Article pubs.acs.org/Langmuir
Microcones and Nanograss: Toward Mechanically Robust Superhydrophobic Surfaces Vitaliy Kondrashov and Jürgen Rühe* Department of Microsystems Engineering - IMTEK, Chemistry and Physics of Interfaces, University of Freiburg, 79110 Freiburg, Germany ABSTRACT: We describe the generation of mechanically robust superhydrophobic surfaces, which carry a hierarchical roughness that is composed of silicon microcones and silicon nanograss. Both micro and nanostructures were fabricated using mask-free dry etching processes. The microcones were obtained utilizing a cryogenic deep reactive ion etching (DRIE) process run in the overpassivation regime. By varying process parameters, surfaces with different microcones geometries and densities were achieved. The nanograss was fabricated using a modified DRIE process with alternating etching and passivation cycles (‘BOSCH process’). All surfaces were covered with a layer of a fluorinated film so that superhydrophobic structures resulted. Depending on microcone geometry and density, the advancing contact angle ranged between 170° and 180°, and roll-off angles of 10 μL drops between 30′ (0.5°) and 6° were observed. The samples were exposed to varying shear loads, and the changes in the morphology were recorded by using electron microscopy. The wetting angles of the mechanically challenged surfaces were recorded and correlated with the mechanical properties of the samples.
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INTRODUCTION In the past decade, a large number of reports have been published on strategies on how to fabricate superhydrophobic (SH) and self-cleaning surfaces.1−3 It is commonly hoped that SH surfaces may have many practical applications in the area of textiles,4,5 for self-cleaning paints,6,7 laminar flow enhancement,8−10 antifogging,11 and anti-icing12−14 applications to name just a few examples. The term “superhydrophobic” is frequently used for surfaces, which have static water contact angle above 150° and low contact angle hysteresis (CAH) resulting in low roll-off angles.3 To be superhydrophobic, the surface must carry micro- and/or nanoscale roughness features that are coated with low energy material.1−3,15 In such cases, the CAH can be very low and even reach values below 1°, leading to extremely facile water roll off.16−19 In contrast to this, surfaces that carry microstructures only can also be very hydrophobic, but show typically a comparably higher CAH due to contact line pinning to the microstructures edges.2,3,16 One of the major drawbacks of nanostructured surfaces is that they are extremely fragile, especially against shear, which strongly restricts the use of nanostructured superhydrophobic surfaces under almost all practical conditions.16,20−22 Even rather low mechanical stress can lead to partial or full removal of the nanostructures, which leaves an unprotected, usually hydrophilic surface behind, which increases the contact area between surface and water and thus causes a stronger pinning and a rise of the CAH.16,21 Especially nanostructures with high aspect ratios are extremely sensitive to any impacting forces.16 Several publications show that durable superhydrophobic surfaces can be achieved upon integration of functional, but © 2014 American Chemical Society
fragile nanostructures with more mechanically stable microstructures, hence creating a topography with at least two hierarchical levels.5,16,17,20−22 The main idea behind this approach is that microstructures resist the mechanical load while the unaffected nanostructures render the surface superhydrophobic. Jung et al.22 introduced a hierarchical surface, where microstructures were fabricated from an epoxy resin and nanostructures were obtained either using spray coating with multiwall carbon nanotubes (CNT) or by the selfassembly of a so-called Lotus wax. A waterfall/water jet test proved that hierarchical structures with CNT showed much better stability of the superhydrophobic properties when compared to nanostructures. Groten et al.16 presented a study on surfaces that carry both silicon microposts and silicon nanograss. To obtain such surfaces, first 30 μm high silicon microposts with varying size (9−32 μm) and distance (8−64 μm) were fabricated on silicon wafers by using standard lithographic techniques. Then, the microstructured surfaces were subjected to a modified dry etching process to generate the silicon nanograss both between and on the top of the microposts. Finally, the surfaces were covered in a photochemical process with a surface-attached monolayer of a perfluoroacrylate polymer, which resulted in a superhydrophobic surface with a contact angle hysteresis (CAH) value close to 0°. It was found that the dual scale roughness features greatly improved the resistance against shear induced wear compared to nanograss surfaces only. However, Received: January 31, 2014 Revised: March 6, 2014 Published: March 16, 2014 4342
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they also stated that although such a study allowed to understand the basics of the influence of wear in the wetting of superhydrophobic surfaces, for practical applications, postshaped surfaces are not ideal, as they are prone to breakoff at the base of the post, and they suggested to use for practical purposes a combination of nanostructures and coneshaped microstructures. Xiu et. al20 presented a microstructured silicon surface having a two-tier roughness. First, they fabricated silicon pyramids through a standard KOH etching process. Second, they used Au nanoparticles and HF/H2O2 etching, to generate nanoroughness features on the pyramid walls. As the last step, a surface fluorination was performed to obtain a superhydrophobic surface. The mechanical durability was tested by moving the sample on nonwoven wipers under a pressure of 3.5 kPa. Although the application of such load resulted in an increase of contact angle hysteresis, the surface still remained water repellent. However, the fabrication process is rather complicated and requires masking steps for micro- and nanostructure formation. Chen et al.23 have combined silicon nanograss with micropyramids obtained by photolithography and potassium hydroxide (KOH) etching. Dussart et al.24 fabricated silicon microcones in a maskless process. This was achieved by using a cryogenic Deep Reactive Ion Etching (DRIE) process in which the substrate temperatures as well as the flow rates of etching and passivation gases were optimized to give the desired structures. It was stated that at extremely low temperatures the etch rate depends on the crystal orientation, which results in preferential etching along the ⟨111⟩ direction and results in the generation of the microstructure slope. However, no explanation was given of how the masks for microcones were formed and how to control the size and density of microstructures. De Boer et al.25 described that self-assembled masks can also be created by the presence of alumina particles sputtered from the walls of the vacuum chamber. However, it is not known whether these masks lead to the formation of nanograss or microcones and how this process can be controlled in a simple way. In this study we present a process for the mask-free generation of superhydrophobic silicon surfaces with a dualscale roughness, namely, surfaces consisting of silicon microcones and silicon nanograss. We first elucidate the mechanism for the generation of the microcones in a cryogenic deep reactive ion etching process, executed in the overpassivation regime. Second, we add to the generated microcones in all of the surrounding area silicon nanograss, which was obtained by using a modified DRIE process with alternating etching/ passivation cycles (BOSCH process).19,26 In order to characterize the mechanical stability of the obtained dual scale surfaces, wear tests were performed. The influence of the application of shear stress on the topography and wetting properties for both hierarchical and nanograss-only surfaces are discussed.
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Figure 1. Fabrication of mechanically robust superhydrophobic surfaces. Schematic depiction of the cryogenic DRIE reaction chamber and the obtained silicon microcones (left), schematic depiction of the reaction chamber of the BOSCH process and the obtained superhydrophobic silicon nanograss (right), silicon microcones combined with nanograss, fabricated by consecutive cryogenic DRIE and BOSCH processes (bottom). overpassivation regime. In a typical experiment, 45 sccm of SF6 and 14 sccm of O2 were simultaneously applied when the electrode temperature was kept at −105 °C. In order to achieve homogeneous temperature distribution along the wafer, helium backside cooling at a pressure of 13 mBar was applied. The plasma was generated using 700 W coil power at 0.013 mBar pressure. To direct ionic species to the wafer, a capacitive coupled plasma (CCP) source was used. The microcones can be fabricated either by using radio frequency CCP source (RF CCP), operating at 13.56 MHz or a 380 kHz lowfrequency pulsed CCP source (pulsed LF CCP). Reproducible results were achieved by using CCP powers higher than 3 W in the RF and 1 W (3 ms pulse length at 25% duty cycle) in the pulsed LF regime. The process time was 30 min. When variations in microcone size and density were desired electrode temperature, SF6/O2 flow ratio and coil power were varied. Silicon nanograss was fabricated, using the modified “BOSCH process”, where repeated alternation between etching and passivation steps leads to etching of high aspect ratio structures (Figure 1, right). This process was carried out in an ICP reactor (Multiplex from ST Systems, U.K.). The Nanograss recipe was developed according to the “black silicon method”.19,26 To achieve reproducible nanograss formation, the duration of the passivation step was steadily increased, while the duration of etching step was kept constant, until the overpassivation regime was reached and nanograss appeared. In the etching step, 120 sccm of SF6 gas was applied for 5 s. In the passivation step, 120 sccm of C4F8 was applied for 7 s. The coil power of 600 W and platen power of 21 W (13.56 MHz) at a pressure of 0.019 mBar
EXPERIMENTAL SECTION
Dual-Scale Roughness Fabrication. Dual-scale roughness surfaces were fabricated in consecutive mask-free dry etching processes as shown in Figure 1. For all fabrications, bare (100) oriented silicon wafers were used. First, all wafers were cleaned in conc. H2SO4/H2O2 (30%) 3:1(Caro’s acid) and dipped into hydrofluoric (HF) acid to remove the native oxide. The microcones were fabricated using a Cryogenic Deep Reactive Ion Etching (DRIE) process (Figure 1, left), which was carried out in an inductively coupled plasma (ICP) reactor (Plasmalab 100 from Oxford Instruments, U.K.) in the strong 4343
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Figure 2. Microcones sizes and densities versus (a,b) O2 flow rate, (c) ICP power and (d) chuck temperature (standard process conditions: ICP power 700 W, LF CCP 1 W, SF6 = 45 sccm, O2 = 14 sccm, T = −105 °C, 10 mTorr, process duration 30 min). were used for both etching and passivation steps. In total, 150 cycles were needed for reproducible nanograss fabrication. The etching of silicon nanograss on the wafers with already existing microcones led to the combination of both roughness types and resulted in dual-scale roughness surface as shown in Figure 1, bottom. During the Bosch Process the surfaces were coated with a fluorinated plasmapolymer layer originating from the passivation gas, C4F8, so that the finally obtained micro- and nanostructured layers were superhydrophobic. Wear Testing. The wear tests were performed using a rheometer (MCR Anton Paar GmbH, Austria). A metal disk with a diameter of 2 cm was first brought into contact with the sample and a normal force of 1, 2, 5, 10, or 20 N was applied for 30 s. During the next 30 s, the disc was rotated with a rate of 10 rotations/min, in this way introducing a controlled shear stress to the structures. During the wear test, friction causes a moment transfer from the silicon substrate, which carries the roughness features, onto the disk. This friction process was followed using a built-in torque sensor. SEM. SEM images were made using a Supra 60 VP electron microscope (Carl Zeiss AG, Germany) at an electron acceleration voltage of 3 kV and on a DSM 962 (Carl Zeiss AG, Germany) at a voltage of 5 kV and 10 kV. Contact Angle and Roll-off Angle Measurements. Advancing and receding contact angles were measured using an OCA20 measurement system (Dataphysics GmbH, Germany). The angles were recorded during adding/withdrawing water at a rate of 0.1 μL/s introduced by built-in syringe pump. The roll-off angles were measured on 10 μL drops using the same system equipped with a rotating sample stage. All measurements were reproduced five times on different sample spots.
island-like passivation spots left. From the literature some conditions are known, which lead to inhomogeneous thickness of SiOxFy films.25,27 One of the reasons for this is nonuniform substrate cooling as the etch rate decreases strongly with decreasing temperature. Such temperature variation could be caused by incomplete contact to the cooling element due to slight wafer bending caused by mechanical clamping.27 Other possibilities are the presence of particles on the cooling elements or wafer damages, which can also cause local temperature variances, which then result in film heterogeneities. How strong the overpassivation actually is depends on the ratio between the passivating and etching gas, i.e., on the SF6/O2 flow rates ratio and the substrate temperature. According to these parameters, different passivation regimes can be distinguished. In the strong overpassivation regime (SF6/O2 flow rates: 45/ 14 sccm, electrode temperature: −105 °C), a rather thick inhibitor layer results. If under such conditions the etching with low energy ionic species is performed (amplitude of RF CCP power is 0 or 1 W), the removal of the passivation layer is relatively homogeneous. If some initial roughness of the SiOxFy film exists or the etching is not quite homogeneous, during etching of the passivation layer, before it is completely etched away and the silicon surface is reached, tiny passivation spots remain behind. These areas act as nanomasks for further silicon etching, which results in silicon nanograss formation.24 Under such conditions no microcone formation is observed. When the RF CCP power amplitude is increased to 3 or 5 W, while all other parameters are kept constant, etching is much stronger. It can be assumed that due to the higher etch rate much fewer passivation spots remain, which have accordingly a larger distance from each other and further etching results in microcones formation. Along with the microcones, some silicon nanograss is formed on the edges of the wafers and on occasional spots randomly distributed on
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RESULTS AND DISCUSSIONS Microcone Fabrication. Silicon microcones were fabricated using a DRIE process at cryogenic temperatures operated in the strong overpassivation regime. Under such conditions, the growth of a passivation layer dominates over etching, and a patchy SiOxFy passivation film is formed.24,25 To create the etch masks for the subsequent generation of microcones, the larger part of the SiOxFy layer must be removed with only sparse, 4344
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thermally activated process, it is not surprising that the temperature also has a large influence on the obtained structures. Only within a low-temperature window, where the etch rate is sufficiently low so that the process just moves into the overpassivation regime, cone formation is observed. Above that window no cones, below it only very small cones are observed (Figure 2d). It should be noted that at the highest density the biggest structures were also obtained, which can make the surface fabricated using such parameters potentially more robust (Figure 3a,b).
the wafers. This is due to formation of a nonuniform passivation film thickness along the wafer. The thickest film is formed on the coldest spots, which are those where the sample is mechanically clamped to the cooling stage. When the RF CCP power amplitude is further increased to 8 W, only microcone formation occurs, without the formation of any nanograss spots. Neither microcones nor nanograss can be observed when the power amplitude is further increased to 12 W. Under weaker overpassivation conditions (SF6/O2 flow rates: 45/14 sccm, electrode temperature: −85 °C), an ion self-bias generated by 1 W of RF CCP power was already enough to form nanomasks for microcones etching. Under these conditions the density of the microcones increased slightly from wafer edge to the center, i.e. from the coldest to the warmest area (caused by wafer bending due to mechanical clamping). However, when a pulsed LF CCP power was applied, the conditions for the formation of microcones were found to be quite different. In these experiments, we applied an LF CCP power with a 3 ms pulse length and a 25% duty cycle with an amplitude of 1 W. Here, microcones formed only at strong overpassivation conditions (SF6/O2 flow rates: 45/14 sccm, electrode temperature: −105 °C), which, when an RF CCP power of the same amplitude was applied, led exclusively to black silicon formation. Apparently, the application of a pulsed ion self-bias led to a less homogeneous removal of the passivation layer. It is found that using pulsed LF CCP power control of size and density of the microcones is more easily achieved than using RF CCP power. Figure 2 shows the influence of various process parameters on the cone size and density. From Figure 2a, it is seen that the size of microstructures decreases strongly when the flow rate of oxygen rises, and thus increasing O2/SF6 flow rate ratio (as the SF6 flow rate remains constant). The higher passivation rate requires that more time is needed to overcome the SiOxFy passivation layer and to start to etch silicon so that less material is etched away in a given time frame of the experiment. Thus smaller microcones result. The ICP power controls the amount of ionic species in the plasma. The higher coil power is, the more ions get involved in etching, and the etch rate rises so that larger, microcones are obtained (Figure 2c). When the microcone density is viewed, a somewhat different picture is obtained (Figure 2b−d). When the low concentration of oxygen causes a low rate of passivation and the reaction is essentially run out of the overpassivation regime, no cones are observed at all (Figure 2b). As the oxygen contents and thus the passivation rate are increased, the reaction moves into the overpassivation regime, which means that at some critical value first islands of passivation film remain on the surface and, as a result of this, the first cones appear. However, right at the borderline to the overpassivation regime, the cone density is somewhat hard to control. Under these conditions even small fluctuations will cause large differences in the distance between the remaining passivation islands, as this is the case at any critical transition. With increasing oxygen flow rate the density of the cones increases, until at some critical flow rate value the layer becomes thicker and more homogeneous and the cone density starts to decrease again. The same argument holds for the maximum of the cone density, which is observed when the ICP power is varied (Figure 2c), only that with increasing ICP power the etch rate is varied. As the etching of silicon and silicon oxides is a
Figure 3. SEM images of a silicon surface with microcones (a) overview (b) close up image, (c−e) images of individual microcones with different opening angles, which were fabricated using different SF6/O2 ratios given in the legend. Scale bars: 100 μm (a,b) and 5 μm (c−e).
It was observed that the opening angle of microcones angles typically varied from 27° to 60° (Figure 3c-e). The opening angle is strongly correlated with the SF6/O2 ratio. A high ratio results in faster lateral etching, which in turn lowers the opening angle. Therefore, the etching does not exactly follow the crystal planes in all cases as it was previously reported.24 Wear Test Results. In order to test the mechanical stability of the obtained samples, they were exposed to defined shear stress, and the topography change of the resulting surfaces was recorded. In the wear test experiments, a defined normal load was applied, and the sample was exposed to shear. After the wear experiments it was analyzed how topography and wettability were changed with respect to the applied loads. It should be noted that the pressure during the shear experiments could not be calculated directly, since the density of the structures varied across the sample and the contact area of the load with the micro- and nanostructures changed with time as some breaking of the micro- and nanostructures occurred. Therefore, below, only the forces applied to the structures will be discussed. Investigated Surfaces. Four surfaces were exposed to wear tests: silicon nanograss (NS) and three combined surfaces containing both microcones and nanograss with different microcone sizes and densities (MC_LD (MicroCones_LargeDense), MC_SS (MicroCones_SmallSparse), MC_SD (MicroCones_SmallDense)). The opening angles of microcones were in the range of 35° (±2°). The respective microcone sizes and 4345
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to the fact that the debris formed by the shaved off microcones grinds into the surface. A completely different reaction to the shear load, however, was observed when higher density cone structures were investigated. To this firstly MC_LD surface was tested, which featured the biggest microcones and had an intermediate cone density (Figure 4c). After application of a load of 20 N, the microstructures were still in place. However, the shear force let to a truncation of the cones. It was found that after application of shear forces of 10 N, the microcones lost on average about 30% of their height, and after application of 20 N, the loss was found to be about 50%. In both cases, the neighboring nanograss remained unaffected. When the cone density was even higher (sample MC_SD) even after application of a force of 10 N, most of microstructures seemed to not be affected at all by the shear; only a few broken structures were found. However, when a load of 20 N was applied, all microstructures were destroyed. The nanograss mostly remained intact, but we also found many spots with broken nanoneedles (Figure 4d). The interplay between structural stability, load, structure size and structure density is rather complex. The true contact area depends on these parameters and changes within the course of the experiment as some part of the micro- or nanostructures is removed and debris is formed. In order to understand the changes of the topography of the surface caused by the wear, the mechanical moment transferred from the substrate onto the metal disc sliding on top of it was followed during the wear experiment using a build-in torque sensor (Figure 5). The friction or, more precisely, the mechanical moment transfer in these experiments depends on the applied force and the true contact area between metal disc and test sample. In Figure 5a, where a pure nanograss surface is sheared, it can be seen that (a) the moment transfer is rather high, in agreement with the large contact area between nanograss and the shearing steel ring, and (b) that the slopes of moment changes induced by shearing of the nanograss were always negative. In addition, the slope values decreased as the force was raised. This can be explained by a permanent decrease of the contact area, caused by breaking of some of the structures. A somewhat similar situation was observed during the wear tests with the MC_LD surface, which featured the biggest microcones (Figure 5b). In this case, however, the moment transfer during application of a load of 1 N in the shear experiment was much lower than one measured when the same shear force was applied to the NS surface. This shows that the shearing surface was in contact only with the top of the microcones, whose density is much lower than the density of nanograss, which in turn results in lower friction and lower torque. The slope values of the friction occurring at a load of 1 and 5 N are almost equal, which shows that under such forces, the shear effect on surface topography was not very strong. However during application of loads of 10 and 20 N, the gradient of the slopes decreased considerably, clearly indicating severe microstructures damages. A qualitatively different picture was obtained, when wear tests were performed on MC_SD surfaces (Figure 5c), where the densest distribution of microcones was present. Here during application of a load of 5 N, the slope was negative, at 10 N was almost flat, and at 20 N was even slightly positive. This change from negative to positive slope during application of an increasingly stronger shear force clearly indicates that the contact area increased with increasing force. At a shear load of 20 N, most of microcones were removed, and the underlying
densities are shown in Table 1. On all tested surfaces, the nanograss had a height of 10 μm, bottom diameter of 200 nm, and density of >1 000 000 needles per mm2. Table 1. List of Samples with Combined Microcone and Nanograss Structures, Their Sizes, and Densities sample name
microcone height μm
microcone diameter μm
microcones density per/mm2
MC_LD MC_SS MC_SD
35 20 22
28 22 25
241 86 355
The hierarchical samples were chosen such that the influence of microstructures size and density on surface mechanical durability can be separately investigated. Figure 4 shows SEM
Figure 4. SEM images of various superhydrophobic surfaces before and after performing the wear tests with forces given in the figure (a) nanograss surface (NS) (b,c,d) microcone and nanograss combinations: MC_SS, MC_LD, MC_SD, respectively. The description of the samples is given in Table 1. Scale bar: 5 μm (a), 20 μm (b−d).
images of the samples before and after application of forces with an amplitude of 5 N, 10 and 20 N. In Figure 4a, it can be seen that application of a force of 10 N was enough to break most of the nanoneedles on the NS surface. After application of a force of 20 N no remaining nanograss needles could be found. When a surface with the lowest microcones density (MC_SS) was exposed to shearing, the problem was even exacerbated. Already after application of a force of 5 N, most of the microcones were removed; they were completely ripped off, so that even the footprints of the cones were no longer visible in the SEM images. This is probably due to the fact that the contact pressure on a very small number of cones becomes extremely high, indeed higher than what the structures can sustain. The nanograss, which was located closely adjacent to the microcones, was also partially destroyed. The rest of the needles, however, seemed largely unaffected. After applying a load of 10 N during the shearing process strong damage of the surface structures was observed (Figure 4b). This might be due 4346
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Figure 5. Time dependence of the moment induced on a steel disk rubbing on the different surfaces during wear tests. The measurements were performed during application of 1, 5, 10 and 20 N shear forces on the nanograss and hierarchical surfaces described in Table 1: (a) nanograss (NS); (b,c,d) various microcone and nanograss combinations: MC_LD, MC_SD, MC_SS, respectively. “S” indicates the slope of a linear fit of the measurements; positive values indicate an increase of friction with time; negative values indicate a decrease.
Figure 6. Influence of the shear load on (a) advancing, (b) receding contact angles (CA), (c) CA hysteresis, and (d) roll-off angle of the different superhydrophobic surfaces studied in Figures 4 and 5.
nanograss was partially reached. When the sample with the smallest and most sparsely distributed microcones is viewed at a shear load of 10 N, a drastic increase in torque within the first 5 s of the experiment was observed. The absolute values of the torque reached the level of the moments measured on NS at 10 N (MC_SS, Figure 5d). At lower loads, the momentum changes, however, were small and had slopes that were in close
agreement with those on the other microcone surfaces, indicating that no contact with the nanograss occurred. When the SEM images of shear-exposed, i.e., worn, surfaces and the moment measurements during the shear experiments are correlated with each other, it can be concluded that microcones can significantly improve the mechanical durability of the nanograss-based surfaces by preventing mechanical 4347
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Again, the slight increase of the RA as well as of the CAH can be explained by pinning induced through the generation of hydrophilic areas, which are caused by truncation of the cones. Surprisingly, MC_SD showed the least changes of the wetting properties after strong shear exposure of the surfaces. It shows a stable RA of average 6.3° after exposure of the sample to shear forces of 5 N, 10 and 20 N. However, according to the SEM images (Figure 4d), the microcones were not worn down as big microcones on MC_LD surface, but almost completely shaved away when strong enough shear load was applied. Surprisingly, after application of a 20 N load, where no remaining cones were found, and the nanograss was not seriously damaged and preserved low CAH. It could be hypothesized that the broken off cones are still present in some part of the shearing system and still protect the nanograss from wear as long as they are not completely removed from the contact zone. Thus it can be concluded that microcone-decorated surfaces show a better preservation of superhydrophobicity after exposure to high shear loads in comparison to the surfaces coated with nanograss only. A prerequisite for this, however, is that the cone density is not too low, as very low cone densities lead to excessive contact pressures. Therefore, to design an optimal surface, which has good shear resistance and superhydrophobic properties, a suitable microcones size and density must be chosen. Small and sparsely distributed microcones give surfaces with wetting properties that are rather close to those of nanograss. However, such structures lose the superhydrophobicity, even when moderate shear loads are applied. Thus, in such samples, the excellent wetting properties are combined with only moderate wear protection. Very dense/large microcones, on the other hand, give excellent protection against shear-induced wear, however, at the price of a slight increase of the contact line pinning so that compared to the nanostructured surfaces only moderate RA between 6° and 13° are observed.
contact between the nanograss and shearing surface. Size and density of the microcones are determining factors for the ultimate surface mechanical stability. During shearing of the surfaces, the cones take the brunt of the force. If a strong shear force is applied and the cone density and/or the size of them are rather small, they are very strongly damaged or even in many cases completely destroyed. However, if they are large in size and/or large in number, high shear forces lead only to a truncation of the cone. If an excessive force is applied, the tip of the cones breaks off. After loss of the tip, however, the contact area is strongly increased (and the shear pressure thus accordingly decreased). Thus the cones can now to support an even higher load, so that the damage is self-limiting. The more cones broken off, the higher the force required to shear off some more material. This wear behavior of the cone decorated nanograss surfaces, however, also has an important influence on the wetting properties of the surfaces against water, (which will be the only liquid discussed in the following). Pure nanograss coated with a fluoropolymer on top is extremely hydrophobic and has a contact angle (CA) of approximately 179° and very low contact angle hysteresis (CAH) (the difference between advancing and receding CA), so that the roll off angles (RA) are very small and assume values well below 1° (Figure 6). However, after application of even a moderate force, the contact angle hysteresis and the roll off angles are strongly increased as the nanostructures are worn away. After application of a shear force of 1 N, the roll-off angle is already increased to 14°, and after application of 5 N, no more roll-off is observed. When the cone-decorated surfaces are viewed (Figure 6), it is seen that the initial CAH and RA of these microstructure containing surfaces are somewhat higher than that of the pure nanograss, in agreement with the expectations.18 This is due to pinning of the drop contact line to the edges of microcones. However, the roll-off angles are between 3° and 8°, which is still very low, compared to flat surfaces or surfaces completely covered by microstructures, which have a similar surface chemistry.28 Generally, the higher the cone density and the larger the cone sizes the more they dominate the wetting behavior. (MC_LD and MC_SD samples). After application of a shear load of 1 and 5 N, the CAH of all tested microstructured surfaces slightly increased, due to partial rupture of the tip of the microstructures. As the truncation of the cones occurs, in the ruptured areas fresh SiO2 is generated, which is quite strongly hydrophilic. Exposure to those hydrophilic areas causes additional contact line pinning. However, compared to the nanograss surface this increase was much smaller, and after application of 5 N, the CAH of the mechanically challenged microcone/nanograss combination still stayed within 10° and was much lower than that of the pure nanograss treated under the same conditions (CAH of 27°). After application of 10 N shear load to the sample MC_SS, which lead to microcone and nanograss removal (Figure 4b), the superhydrophobicity was completely lost, and the CAH increased to 97°. This indicates that the contact lines of the drops are strongly pinned. However, for samples MC_SD and MC_LD, it is observed that they survive even a shear load of 20 N, without losing their superhydrophobic properties. The RA of MC_LD increased slightly with increasing strength of the mechanical treatment: 8.7° after 5 N, 9.3° after 10 N, and 14° after application of 20 N shear load. However, the material remains strongly hydrophobic even after exposure to the strongest mechanical stress.
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CONCLUSIONS In this study, we described how to fabricate surfaces containing simultaneously silicon microcones and silicon nanograss in a very reproducible way. The fabrication of both micro- and nanostructures does not require any prerequisite patterning step, e.g. photolithography. The microcones fabrication process is based on a low-temperature deep reactive ion etching step, which is run in the overpassivation regime. The height and diameter of the microcones depends on the relative rates of etching and passivation and increases with increasing surface temperature and ICP power. The microcones density can be controlled via adjusting the electrode temperature and the SF6/ O2 ratio; however, the dependence is not linear, and at certain sets of parameters, pronounced maxima of the cone density occur. In a second DRIE step, then the remainder of the surfaces is covered with nanograss in a standard BOSCH process. The thus obtained dual-scale surfaces, which combine microcone and nanograss structures, show a greatly improved mechanical durability, especially against shear, in comparison with surfaces containing nanostructures only. The samples with the biggest microstructures and those that possessed the highest density of microcones have shown excellent wear protection. When the shear stress becomes too high, the cones are truncated. This, however, increases the contact area and thus decreases the shear stress, so that further wear is prevented. 4348
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capacitive coupled plasma; RF radio frequency; LF lowfrequency
The improvement of the surface wear properties by following such a dual-scale concept becomes also evident when the wetting properties of the obtained surfaces are viewed. While nanostructure-only surfaces already lose the superhydrophobic properties completely when a rather moderate shear stress is applied, the wetting behavior of the dual scale surfaces is under the same conditions preserved, and under most conditions the wetting behavior is more robust against shear-induced mechanical wear. If the size and surface density of the microstructures are not too large, the wetting properties are still dominated by the nanostructures. In such a situation, the microcones carry the brunt of the mechanical stress and protect the nanostructures from wear. With increasing cone density, the protection surface against shear-induced wear improves while the contact line pinning slightly increases. This way, superhydrophobic, yet wear resistant, surfaces can be generated. When extremely low CAH and RAs are desired even during or after exposure to average mechanical stresses, smaller microstructures with higher density should be used. However, when high shear loads are likely to occur, bigger microstructures should be generated. To further enhance the wetting properties of mechanically challenged surfaces, one has to address the problem that, after breaking of some fraction of the cones, the stump areas remaining behind are hydrophilic. To mitigate this effect onto the superhydrophobic properties of the substrates, one could use a material that has a better resistance against breaking, change the opening angle of the cones, or generate nanostructured surfaces carrying microcones from an intrinsically hydrophobic material, so that even after cone truncation a hydrophobic spot remains.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*Mailing address: University of Freiburg Department of Microsystems Engineering- IMTEK Chemistry and Physics of Interfaces Georges-Koehler-Allee 103 D-79110 Freiburg, Germany. Tel.: +49 761 203 7160. Fax: +49 761 203 7162. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Prof. Dr. Reinecke, HSG-IMIT and IMTEK, for the opportunity to use the dry etching facilities at HSGIMIT (Villingen-Schwenningen, Germany) and the SEM at IMTEK. We also appreciate the very helpful assistance of Johannes Auber, Karsten Kurschat, and other HSG-IMIT clean room staff in the Cryogenic DRIE processes. We would like to thank Tobias Amann from Fraunhofer IWM for his help in performing wear tests and Armin Baur, IMTEK, for SEM pictures. This work was in part funded by the Federal Ministry of Education and Research (BMBF), Germany; project “Nanotau”, project-ID 16SV5348.
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ABBREVIATIONS SH superhydrophobic; CA contact angle; ACA advancing contact angle; RCA receding contact angle; CAH contact angle hysteresis; RA roll-off angle; CNT carbon nanotubes; AFM atomic force microscope; SEM scanning electron microscope; KOH potassium hydroxide; HF hydrofluoric acid; DRIE deep reactive ion etching; ICP inductively coupled plasma; CCP 4349
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(25) de Boer, M. J.; Gardeniers, J. G. E.; Jansen, H. V.; Smulders, E.; Gilde, M.-J.; Roelofs, G.; Sasserath, J. N.; Elwenspoek, M. Guidelines for etching silicon MEMS structures using fluorine high-density plasmas at cryogenic temperatures. J. MEMS 2002, 11, 385−401. (26) Jansen, H. V.; de Boer, M.; Legtenberg, R.; Elwespoek, M. The black silicon method - A universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control. J. Micromech. Microeng. 1995, 5, 115−120. (27) Jansen, H. V.; de Boer, M. J.; Unnikrishnan, S.; Louwerse, M. C.; Elwenspoek, M. C. Black silicon method X: A review on high speed and selective plasma etching of silicon with profile control: an in-depth comparison between Bosch and cryostat DRIE processes as a roadmap to next generation equipment. J. Micromech. Microeng. 2009, 19, 1−41. (28) Dorrer, C.; Ruhe, J. Advancing and receding motion of droplets on ultrahydrophobic post surfaces. Langmuir 2006, 22, 7652−7657.
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Microcones and Nanograss: Toward Mechanically Robust Superhydrophobic Surfaces Vitaliy Kondrashov and Jürgen Rühe* Department of Microsystems Engineering - IMTEK, Chemistry and Physics of Interfaces, University of Freiburg, 79110 Freiburg, Germany ABSTRACT: We describe the generation of mechanically robust superhydrophobic surfaces, which carry a hierarchical roughness that is composed of silicon microcones and silicon nanograss. Both micro and nanostructures were fabricated using mask-free dry etching processes. The microcones were obtained utilizing a cryogenic deep reactive ion etching (DRIE) process run in the overpassivation regime. By varying process parameters, surfaces with different microcones geometries and densities were achieved. The nanograss was fabricated using a modified DRIE process with alternating etching and passivation cycles (‘BOSCH process’). All surfaces were covered with a layer of a fluorinated film so that superhydrophobic structures resulted. Depending on microcone geometry and density, the advancing contact angle ranged between 170° and 180°, and roll-off angles of 10 μL drops between 30′ (0.5°) and 6° were observed. The samples were exposed to varying shear loads, and the changes in the morphology were recorded by using electron microscopy. The wetting angles of the mechanically challenged surfaces were recorded and correlated with the mechanical properties of the samples.
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INTRODUCTION In the past decade, a large number of reports have been published on strategies on how to fabricate superhydrophobic (SH) and self-cleaning surfaces.1−3 It is commonly hoped that SH surfaces may have many practical applications in the area of textiles,4,5 for self-cleaning paints,6,7 laminar flow enhancement,8−10 antifogging,11 and anti-icing12−14 applications to name just a few examples. The term “superhydrophobic” is frequently used for surfaces, which have static water contact angle above 150° and low contact angle hysteresis (CAH) resulting in low roll-off angles.3 To be superhydrophobic, the surface must carry micro- and/or nanoscale roughness features that are coated with low energy material.1−3,15 In such cases, the CAH can be very low and even reach values below 1°, leading to extremely facile water roll off.16−19 In contrast to this, surfaces that carry microstructures only can also be very hydrophobic, but show typically a comparably higher CAH due to contact line pinning to the microstructures edges.2,3,16 One of the major drawbacks of nanostructured surfaces is that they are extremely fragile, especially against shear, which strongly restricts the use of nanostructured superhydrophobic surfaces under almost all practical conditions.16,20−22 Even rather low mechanical stress can lead to partial or full removal of the nanostructures, which leaves an unprotected, usually hydrophilic surface behind, which increases the contact area between surface and water and thus causes a stronger pinning and a rise of the CAH.16,21 Especially nanostructures with high aspect ratios are extremely sensitive to any impacting forces.16 Several publications show that durable superhydrophobic surfaces can be achieved upon integration of functional, but © 2014 American Chemical Society
fragile nanostructures with more mechanically stable microstructures, hence creating a topography with at least two hierarchical levels.5,16,17,20−22 The main idea behind this approach is that microstructures resist the mechanical load while the unaffected nanostructures render the surface superhydrophobic. Jung et al.22 introduced a hierarchical surface, where microstructures were fabricated from an epoxy resin and nanostructures were obtained either using spray coating with multiwall carbon nanotubes (CNT) or by the selfassembly of a so-called Lotus wax. A waterfall/water jet test proved that hierarchical structures with CNT showed much better stability of the superhydrophobic properties when compared to nanostructures. Groten et al.16 presented a study on surfaces that carry both silicon microposts and silicon nanograss. To obtain such surfaces, first 30 μm high silicon microposts with varying size (9−32 μm) and distance (8−64 μm) were fabricated on silicon wafers by using standard lithographic techniques. Then, the microstructured surfaces were subjected to a modified dry etching process to generate the silicon nanograss both between and on the top of the microposts. Finally, the surfaces were covered in a photochemical process with a surface-attached monolayer of a perfluoroacrylate polymer, which resulted in a superhydrophobic surface with a contact angle hysteresis (CAH) value close to 0°. It was found that the dual scale roughness features greatly improved the resistance against shear induced wear compared to nanograss surfaces only. However, Received: January 31, 2014 Revised: March 6, 2014 Published: March 16, 2014 4342
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they also stated that although such a study allowed to understand the basics of the influence of wear in the wetting of superhydrophobic surfaces, for practical applications, postshaped surfaces are not ideal, as they are prone to breakoff at the base of the post, and they suggested to use for practical purposes a combination of nanostructures and coneshaped microstructures. Xiu et. al20 presented a microstructured silicon surface having a two-tier roughness. First, they fabricated silicon pyramids through a standard KOH etching process. Second, they used Au nanoparticles and HF/H2O2 etching, to generate nanoroughness features on the pyramid walls. As the last step, a surface fluorination was performed to obtain a superhydrophobic surface. The mechanical durability was tested by moving the sample on nonwoven wipers under a pressure of 3.5 kPa. Although the application of such load resulted in an increase of contact angle hysteresis, the surface still remained water repellent. However, the fabrication process is rather complicated and requires masking steps for micro- and nanostructure formation. Chen et al.23 have combined silicon nanograss with micropyramids obtained by photolithography and potassium hydroxide (KOH) etching. Dussart et al.24 fabricated silicon microcones in a maskless process. This was achieved by using a cryogenic Deep Reactive Ion Etching (DRIE) process in which the substrate temperatures as well as the flow rates of etching and passivation gases were optimized to give the desired structures. It was stated that at extremely low temperatures the etch rate depends on the crystal orientation, which results in preferential etching along the ⟨111⟩ direction and results in the generation of the microstructure slope. However, no explanation was given of how the masks for microcones were formed and how to control the size and density of microstructures. De Boer et al.25 described that self-assembled masks can also be created by the presence of alumina particles sputtered from the walls of the vacuum chamber. However, it is not known whether these masks lead to the formation of nanograss or microcones and how this process can be controlled in a simple way. In this study we present a process for the mask-free generation of superhydrophobic silicon surfaces with a dualscale roughness, namely, surfaces consisting of silicon microcones and silicon nanograss. We first elucidate the mechanism for the generation of the microcones in a cryogenic deep reactive ion etching process, executed in the overpassivation regime. Second, we add to the generated microcones in all of the surrounding area silicon nanograss, which was obtained by using a modified DRIE process with alternating etching/ passivation cycles (BOSCH process).19,26 In order to characterize the mechanical stability of the obtained dual scale surfaces, wear tests were performed. The influence of the application of shear stress on the topography and wetting properties for both hierarchical and nanograss-only surfaces are discussed.
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Figure 1. Fabrication of mechanically robust superhydrophobic surfaces. Schematic depiction of the cryogenic DRIE reaction chamber and the obtained silicon microcones (left), schematic depiction of the reaction chamber of the BOSCH process and the obtained superhydrophobic silicon nanograss (right), silicon microcones combined with nanograss, fabricated by consecutive cryogenic DRIE and BOSCH processes (bottom). overpassivation regime. In a typical experiment, 45 sccm of SF6 and 14 sccm of O2 were simultaneously applied when the electrode temperature was kept at −105 °C. In order to achieve homogeneous temperature distribution along the wafer, helium backside cooling at a pressure of 13 mBar was applied. The plasma was generated using 700 W coil power at 0.013 mBar pressure. To direct ionic species to the wafer, a capacitive coupled plasma (CCP) source was used. The microcones can be fabricated either by using radio frequency CCP source (RF CCP), operating at 13.56 MHz or a 380 kHz lowfrequency pulsed CCP source (pulsed LF CCP). Reproducible results were achieved by using CCP powers higher than 3 W in the RF and 1 W (3 ms pulse length at 25% duty cycle) in the pulsed LF regime. The process time was 30 min. When variations in microcone size and density were desired electrode temperature, SF6/O2 flow ratio and coil power were varied. Silicon nanograss was fabricated, using the modified “BOSCH process”, where repeated alternation between etching and passivation steps leads to etching of high aspect ratio structures (Figure 1, right). This process was carried out in an ICP reactor (Multiplex from ST Systems, U.K.). The Nanograss recipe was developed according to the “black silicon method”.19,26 To achieve reproducible nanograss formation, the duration of the passivation step was steadily increased, while the duration of etching step was kept constant, until the overpassivation regime was reached and nanograss appeared. In the etching step, 120 sccm of SF6 gas was applied for 5 s. In the passivation step, 120 sccm of C4F8 was applied for 7 s. The coil power of 600 W and platen power of 21 W (13.56 MHz) at a pressure of 0.019 mBar
EXPERIMENTAL SECTION
Dual-Scale Roughness Fabrication. Dual-scale roughness surfaces were fabricated in consecutive mask-free dry etching processes as shown in Figure 1. For all fabrications, bare (100) oriented silicon wafers were used. First, all wafers were cleaned in conc. H2SO4/H2O2 (30%) 3:1(Caro’s acid) and dipped into hydrofluoric (HF) acid to remove the native oxide. The microcones were fabricated using a Cryogenic Deep Reactive Ion Etching (DRIE) process (Figure 1, left), which was carried out in an inductively coupled plasma (ICP) reactor (Plasmalab 100 from Oxford Instruments, U.K.) in the strong 4343
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Figure 2. Microcones sizes and densities versus (a,b) O2 flow rate, (c) ICP power and (d) chuck temperature (standard process conditions: ICP power 700 W, LF CCP 1 W, SF6 = 45 sccm, O2 = 14 sccm, T = −105 °C, 10 mTorr, process duration 30 min). were used for both etching and passivation steps. In total, 150 cycles were needed for reproducible nanograss fabrication. The etching of silicon nanograss on the wafers with already existing microcones led to the combination of both roughness types and resulted in dual-scale roughness surface as shown in Figure 1, bottom. During the Bosch Process the surfaces were coated with a fluorinated plasmapolymer layer originating from the passivation gas, C4F8, so that the finally obtained micro- and nanostructured layers were superhydrophobic. Wear Testing. The wear tests were performed using a rheometer (MCR Anton Paar GmbH, Austria). A metal disk with a diameter of 2 cm was first brought into contact with the sample and a normal force of 1, 2, 5, 10, or 20 N was applied for 30 s. During the next 30 s, the disc was rotated with a rate of 10 rotations/min, in this way introducing a controlled shear stress to the structures. During the wear test, friction causes a moment transfer from the silicon substrate, which carries the roughness features, onto the disk. This friction process was followed using a built-in torque sensor. SEM. SEM images were made using a Supra 60 VP electron microscope (Carl Zeiss AG, Germany) at an electron acceleration voltage of 3 kV and on a DSM 962 (Carl Zeiss AG, Germany) at a voltage of 5 kV and 10 kV. Contact Angle and Roll-off Angle Measurements. Advancing and receding contact angles were measured using an OCA20 measurement system (Dataphysics GmbH, Germany). The angles were recorded during adding/withdrawing water at a rate of 0.1 μL/s introduced by built-in syringe pump. The roll-off angles were measured on 10 μL drops using the same system equipped with a rotating sample stage. All measurements were reproduced five times on different sample spots.
island-like passivation spots left. From the literature some conditions are known, which lead to inhomogeneous thickness of SiOxFy films.25,27 One of the reasons for this is nonuniform substrate cooling as the etch rate decreases strongly with decreasing temperature. Such temperature variation could be caused by incomplete contact to the cooling element due to slight wafer bending caused by mechanical clamping.27 Other possibilities are the presence of particles on the cooling elements or wafer damages, which can also cause local temperature variances, which then result in film heterogeneities. How strong the overpassivation actually is depends on the ratio between the passivating and etching gas, i.e., on the SF6/O2 flow rates ratio and the substrate temperature. According to these parameters, different passivation regimes can be distinguished. In the strong overpassivation regime (SF6/O2 flow rates: 45/ 14 sccm, electrode temperature: −105 °C), a rather thick inhibitor layer results. If under such conditions the etching with low energy ionic species is performed (amplitude of RF CCP power is 0 or 1 W), the removal of the passivation layer is relatively homogeneous. If some initial roughness of the SiOxFy film exists or the etching is not quite homogeneous, during etching of the passivation layer, before it is completely etched away and the silicon surface is reached, tiny passivation spots remain behind. These areas act as nanomasks for further silicon etching, which results in silicon nanograss formation.24 Under such conditions no microcone formation is observed. When the RF CCP power amplitude is increased to 3 or 5 W, while all other parameters are kept constant, etching is much stronger. It can be assumed that due to the higher etch rate much fewer passivation spots remain, which have accordingly a larger distance from each other and further etching results in microcones formation. Along with the microcones, some silicon nanograss is formed on the edges of the wafers and on occasional spots randomly distributed on
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RESULTS AND DISCUSSIONS Microcone Fabrication. Silicon microcones were fabricated using a DRIE process at cryogenic temperatures operated in the strong overpassivation regime. Under such conditions, the growth of a passivation layer dominates over etching, and a patchy SiOxFy passivation film is formed.24,25 To create the etch masks for the subsequent generation of microcones, the larger part of the SiOxFy layer must be removed with only sparse, 4344
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thermally activated process, it is not surprising that the temperature also has a large influence on the obtained structures. Only within a low-temperature window, where the etch rate is sufficiently low so that the process just moves into the overpassivation regime, cone formation is observed. Above that window no cones, below it only very small cones are observed (Figure 2d). It should be noted that at the highest density the biggest structures were also obtained, which can make the surface fabricated using such parameters potentially more robust (Figure 3a,b).
the wafers. This is due to formation of a nonuniform passivation film thickness along the wafer. The thickest film is formed on the coldest spots, which are those where the sample is mechanically clamped to the cooling stage. When the RF CCP power amplitude is further increased to 8 W, only microcone formation occurs, without the formation of any nanograss spots. Neither microcones nor nanograss can be observed when the power amplitude is further increased to 12 W. Under weaker overpassivation conditions (SF6/O2 flow rates: 45/14 sccm, electrode temperature: −85 °C), an ion self-bias generated by 1 W of RF CCP power was already enough to form nanomasks for microcones etching. Under these conditions the density of the microcones increased slightly from wafer edge to the center, i.e. from the coldest to the warmest area (caused by wafer bending due to mechanical clamping). However, when a pulsed LF CCP power was applied, the conditions for the formation of microcones were found to be quite different. In these experiments, we applied an LF CCP power with a 3 ms pulse length and a 25% duty cycle with an amplitude of 1 W. Here, microcones formed only at strong overpassivation conditions (SF6/O2 flow rates: 45/14 sccm, electrode temperature: −105 °C), which, when an RF CCP power of the same amplitude was applied, led exclusively to black silicon formation. Apparently, the application of a pulsed ion self-bias led to a less homogeneous removal of the passivation layer. It is found that using pulsed LF CCP power control of size and density of the microcones is more easily achieved than using RF CCP power. Figure 2 shows the influence of various process parameters on the cone size and density. From Figure 2a, it is seen that the size of microstructures decreases strongly when the flow rate of oxygen rises, and thus increasing O2/SF6 flow rate ratio (as the SF6 flow rate remains constant). The higher passivation rate requires that more time is needed to overcome the SiOxFy passivation layer and to start to etch silicon so that less material is etched away in a given time frame of the experiment. Thus smaller microcones result. The ICP power controls the amount of ionic species in the plasma. The higher coil power is, the more ions get involved in etching, and the etch rate rises so that larger, microcones are obtained (Figure 2c). When the microcone density is viewed, a somewhat different picture is obtained (Figure 2b−d). When the low concentration of oxygen causes a low rate of passivation and the reaction is essentially run out of the overpassivation regime, no cones are observed at all (Figure 2b). As the oxygen contents and thus the passivation rate are increased, the reaction moves into the overpassivation regime, which means that at some critical value first islands of passivation film remain on the surface and, as a result of this, the first cones appear. However, right at the borderline to the overpassivation regime, the cone density is somewhat hard to control. Under these conditions even small fluctuations will cause large differences in the distance between the remaining passivation islands, as this is the case at any critical transition. With increasing oxygen flow rate the density of the cones increases, until at some critical flow rate value the layer becomes thicker and more homogeneous and the cone density starts to decrease again. The same argument holds for the maximum of the cone density, which is observed when the ICP power is varied (Figure 2c), only that with increasing ICP power the etch rate is varied. As the etching of silicon and silicon oxides is a
Figure 3. SEM images of a silicon surface with microcones (a) overview (b) close up image, (c−e) images of individual microcones with different opening angles, which were fabricated using different SF6/O2 ratios given in the legend. Scale bars: 100 μm (a,b) and 5 μm (c−e).
It was observed that the opening angle of microcones angles typically varied from 27° to 60° (Figure 3c-e). The opening angle is strongly correlated with the SF6/O2 ratio. A high ratio results in faster lateral etching, which in turn lowers the opening angle. Therefore, the etching does not exactly follow the crystal planes in all cases as it was previously reported.24 Wear Test Results. In order to test the mechanical stability of the obtained samples, they were exposed to defined shear stress, and the topography change of the resulting surfaces was recorded. In the wear test experiments, a defined normal load was applied, and the sample was exposed to shear. After the wear experiments it was analyzed how topography and wettability were changed with respect to the applied loads. It should be noted that the pressure during the shear experiments could not be calculated directly, since the density of the structures varied across the sample and the contact area of the load with the micro- and nanostructures changed with time as some breaking of the micro- and nanostructures occurred. Therefore, below, only the forces applied to the structures will be discussed. Investigated Surfaces. Four surfaces were exposed to wear tests: silicon nanograss (NS) and three combined surfaces containing both microcones and nanograss with different microcone sizes and densities (MC_LD (MicroCones_LargeDense), MC_SS (MicroCones_SmallSparse), MC_SD (MicroCones_SmallDense)). The opening angles of microcones were in the range of 35° (±2°). The respective microcone sizes and 4345
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to the fact that the debris formed by the shaved off microcones grinds into the surface. A completely different reaction to the shear load, however, was observed when higher density cone structures were investigated. To this firstly MC_LD surface was tested, which featured the biggest microcones and had an intermediate cone density (Figure 4c). After application of a load of 20 N, the microstructures were still in place. However, the shear force let to a truncation of the cones. It was found that after application of shear forces of 10 N, the microcones lost on average about 30% of their height, and after application of 20 N, the loss was found to be about 50%. In both cases, the neighboring nanograss remained unaffected. When the cone density was even higher (sample MC_SD) even after application of a force of 10 N, most of microstructures seemed to not be affected at all by the shear; only a few broken structures were found. However, when a load of 20 N was applied, all microstructures were destroyed. The nanograss mostly remained intact, but we also found many spots with broken nanoneedles (Figure 4d). The interplay between structural stability, load, structure size and structure density is rather complex. The true contact area depends on these parameters and changes within the course of the experiment as some part of the micro- or nanostructures is removed and debris is formed. In order to understand the changes of the topography of the surface caused by the wear, the mechanical moment transferred from the substrate onto the metal disc sliding on top of it was followed during the wear experiment using a build-in torque sensor (Figure 5). The friction or, more precisely, the mechanical moment transfer in these experiments depends on the applied force and the true contact area between metal disc and test sample. In Figure 5a, where a pure nanograss surface is sheared, it can be seen that (a) the moment transfer is rather high, in agreement with the large contact area between nanograss and the shearing steel ring, and (b) that the slopes of moment changes induced by shearing of the nanograss were always negative. In addition, the slope values decreased as the force was raised. This can be explained by a permanent decrease of the contact area, caused by breaking of some of the structures. A somewhat similar situation was observed during the wear tests with the MC_LD surface, which featured the biggest microcones (Figure 5b). In this case, however, the moment transfer during application of a load of 1 N in the shear experiment was much lower than one measured when the same shear force was applied to the NS surface. This shows that the shearing surface was in contact only with the top of the microcones, whose density is much lower than the density of nanograss, which in turn results in lower friction and lower torque. The slope values of the friction occurring at a load of 1 and 5 N are almost equal, which shows that under such forces, the shear effect on surface topography was not very strong. However during application of loads of 10 and 20 N, the gradient of the slopes decreased considerably, clearly indicating severe microstructures damages. A qualitatively different picture was obtained, when wear tests were performed on MC_SD surfaces (Figure 5c), where the densest distribution of microcones was present. Here during application of a load of 5 N, the slope was negative, at 10 N was almost flat, and at 20 N was even slightly positive. This change from negative to positive slope during application of an increasingly stronger shear force clearly indicates that the contact area increased with increasing force. At a shear load of 20 N, most of microcones were removed, and the underlying
densities are shown in Table 1. On all tested surfaces, the nanograss had a height of 10 μm, bottom diameter of 200 nm, and density of >1 000 000 needles per mm2. Table 1. List of Samples with Combined Microcone and Nanograss Structures, Their Sizes, and Densities sample name
microcone height μm
microcone diameter μm
microcones density per/mm2
MC_LD MC_SS MC_SD
35 20 22
28 22 25
241 86 355
The hierarchical samples were chosen such that the influence of microstructures size and density on surface mechanical durability can be separately investigated. Figure 4 shows SEM
Figure 4. SEM images of various superhydrophobic surfaces before and after performing the wear tests with forces given in the figure (a) nanograss surface (NS) (b,c,d) microcone and nanograss combinations: MC_SS, MC_LD, MC_SD, respectively. The description of the samples is given in Table 1. Scale bar: 5 μm (a), 20 μm (b−d).
images of the samples before and after application of forces with an amplitude of 5 N, 10 and 20 N. In Figure 4a, it can be seen that application of a force of 10 N was enough to break most of the nanoneedles on the NS surface. After application of a force of 20 N no remaining nanograss needles could be found. When a surface with the lowest microcones density (MC_SS) was exposed to shearing, the problem was even exacerbated. Already after application of a force of 5 N, most of the microcones were removed; they were completely ripped off, so that even the footprints of the cones were no longer visible in the SEM images. This is probably due to the fact that the contact pressure on a very small number of cones becomes extremely high, indeed higher than what the structures can sustain. The nanograss, which was located closely adjacent to the microcones, was also partially destroyed. The rest of the needles, however, seemed largely unaffected. After applying a load of 10 N during the shearing process strong damage of the surface structures was observed (Figure 4b). This might be due 4346
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Figure 5. Time dependence of the moment induced on a steel disk rubbing on the different surfaces during wear tests. The measurements were performed during application of 1, 5, 10 and 20 N shear forces on the nanograss and hierarchical surfaces described in Table 1: (a) nanograss (NS); (b,c,d) various microcone and nanograss combinations: MC_LD, MC_SD, MC_SS, respectively. “S” indicates the slope of a linear fit of the measurements; positive values indicate an increase of friction with time; negative values indicate a decrease.
Figure 6. Influence of the shear load on (a) advancing, (b) receding contact angles (CA), (c) CA hysteresis, and (d) roll-off angle of the different superhydrophobic surfaces studied in Figures 4 and 5.
nanograss was partially reached. When the sample with the smallest and most sparsely distributed microcones is viewed at a shear load of 10 N, a drastic increase in torque within the first 5 s of the experiment was observed. The absolute values of the torque reached the level of the moments measured on NS at 10 N (MC_SS, Figure 5d). At lower loads, the momentum changes, however, were small and had slopes that were in close
agreement with those on the other microcone surfaces, indicating that no contact with the nanograss occurred. When the SEM images of shear-exposed, i.e., worn, surfaces and the moment measurements during the shear experiments are correlated with each other, it can be concluded that microcones can significantly improve the mechanical durability of the nanograss-based surfaces by preventing mechanical 4347
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Again, the slight increase of the RA as well as of the CAH can be explained by pinning induced through the generation of hydrophilic areas, which are caused by truncation of the cones. Surprisingly, MC_SD showed the least changes of the wetting properties after strong shear exposure of the surfaces. It shows a stable RA of average 6.3° after exposure of the sample to shear forces of 5 N, 10 and 20 N. However, according to the SEM images (Figure 4d), the microcones were not worn down as big microcones on MC_LD surface, but almost completely shaved away when strong enough shear load was applied. Surprisingly, after application of a 20 N load, where no remaining cones were found, and the nanograss was not seriously damaged and preserved low CAH. It could be hypothesized that the broken off cones are still present in some part of the shearing system and still protect the nanograss from wear as long as they are not completely removed from the contact zone. Thus it can be concluded that microcone-decorated surfaces show a better preservation of superhydrophobicity after exposure to high shear loads in comparison to the surfaces coated with nanograss only. A prerequisite for this, however, is that the cone density is not too low, as very low cone densities lead to excessive contact pressures. Therefore, to design an optimal surface, which has good shear resistance and superhydrophobic properties, a suitable microcones size and density must be chosen. Small and sparsely distributed microcones give surfaces with wetting properties that are rather close to those of nanograss. However, such structures lose the superhydrophobicity, even when moderate shear loads are applied. Thus, in such samples, the excellent wetting properties are combined with only moderate wear protection. Very dense/large microcones, on the other hand, give excellent protection against shear-induced wear, however, at the price of a slight increase of the contact line pinning so that compared to the nanostructured surfaces only moderate RA between 6° and 13° are observed.
contact between the nanograss and shearing surface. Size and density of the microcones are determining factors for the ultimate surface mechanical stability. During shearing of the surfaces, the cones take the brunt of the force. If a strong shear force is applied and the cone density and/or the size of them are rather small, they are very strongly damaged or even in many cases completely destroyed. However, if they are large in size and/or large in number, high shear forces lead only to a truncation of the cone. If an excessive force is applied, the tip of the cones breaks off. After loss of the tip, however, the contact area is strongly increased (and the shear pressure thus accordingly decreased). Thus the cones can now to support an even higher load, so that the damage is self-limiting. The more cones broken off, the higher the force required to shear off some more material. This wear behavior of the cone decorated nanograss surfaces, however, also has an important influence on the wetting properties of the surfaces against water, (which will be the only liquid discussed in the following). Pure nanograss coated with a fluoropolymer on top is extremely hydrophobic and has a contact angle (CA) of approximately 179° and very low contact angle hysteresis (CAH) (the difference between advancing and receding CA), so that the roll off angles (RA) are very small and assume values well below 1° (Figure 6). However, after application of even a moderate force, the contact angle hysteresis and the roll off angles are strongly increased as the nanostructures are worn away. After application of a shear force of 1 N, the roll-off angle is already increased to 14°, and after application of 5 N, no more roll-off is observed. When the cone-decorated surfaces are viewed (Figure 6), it is seen that the initial CAH and RA of these microstructure containing surfaces are somewhat higher than that of the pure nanograss, in agreement with the expectations.18 This is due to pinning of the drop contact line to the edges of microcones. However, the roll-off angles are between 3° and 8°, which is still very low, compared to flat surfaces or surfaces completely covered by microstructures, which have a similar surface chemistry.28 Generally, the higher the cone density and the larger the cone sizes the more they dominate the wetting behavior. (MC_LD and MC_SD samples). After application of a shear load of 1 and 5 N, the CAH of all tested microstructured surfaces slightly increased, due to partial rupture of the tip of the microstructures. As the truncation of the cones occurs, in the ruptured areas fresh SiO2 is generated, which is quite strongly hydrophilic. Exposure to those hydrophilic areas causes additional contact line pinning. However, compared to the nanograss surface this increase was much smaller, and after application of 5 N, the CAH of the mechanically challenged microcone/nanograss combination still stayed within 10° and was much lower than that of the pure nanograss treated under the same conditions (CAH of 27°). After application of 10 N shear load to the sample MC_SS, which lead to microcone and nanograss removal (Figure 4b), the superhydrophobicity was completely lost, and the CAH increased to 97°. This indicates that the contact lines of the drops are strongly pinned. However, for samples MC_SD and MC_LD, it is observed that they survive even a shear load of 20 N, without losing their superhydrophobic properties. The RA of MC_LD increased slightly with increasing strength of the mechanical treatment: 8.7° after 5 N, 9.3° after 10 N, and 14° after application of 20 N shear load. However, the material remains strongly hydrophobic even after exposure to the strongest mechanical stress.
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CONCLUSIONS In this study, we described how to fabricate surfaces containing simultaneously silicon microcones and silicon nanograss in a very reproducible way. The fabrication of both micro- and nanostructures does not require any prerequisite patterning step, e.g. photolithography. The microcones fabrication process is based on a low-temperature deep reactive ion etching step, which is run in the overpassivation regime. The height and diameter of the microcones depends on the relative rates of etching and passivation and increases with increasing surface temperature and ICP power. The microcones density can be controlled via adjusting the electrode temperature and the SF6/ O2 ratio; however, the dependence is not linear, and at certain sets of parameters, pronounced maxima of the cone density occur. In a second DRIE step, then the remainder of the surfaces is covered with nanograss in a standard BOSCH process. The thus obtained dual-scale surfaces, which combine microcone and nanograss structures, show a greatly improved mechanical durability, especially against shear, in comparison with surfaces containing nanostructures only. The samples with the biggest microstructures and those that possessed the highest density of microcones have shown excellent wear protection. When the shear stress becomes too high, the cones are truncated. This, however, increases the contact area and thus decreases the shear stress, so that further wear is prevented. 4348
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capacitive coupled plasma; RF radio frequency; LF lowfrequency
The improvement of the surface wear properties by following such a dual-scale concept becomes also evident when the wetting properties of the obtained surfaces are viewed. While nanostructure-only surfaces already lose the superhydrophobic properties completely when a rather moderate shear stress is applied, the wetting behavior of the dual scale surfaces is under the same conditions preserved, and under most conditions the wetting behavior is more robust against shear-induced mechanical wear. If the size and surface density of the microstructures are not too large, the wetting properties are still dominated by the nanostructures. In such a situation, the microcones carry the brunt of the mechanical stress and protect the nanostructures from wear. With increasing cone density, the protection surface against shear-induced wear improves while the contact line pinning slightly increases. This way, superhydrophobic, yet wear resistant, surfaces can be generated. When extremely low CAH and RAs are desired even during or after exposure to average mechanical stresses, smaller microstructures with higher density should be used. However, when high shear loads are likely to occur, bigger microstructures should be generated. To further enhance the wetting properties of mechanically challenged surfaces, one has to address the problem that, after breaking of some fraction of the cones, the stump areas remaining behind are hydrophilic. To mitigate this effect onto the superhydrophobic properties of the substrates, one could use a material that has a better resistance against breaking, change the opening angle of the cones, or generate nanostructured surfaces carrying microcones from an intrinsically hydrophobic material, so that even after cone truncation a hydrophobic spot remains.
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AUTHOR INFORMATION
Corresponding Author
*Mailing address: University of Freiburg Department of Microsystems Engineering- IMTEK Chemistry and Physics of Interfaces Georges-Koehler-Allee 103 D-79110 Freiburg, Germany. Tel.: +49 761 203 7160. Fax: +49 761 203 7162. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Prof. Dr. Reinecke, HSG-IMIT and IMTEK, for the opportunity to use the dry etching facilities at HSGIMIT (Villingen-Schwenningen, Germany) and the SEM at IMTEK. We also appreciate the very helpful assistance of Johannes Auber, Karsten Kurschat, and other HSG-IMIT clean room staff in the Cryogenic DRIE processes. We would like to thank Tobias Amann from Fraunhofer IWM for his help in performing wear tests and Armin Baur, IMTEK, for SEM pictures. This work was in part funded by the Federal Ministry of Education and Research (BMBF), Germany; project “Nanotau”, project-ID 16SV5348.
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ABBREVIATIONS SH superhydrophobic; CA contact angle; ACA advancing contact angle; RCA receding contact angle; CAH contact angle hysteresis; RA roll-off angle; CNT carbon nanotubes; AFM atomic force microscope; SEM scanning electron microscope; KOH potassium hydroxide; HF hydrofluoric acid; DRIE deep reactive ion etching; ICP inductively coupled plasma; CCP 4349
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