구조, Metal-assisted etching

Silicon nanowires (Si NWs) have attracted much attention due to the high potential for their applications to multifunctional devices such as transistors, photodetectors, memories, and processors [16]. To achieve this goal, it is important to clearly define a structure of Si NWs. In particular, the surface structure of Si NWs significantly affects an electrical and thermal properties [7]. To prepare high-quality Si NWs, metal-assisted etching (MaE) is widely studied, which is the etching method using noble metal catalysts and chemical solutions [810]. Recently, Yang et al. showed the controllable electrical and thermal conductivities of porous Si NWs can be utilized as a building block of thermoelectric devices [11]. S. J. Rezvani et al. and Y. Qu et al. show that an electrical conductivity of porous Si NW is much lower than that of conventional Si NW with a smooth surface. [12,13]. In addition, J. M. Weisse shows that a thermal conductivity of Si NW is negatively proportional to a porosity [14]. Here, I investigate the effect of noble metal structure on the silicon nanowire surface. Two Au meshes are prepared by different patterning methods. One is the usage of an anodic aluminum oxide (AAO) disk as a patterning template. The other is the patterning using polystyrene (PS) nanospheres. Two kinds of silicon nanowires fabricated by MaE using each Au mesh show different surface structures.

The Au mesh with holes were prepared by two different methods. First, the AAO disk is prepared as a patterning template (Fig.1) [15]. The AAO disk has long channels with a hexagonal alignment from a pore mouth to a back side. Then both side can be used to pattern the Au mesh. The Au of 50 nm is deposited on the AAO disk by metal sputtering. Then the AAO is selectively dissolved on KOH for 5 minutes. Then the Au mesh is floated on KOH, and subsequently KOH is replaced to DI-water. To remove Au particles on the back side of Au mesh, the Au film is transferred to aqua regia (HCl/HNO3 = 3:1) for 5 s. The Au mesh is transferred to Di-water to clean the backside again, then the Au mesh is finally transferred to moderated-doped Si wafer.

Figure 1. (Color online) Schematic illustration to show the fabrication processes of Au meshes using AAO and PS beads. Au meshes are utilized as catalyst of MaE to etch down the Si, resulting in the formation of Si NWs.

Second, PS beads are used to patterning masks on a Si wafer (Fig. 1) [16]. The PS beads are dropped on DI-water carefully, then the monolayer of PS beads are prepared on Di-water. Sodium is dropped on DI-water, then the PS beads are closely packed, resulting in the generation of hexagonal aligned PS beads layer. The PS beads layer is transferred on the moderated-doped Si wafer. To reduce the size of PS beads, the O2 plasma is treated. Subsequently the Au of 50 nm is deposited on the prepared sample by metal sputtering. The PS beads are selectively removed by acetone for several minutes. The Au mesh with hole arrays is remained on the Si wafer.

The prepared Si wafers with two kinds of Au meshes are dipped in HF, H2O2, and H2O (v/v/v = 1/0.1/2) for several minutes at the room temperature. Then vertical Si NW arrays are generated (Fig. 1).

Two kinds of Au meshes are structurally investigated by scanning electron microscopy (SEM). The fabricated Au mesh by using AAO has hole arrays with a hexagonal alignment, which corresponds to the structure of AAO. The SEM image of the head of Au mesh shows that pores are closed by Au particles (Fig. 2a). The SEM image of the tail of Au mesh shows the closed pores by tangled Au particles (Fig. 2b). To open the pores of Au mesh, the tangled Au particles on the tail of Au mesh are removed by aqua regia. After the treatment of aqua regia, the pores of Au mesh are clearly opened (Fig. 2c). The SEM image of tail of Au mesh also shows the opened pores without the Au nanoparticles (Fig. 2d). The edge shape of pore is clean without residues.

Figure 2. (Color online) (a) and (b) The SEM images of a head and a tail of Au mesh, respectively. Au nanoparticles are observed through the holes of Au mesh. The tail of Au mesh shows closed holes clearly. (c) and (d) The SEM images of a head and a tail of Au mesh, respectively, which is treated by aqua regia. The holes of Au mesh is clearly punched without nanopartilces.

The fabrication process using PS beads is investigated by SEM. The PS beads on the Si wafer are closely packed each other and form the monolayer (Fig. 3a). After the O2 plasma treatment, the diameter of PS beads is reduced as half of the original diameter (Fig. 3b). The surface of reduced PS beads is quite rough because of hardening by O2 plasma. The Au film of 50 nm is deposited onto the prepared sample by metal sputtering (Fig. 3c). To generate the Au mesh, the Au cup/PS bead arrays are dissolved by acetone. The remained Au structure on the Si wafer is the mesh with hole arrays (Fig. 3d). There are many residues around the holes, which may metal nano particles or polymer particles. Even though the the edge structure of hole is more circular than that of the Au mesh from AAO, there are many cracks around the hole, which reflects the rough surface of O2 plasma treated PS beads.

Figure 3. (Color online) (a) The SEM image of PS beads. PS beads are packed with a hexagonal arrangement. (b) The SEM image of PS beads with a reduced size. The O2 plasma treated PS beads shows a rough surface. (c) The SEM image of Au deposited PS bead arrays. (d) The SEM image of Au mesh with hole arrays. Residues are remained around holes after removing PS beads.

Two kinds of Si NW arrays are fabricated by MaE using each prepared Au mesh. The Au mesh loaded Si wafer is dipped in HF/H2O2/H2O. In MaE, the Si surface underneath the Au is oxidized because the Au extracts electrons from the Si surface, then consuming H2O2. Then oxidized Si surface is dissolved in the solution by HF. The cyclic reaction gives the result of the etching of Si surface down, generating Si NWs through the holes of Au mesh. As a result, vertically aligned Si NW arrays are generated from both samples (Fig. 4a and c). The high-resolution SEM images show the details of Si NW surfaces (Fig. 4b and d). In the case of Si NWs fabricated by using the Au mesh from AAO, the surface of NW is extremely smooth. It is hard to observe a roughness of surface in the SEM image. On the other hand, the Si NW fabricated by using the Au mesh from PS beads show the quite rough surface. In particular, there are striped vertical lines on the surface of Si NWs. On the contrary, the Si wafer under the Au mesh shows very smooth surface where is not etched during MaE. According to the mechanism of MaE, the Si wafer is etched down with the exact same shape of the Au mesh. Then the cracked hole structure of Au mesh makes the striped lines on the Si NW surface. Two kinds of Si NWs have different diameters, however it seems that there is no correlation between the diameter and surface structures. The hole shape dominantly affects the Si NW surface structure. This result suggests that the structure of noble metal as catalyst in MaE is important to prepare a high quality Si NWs with a smooth surface.

Figure 4. (Color online) (a) The SEM image of Si NWs fabricated by MaE using the Au mesh patterned by AAO. (b) The SEM image of Si NW roots in (a). The surface of Si NW is smooth. (c) The SEM image of Si NWs fabricated by MaE using the Au mesh patterned by PS beads. (d) The SEM image of Si NW roots in (c). The surface of Si NW is rough with vertical striped structure.

In this work, I demonstrated the fabrication of two kinds of Si NWs by MaE using two Au meshes that are prepared by different methods. One is the usage of AAO disk as a patterning template, and the other is the patterning method using aligned PS bead arrays as a patterning mask. The Au mesh from AAO shows very clean hole structures without any other nano particles, however the Au mesh from PS beads shows the holes with the cracked edge structure and residues. The MaE using the Au mesh from AAO enables to prepare Si NWs with the extremely smooth surface, while the Si NWs fabricated by MaE using the Au mesh from PS beads has the quite rough surface with stripped vertical lines. I believe that the MaE using catalyst with well-defined structure is important to fabricate high-quality Si NWs, and it will be helpful to secure high electrical and thermal conductivities of Si NWs.

This work was supported by the research grant of Jeju National University in 2021.

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