Micro-nano hybrid structures with manipulated wettability using a two-step silicon etching on a large area
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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°.
KeywordsContact Angle Boiling Heat Transfer Silicon Etching Surface Contact Angle Bosch Process
critical heat flux
deep trench reactive ion etching
energy dispersive spectrometer
field emission scanning electron microscope
micro-nano hybrid structures
In general, boiling heat transfer is considered to be very effective mechanism for cooling the high heat-generating devices due to the large latent heat by phase transition accompanied by fast transport of gas-phased bubbles. In many industrial fields related to energy conversion, e.g., nuclear power plants, heat pump, and electronics, improving the performance of boiling heat transfer based on surface treatment and modification is a key issue [1, 2, 3]. Both microstructures and nanostructures are often used to enhance the performance of boiling heat transfer by controlling and modifying structure geometries.
There have been numerous studies on boiling heat transfer improvements obtained by microscale structure fabrication using artificial structures, such as patterned circular/rectangular holes/pillars, and conical/cylindrical cavities [4, 5]. With the development of feasible nanoscale fabrication technique including nanostructure patterning by conventional photolithography  or maskless method  that can easily manipulate the surface wettability [8, 9], nanoscale surface treatments could also be applied to boiling heat transfer enhancement. 3D macro-porous metallic surface layer with nanoscale porous structures-enhanced heat transfer coefficient, especially at low heat flux of 1 W · cm-2, over 17 times compared to the plain surface . Top-down etched silicon nanowires (SiNWs) and electrodeposited copper nanowires improved the boiling performance by up to 100% compared to a plain silicon surface, by increasing surface wettability where the nanowires exhibited superhydrophilic behavior . With tilted copper nanorods, synthesized by an electron-beam evaporator, pool boiling heat transfer characteristics were also enhanced by the increased wettability and the nucleation sites resulting from the intrinsic nature of the dense nanowires . Additionally, some studies report the increase in boiling behavior using carbon nanotube-coated surfaces . It may be inferred from these references that nanoscale structures greatly increase the surface area and wettability and lead to the enhancement of boiling behavior by supplying adequate liquid to the boiling surface and extending the burn-out limit of the surfaces.
In light of previous efforts to enhance boiling performance by increasing the nucleation sites and the surface wettability, micro-nano hybrid structures (MNHS) [14, 15] may offer extraordinary boiling heat transfer performance. Specifically, hierarchical MNHS can significantly increase the boiling surface area, the surface roughness and the surface wettability, compared to single-scale structures. However, there has been relatively little research on the fabrication and the application of hierarchical MNHS to enhance the performance of boiling heat transfer further.
For the fabrication of MNHS, nanowire-adorned microstructures by selective electrochemical growth of nanowires , using a porous anodic alumina template  and the dual-scale hierarchical structures with SU-8 photoresist (PR) using capillary force lithography  were reported. However, for boiling applications there are some specialized requirements which should be met in prior. First, the boiling surface must have good thermal properties, including high thermal conductivity and durability under high heat flux conditions. Second, the surface area and roughness should be increased further by modifying surface geometries, for example, combining microscale patterns with nanoscale structures, to expel the heat from the surface sufficiently and to act as a bubble nucleation site. Third, the fabrication technique must be simple, and must enable one to simultaneously synthesize nanoscale structures and microstructures over a large area. Fourth, a hydrophilic surface, which can readily attract and supply the cooling agent to the boiling surface, is desirable to prevent a film formation on heated surface for boiling applications. In view of these requirements, previous MNHS fabrication techniques, which were complicated by the use of templates and additional electrode layers, or were intended for low thermal conductivity and hydrophobicity based on polymer materials, may be inappropriate for boiling applications.
In this study, we focus on the hierarchical structure formations by fabricating hierarchical MNHS that meet the boiling heat transfer requirements mentioned above. In particular, we propose a simple fabrication process using two-step silicon etching: silicon deep trench reactive ion etching (DRIE) for microstructure fabrication and electroless silicon etching for nanowire formation. These processes are feasible and robust. In particular, the electroless silicon etching process enables uniform nanowires to be readily fabricated over a large area at room temperature, without any catalysts or templates [18, 19, 20]. By this simple technique, we fabricated wafer-scale hierarchical MNHS made up of micropillars/cavities covered with uniformly grown nanowires. By controlling the removal of natively coated polymeric passivation layers during DRIE (or Bosch process) , we obtained various combined structures by two-step silicon etching process. Especially, by removing the polymeric passivation layers which induce the conglomeration of nanowires at the boundary of micropatterns and thus make hydrophobic surface by prohibiting the cooling agent from spreading or wicking, hierarchical MNHS can readily serve to pump the water cooling agent to the surface due to the superhydrophilic characteristics. We validated that the surface wettability of those surfaces with hierarchical MNHS by measuring the surface contact angle for deionized water. Based on our results, we believe that the surfaces with hierarchical MNHS may be candidates for boiling heat transfer applications.
Sample preparation: top-down SiNWs
MNHS fabrication using two-step silicon etching process
For the characterizations in this study, FE-SEM images were taken by JEOL-JSM-6700F scanning electron microscope. In addition, determining the polymeric residuals on the sidewalls of MNHS were also performed by energy dispersive spectrometer (EDS) equipped with the same SEM.
Contact angle measurement
All measurements of surface contact angle were conducted using KSV CAM-200 (KSV Ins.). The value of contact angle on each fabricated substrate was automatically calculated based on the calibrating program, KSV Contact Angle Measurement System. We used DI water droplet having volume of 2 μl and captured droplet images with frame interval of 2 ms using CCD camera with resolution of 512 × 480 pixels. The presented values of contact angle in this article are averaged value obtained by measurements more than three times on the same substrate but not on the same local spot.
Results and discussion
Micro-nano hybrid structures: micropillars and nanowires
Micro-nano hybrid structures: microcavities and nanowires
However, nanowires are locally formed at the bottoms of the microholes. As the cross-sectional view of Figure 5b shows, the nanowires were fabricated in the center region of the bottom surface, but do not appear on the surface near the sidewalls. Additional fabrications were carried out, but very little uniformity, symmetry, or repeatability was noted in the fabricated nanowires on the bottom surfaces. During the electroless etching process, we have often observed that when the etching solution is poured over the silicon substrate, air bubbles are initially formed on the square cavity patterns. As the etching continues for over an hour, these bubbles may decay. It may be difficult for the etching solution to make contact with the bottom surface of the microcavity, and for silver ions to adhere to that surface. Moreover, even if silver ions initially manage to attach themselves to the surface, as the etching progresses, it becomes increasingly difficult to replenish the etching solution to etch the oxidized silicon layer (SiO2) under the attached silver particles on the silicon through the silver dendrites that cling to the surface and spread over the whole silicon substrate. During the electroless silicon etching process, we did not use any artificial stirrer or sonicator to mix and supply the solution near the etching surface or to destroy the initial air bubbles inside the microcavities.
Surface wettability and potential for boiling applications
In the light of microscale structures (not nanoscale ones), for example, r should be increased by increasing h then the surface would be more hydrophilic by decreasing surface contact angle. In addition, when the spacing between the pillars (b) decreases on structures with fixed width (a) and height (h), contact angle also decreases due to the increased roughness factor.
where θc and φS indicate the criteria contact angle to induce an intermediate condition between the water spreading and the imbibition through the surface (θ = 0 and θ < π/2, respectively), and the solid fraction remaining dry, respectively. When we assume that the nanowires with 10 μm length have the width of an edge and the distance between nanowires are both 100 nm, the rough surface with the nanowires readily satisfies the criteria for wicking and thus leads to superhydrophilic characteristics.
Contact angle (θ) measurements for the MNHS.
We fabricated hierarchical MNHS using a two-step silicon etching process, consisting of dry etching (DRIE or Bosch process) for micropattern formation and electroless silicon wet etching for synthesizing nanowires. MNHS that lead to geometrically rough and superhydrophilic advantages for boiling application form hierarchical structures having micropillars or microcavities with high-aspect-ratio nanowires. The fabrication process is simple and cost effective, and can readily produce MNHS over the area of an entire wafer. By controlling polymer-removal technique, we can create an artificial surface with microscale nucleation sites favorable for bubble generation in boiling heat transfer. Specifically, MNHS will have a conspicuously large boiling surface area and superhydrophilic characteristics. It will improve the boiling performance over a broad heat flux range by introducing hydrophilic regions, removing hydrophobic sidewall structures that are usually formed in silicon dry etching process. MNHS having superhydrophilic characteristics can supply and refresh water to the surface more collaboratively than one having hydrophobic barrier structures. Thus, the burn-out of the surface can be retarded according to the increasing of a CHF limit and preventing film boiling. In view of these characteristics, boiling performance could be improved by MNHS that are accompanied by the geometrical combination of micro- and nanoscale structures and the superhydrophilic surface wettability. A design study for optimal MNHS, as well as an experimental evaluation of the performance of boiling heat transfer should be topics for future research, and are currently under investigation.
This study was supported by Mid-career Researcher Program through NRF Grant funded by the MEST (No. 2011-0000252). The author, B. S. Kim is grateful for the Seoul Science Fellowship by Seoul Metropolitan Government.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.