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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

SEM images of synthesized SiNWs by the electroless etching method: (a) a top view of nanowire surface; (b) a cross-sectional view of the nanowires.
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Figure 1: SEM images of synthesized SiNWs by the electroless etching method: (a) a top view of nanowire surface; (b) a cross-sectional view of the nanowires.

Mentions: Using electroless metal deposition and anisotropic silicon etching, SiNWs with very high aspect ratios can readily be synthesized on a wafer-scale area of a silicon substrate. We refer to previous studies for detailed electrolyte recipes and fabrication processes [11,19,20]. Prior to the formation of micropatterns and nanowires, the silicon wafer was thoroughly cleaned. Here, we used an n-type silicon wafer (phosphorous-doped) with a (100) orientation, a resistivity between 1 and 10 Ω cm, and a thickness of 500 μm. To begin with, a 4-inch silicon wafer was cleaned for 40 min in an H2O2 and H2SO4 solution with a volume ratio of 1:3, to remove organic materials. This was followed by an additional cleaning process with acetone and methanol for 5 min each in turn, using a sonicator. For fabricating SiNWs by electroless etching, the wafer was immersed in an aqueous solution of 0.02 M AgNO3 and 5 M HF for 70 min at room temperature. When diluted AgNO3 and HF solution are used for electroless silicon etching, Ag+ ions are attracted to the silicon surface by galvanic displacement. At the interface between the Ag+ and the silicon, oxidization of the silicon takes place, and, subsequently, the oxidized layer is etched by the hydrofluoric acid. Throughout the electroless silicon wet etching process, SiNWs were uniformly formed on the whole wafer surface, and silver dendrites covered the entire substrate [15]. After silicon etching, the wafer was immersed in HNO3 solution (70%) for 80 min to remove the silver dendrites and reveal the nanowire arrays. Finally, the sample was thoroughly rinsed with deionized water and dried under ambient conditions. Figure 1 shows field emission scanning electron microscope (FE-SEM) images of nanowires aligned vertically on a silicon substrate. In our fabrication process, the lengths and diameters of the nanowires were about 10 μm and 100 nm, respectively.


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)

SEM images of synthesized SiNWs by the electroless etching method: (a) a top view of nanowire surface; (b) a cross-sectional view of the nanowires.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: SEM images of synthesized SiNWs by the electroless etching method: (a) a top view of nanowire surface; (b) a cross-sectional view of the nanowires.
Mentions: Using electroless metal deposition and anisotropic silicon etching, SiNWs with very high aspect ratios can readily be synthesized on a wafer-scale area of a silicon substrate. We refer to previous studies for detailed electrolyte recipes and fabrication processes [11,19,20]. Prior to the formation of micropatterns and nanowires, the silicon wafer was thoroughly cleaned. Here, we used an n-type silicon wafer (phosphorous-doped) with a (100) orientation, a resistivity between 1 and 10 Ω cm, and a thickness of 500 μm. To begin with, a 4-inch silicon wafer was cleaned for 40 min in an H2O2 and H2SO4 solution with a volume ratio of 1:3, to remove organic materials. This was followed by an additional cleaning process with acetone and methanol for 5 min each in turn, using a sonicator. For fabricating SiNWs by electroless etching, the wafer was immersed in an aqueous solution of 0.02 M AgNO3 and 5 M HF for 70 min at room temperature. When diluted AgNO3 and HF solution are used for electroless silicon etching, Ag+ ions are attracted to the silicon surface by galvanic displacement. At the interface between the Ag+ and the silicon, oxidization of the silicon takes place, and, subsequently, the oxidized layer is etched by the hydrofluoric acid. Throughout the electroless silicon wet etching process, SiNWs were uniformly formed on the whole wafer surface, and silver dendrites covered the entire substrate [15]. After silicon etching, the wafer was immersed in HNO3 solution (70%) for 80 min to remove the silver dendrites and reveal the nanowire arrays. Finally, the sample was thoroughly rinsed with deionized water and dried under ambient conditions. Figure 1 shows field emission scanning electron microscope (FE-SEM) images of nanowires aligned vertically on a silicon substrate. In our fabrication process, the lengths and diameters of the nanowires were about 10 μm and 100 nm, respectively.

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