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Micro-nano hybrid structures with manipulated wettability using a two-step silicon etching on a large area.

Kim BS, Shin S, Shin SJ, Kim KM, Cho HH - Nanoscale Res Lett (2011)

Bottom Line: The fabrication process is readily capable of producing MNHS covering a wafer-scale area.By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability.MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, Yonsei University, 262, Seongsanno, Seodaemun-gu, Seoul 120-749, Korea. hhcho@yonsei.ac.kr.

ABSTRACT
Nanoscale surface manipulation technique to control the surface roughness and the wettability is a challenging field for performance enhancement in boiling heat transfer. In this study, micro-nano hybrid structures (MNHS) with hierarchical geometries that lead to maximizing of surface area, roughness, and wettability are developed for the boiling applications. MNHS structures consist of micropillars or microcavities along with nanowires having the length to diameter ratio of about 100:1. MNHS is fabricated by a two-step silicon etching process, which are dry etching for micropattern and electroless silicon wet etching for nanowire synthesis. The fabrication process is readily capable of producing MNHS covering a wafer-scale area. By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability. MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

No MeSH data available.


Related in: MedlinePlus

MNHS with microcavities having a hole width of 200 μm and distance between cavities of 200 μm: (a, b) acetone-mediated MNHS. The thin sidewall structure is clearly shown; (c, d) asher-mediated MNHS without any substrate rotation. The inset in (d) is a close-up image of the side region; (e, f) with 45° rotation of the substrate. The inset in (f) is a top view of laterally grown nanowires on the sides of the microcavities.
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Figure 5: MNHS with microcavities having a hole width of 200 μm and distance between cavities of 200 μm: (a, b) acetone-mediated MNHS. The thin sidewall structure is clearly shown; (c, d) asher-mediated MNHS without any substrate rotation. The inset in (d) is a close-up image of the side region; (e, f) with 45° rotation of the substrate. The inset in (f) is a top view of laterally grown nanowires on the sides of the microcavities.

Mentions: Figure 5 shows SEM images of microscale square cavities with nanowires. For the microscale cavities, the width and the depth of a square, and the distance between cavities is 200, 30, and 200 μm, respectively. Using the acetone wet-cleaning process instead of the asher process, it is also possible to make a thin sharp sidewall barrier that surrounds the microcavity structures. Figure 5a,b shows a fabricated structure with thin sidewalls. However, it should be noted that the specimens shown in Figure 5c,d,e,f was fabricated using the asher process to remove the polymeric passivation layer. By contrast, unlike the previous structures (Figure 5,b), when we fabricated MNHS using the asher process instead of the acetone process after the silicon dry etching, no thin, sharp sidewalls are observed at the boundaries of the microcavities. The boundaries are also etched out by 1-h silicon wet etching in diluted HF and AgNO3 solution for nanowire synthesis. From the top view shown in Figure 5c,d,e,f, we can infer that the bulk portion of the silicon near the boundary area of the square microcavities could have been etched away, because of the orthogonal silicon crystallographic orientation. In the inset of Figure 5d, which shows a cross-sectional view of the boundary area, we observe that nanowires with specific angles of ± 45° to the flat plane are formed on the sidewall of the microcavity. According to previous studies [22-24], in electroless silicon etching, SiNWs are synthesized with a certain orientation, determined by the crystallographic orientation of the wafer. They reported that SiNWs are synthesized primarily in the normal direction with a (100) wafer crystallographic orientation. In our fabrication process, we used (100) Si wafers and the white arrow lines in Figure 5c,d,e,f indicate the in-plane 〈100〉 direction. Because the silicon is etched parallel to the 〈100〉 orientation, silicon etching from the sidewall and top surface must eventually intersect during the etching process, and thereby the silicon near the boundary will be etched away at the same time. Figure 5e,f shows images of the structure fabricated on the substrate with 45° rotation. As expected, the nanowires on the sidewall surface are normal to the boundaries that are parallel to the (100) substrate crystallographic orientations.


Micro-nano hybrid structures with manipulated wettability using a two-step silicon etching on a large area.

Kim BS, Shin S, Shin SJ, Kim KM, Cho HH - Nanoscale Res Lett (2011)

MNHS with microcavities having a hole width of 200 μm and distance between cavities of 200 μm: (a, b) acetone-mediated MNHS. The thin sidewall structure is clearly shown; (c, d) asher-mediated MNHS without any substrate rotation. The inset in (d) is a close-up image of the side region; (e, f) with 45° rotation of the substrate. The inset in (f) is a top view of laterally grown nanowires on the sides of the microcavities.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211421&req=5

Figure 5: MNHS with microcavities having a hole width of 200 μm and distance between cavities of 200 μm: (a, b) acetone-mediated MNHS. The thin sidewall structure is clearly shown; (c, d) asher-mediated MNHS without any substrate rotation. The inset in (d) is a close-up image of the side region; (e, f) with 45° rotation of the substrate. The inset in (f) is a top view of laterally grown nanowires on the sides of the microcavities.
Mentions: Figure 5 shows SEM images of microscale square cavities with nanowires. For the microscale cavities, the width and the depth of a square, and the distance between cavities is 200, 30, and 200 μm, respectively. Using the acetone wet-cleaning process instead of the asher process, it is also possible to make a thin sharp sidewall barrier that surrounds the microcavity structures. Figure 5a,b shows a fabricated structure with thin sidewalls. However, it should be noted that the specimens shown in Figure 5c,d,e,f was fabricated using the asher process to remove the polymeric passivation layer. By contrast, unlike the previous structures (Figure 5,b), when we fabricated MNHS using the asher process instead of the acetone process after the silicon dry etching, no thin, sharp sidewalls are observed at the boundaries of the microcavities. The boundaries are also etched out by 1-h silicon wet etching in diluted HF and AgNO3 solution for nanowire synthesis. From the top view shown in Figure 5c,d,e,f, we can infer that the bulk portion of the silicon near the boundary area of the square microcavities could have been etched away, because of the orthogonal silicon crystallographic orientation. In the inset of Figure 5d, which shows a cross-sectional view of the boundary area, we observe that nanowires with specific angles of ± 45° to the flat plane are formed on the sidewall of the microcavity. According to previous studies [22-24], in electroless silicon etching, SiNWs are synthesized with a certain orientation, determined by the crystallographic orientation of the wafer. They reported that SiNWs are synthesized primarily in the normal direction with a (100) wafer crystallographic orientation. In our fabrication process, we used (100) Si wafers and the white arrow lines in Figure 5c,d,e,f indicate the in-plane 〈100〉 direction. Because the silicon is etched parallel to the 〈100〉 orientation, silicon etching from the sidewall and top surface must eventually intersect during the etching process, and thereby the silicon near the boundary will be etched away at the same time. Figure 5e,f shows images of the structure fabricated on the substrate with 45° rotation. As expected, the nanowires on the sidewall surface are normal to the boundaries that are parallel to the (100) substrate crystallographic orientations.

Bottom Line: The fabrication process is readily capable of producing MNHS covering a wafer-scale area.By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability.MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, Yonsei University, 262, Seongsanno, Seodaemun-gu, Seoul 120-749, Korea. hhcho@yonsei.ac.kr.

ABSTRACT
Nanoscale surface manipulation technique to control the surface roughness and the wettability is a challenging field for performance enhancement in boiling heat transfer. In this study, micro-nano hybrid structures (MNHS) with hierarchical geometries that lead to maximizing of surface area, roughness, and wettability are developed for the boiling applications. MNHS structures consist of micropillars or microcavities along with nanowires having the length to diameter ratio of about 100:1. MNHS is fabricated by a two-step silicon etching process, which are dry etching for micropattern and electroless silicon wet etching for nanowire synthesis. The fabrication process is readily capable of producing MNHS covering a wafer-scale area. By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability. MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

No MeSH data available.


Related in: MedlinePlus