The International Conference On Metallurgical Coatings And Thin Films ICMCTF 2006 Wetting Behaviors of a-C:H:Si:O Film Coated Nano-scale Structured Surface Tae-Young Kim*,*** , Bialuch Ingmar **, Klaus Bewilogua **, Kyu Hwan Oh ***and Kwang-Ryeol Lee * * Future Technology Research Division, KIST, KOREA ** New Tribological Coating, Fraunhofer IST, GERMANY ***School of Material Science and Engineering, SNU, KOREA Thank you chairman. Good afternoon everybody, my name is tae-young Kim from KIST Korea. The title of my talk is Wetting behavior of a-C:H:Si:O fiilm coated nano-scale structured surface.

Lotus Leaf Surface Property of lotus leaf Water droplet is not spread (static wetting angle reached 150o) Water droplet removed by slight tilting of surface (wetting angle hysteresis is very low) When you visited the lake located near your hometown, you can easily find the lotus on the lake water. And if you carefully observed the leaf of lotus, you’ll find that lotus leaf is not wetted by water and maintain very clean surface. If you compare the lotus leaf with the other water plants, you can understand that this is very interesting phenomenon. So many people want to know the reason of this phenomenon. The property of lotus leaf can be easily observed by dropping water on the leaf surface. When water droplet stayed on the lotus surface the wetting angle reached up to 150o and also easily removed by slight tilting of leaf. Water repellent and self cleaning effect which is known as lotus effect can be thought as the this super-hydrophobic surface property

Applications of Super-hydrophobic Surface Applications related with water droplet moving Water repellent surface Self cleaning of surface Low resistance coating against liquid flow This observation gives some intuition to many scientists and engineers. If super-hydrophobic surface was generated on to real materials, it could be applied to so many field / related with flowing water /such as water repellent surface, self cleaning surface, or low resistance against liquid flow. So they try to make super-hydrophobic surface on to the real surface with mimicking lotus leaf surface.

Microstructure of Lotus Leaf From the microstructural observation, many scientists found that lotus surface was covered by some kind of wax, and it has very rough surface morphology. This observation gives some prediction /that / ‘super-hydrophobic surface may be caused by hydrophobic surface chemicals and surface roughness’. Planta, 202,(1998) 1 Surface Material - cuticular wax Surface morphology – very rough in micrometer scale

Previous Works Langmuir 2004; 20(2); 10015 With this clue, many researchers tried to make many surface structures as shown in these figures. Some cases, they generated the super-hydrophobic surface and found the reasons of super-hydrophobic surface. Although all previous results explain many kinds of super-hydrophobicity, the structure of modeled surface is quite different with real lotus leaf surface. Langmuir 2004; 20(2); 287-290 Langmuir 2006; 22; 2433

Dual Roughness Effect? 20μm Planta, 202,(1998) 1 If we closely observe the microstructure of lotus leaf surface, we will find that the surface morphology consisted with two kinds of roughness. It consists of micrometer scale roughness and also it was covered by nanometer scale hair. The existence of two kinds of roughness on same surface is main difference with precious modeled surface This observation gives some clue to understand the super-hydrophobic surface. Motivation of this work comes from this observation. 20μm Planta, 202,(1998) 1

Motivation of This Work Does dual roughness surface structure (DRS) affect super- hydrophobic property? How much does DRS contribute the hydrophobic property? With observation of dual rough structure in lotus leaf, we can raise questions. Does dual roughness surface structure (DRS) affect super-hydrophobic property? How much does DRS contribute the super-hydrophobic property?

Experimental Analysis Surface structure control :Plasma Si etching technique Nano-meter size metal mask formation Surface chemical control : hydrophobic a-C:H:Si:O film deposition Super-hydrophobic surface To answer this question, we designed the experiments with two kinds of technique such as surface coating technique and surface etching technique. Surface coating gives homogeneity of surface chemical and surface etching supports various surface roughness. The microstructure and wetting behavior was checked by SEM and goniometer. Analysis

Plasma source gas : CF4+O2 Plasma Si Etching Plasma source gas : CF4 Plasma source gas : CF4+O2 CF4 plasma gas CF4+O2 plasma gas Si wafer Si wafer For the structuring of surface, we used Si wafer as a substrate. we did plasma etching of Si with CF4 and mixture gas of CF4 and O2 in RF-PECVD . The RF bias is 300V and etching time is 10 min. In this experiment we can observe the different etching behavior as oxygen is added as precursor gas. RF-PECVD Source gas : Pure CF4 gas and CF4+O2 mixing gas Chamber Pressure : 2 and 5 Pa RF power : 150 and 300 W RF bias : 300V Etching time : 10min

Plasma source gas : CF4+O2 Plasma Si Etching Plasma source gas : CF4 Nano post formation Plasma source gas : CF4+O2 Flat etched surface When we used the pure CF4 plasma gas, nano scale post was formed at the etched surface. But in the case of CF4 and O2 mixture gas, we can get the flat etched surface. 400nm 400nm

Surface Structure Manipulation Without Cu mask CF4+O2 plasma etching CF4 plasma etching Flat Si wafer Small Post Now we can get the two kinds of surface structure by Si etching technique. However just with this etching technique, we can not make the dual roughness that can be compared with mono roughness. So we used one more technique. Si wafer

Nano Size Metal Dot Formation Cu sputtering on Si surface Si wafer Metal film separation by by heat treatment To make dual roughness, we formed nano size mask with conventional metal dot formation technique. We deposited Cu metal thin film on the Si substrate by sputtering, and then annealed sample at 550o. With this process, the morphology of metal changed from film to randomly distributed dot. We used this nano size metal dot as the mask of Si etching process.

Surface Structure Manipulation Without Cu mask With Cu mask CF4+O2 plasma etching CF4 plasma etching Flat Si wafer Si wafer Small Post Metal dot formed Si substrate was etched in same condition previously discussed. Then the shape of substrate would be changed like this. Si wafer Si wafer

Surface Structure Manipulation Without Cu mask With Cu mask CF4+O2 plasma etching CF4 plasma etching Flat Big Post Si wafer Si wafer Small Post Dual Rough Then we simultaneously deposited hydrophobic a-C:H:Si:O film on to all samples. With film deposition, we can get the homogeneity of surface chemical thereby preventing various surface chemical species. With SEM we verified the microstructures of each samples. Si wafer Si wafer

Microstructures Without Cu mask With Cu mask CF4+O2 plasma etching CF4 plasma etching Flat Big Post 500nm 500nm Small Post Dual Rough Microstructures show that the structure of each sample was formed as previously designed. From now, I will call each structure as flat structure, big post structure, small post structure, and dual rough structure. With these samples, we investigated the wetting behavior as different surface structures. 250nm 500nm

Static Wetting Angle 93.2o 103.8o 134.0o 159.6o Without Cu mask With Cu mask CF4+O2 plasma etching CF4 plasma etching Flat 93.2o Big Post 103.8o Small Post Dual Rough 134.0o 159.6o We measured static wetting angle. Volume of water drop is 5μl with smooth dropping. On the flat surface, wetting angle was 93o. When surface structure changed to big post structure, angle increased to 104o. On the small post structure, angle increased to 134o. However, when surface structure changed to dual rough structure, angle increased up to 160o. Just measuring static wetting angle, dual rough structure shows superhydrophobic surface property. Water drop volume : 5μL Gently drop on the surface

Static Wetting Angle Now I will show the movies which can show the static wetting angle as structure variation. Firstly, I will show the static wetting behavior at the small post structure. As you can see in movies, water drop shows very high static wetting angle. Also in case of dual rough structure, wetting angle of water drop also shows very high water wetting angle. But the behavior of droplet moving on each surface are different with each other. At the small post, droplet attached to the surface, but at the dual rough surface you can see the droplet bouncing.

Dynamic Wetting Angle Wetting angle hysteresis To understand the different behavior of moving droplets, we have to consider wetting angel hysteresis. Generally, the advancing side wetting angle of moving droplet is called advancing angle and the receding front angle is called receding angle. Wetting angle hysteresis is the difference between advancing wetting angle and receding wetting angle. Wetting angle hysteresis is known as representative value which reflect how easily water droplet can move.

Wetting angle hysteresis Dynamic Wetting Angle Wetting angle hysteresis High Low For your understanding, I will show the difference of droplet moving behavior as wetting angle hysteresis in this movies. At the high hysteresis, droplet moved very slowly. But at the low hysteresis surface, droplet moved very fast at the similar tilting angle.

Dynamic Wetting Angle Without Cu mask With Cu mask CF4+O2 plasma etching CF4 plasma etching Flat Big Post Small Post Dual Rough This is the results of wetting angle hysteresis at each surfaces. Wetting angle hysteresis of each surface structure was shown in table. Hysteresis of flat surface is 15o. But in mono rough structure, such as Big and small post structure, wetting angle hysteresis increased up to 50o. However at the dual rough structure, it extremely decreased to 5o.

Dynamic Wetting Angle For your understanding, we prepare the movies that show the difference in wetting phenomenon at different wetting angle hysteresis. We will show wetting behavior of small post structure and dual rough structure. I hope you remember the static wetting angle of each surface is very high at both side, but wetting angle hysteresis is quite different. At the small post structure, dropped water attach on the surface and do not moved easily, but in dual rough structure water are bounced and do not stayed on the surface. This movies clearly show super –hydrophobicity of dual rough structure.

Dual Rough Structure Effect Now we can answer the question which was suggested in motivation of this work. First question, does dual rough structure affect super-hydrophobic property? The answer is YES. And how much affects? The answer is dual rough structure increase static wetting angle and decrease wetting angle hysteresis, so it could be very effective structure for moving droplet applications. 134.0 Static wetting angle 159.6 48.7 Wetting angle hysteresis 4.8

Conclusions We fabricated various structures with mono and dual roughness through nano structuring of Si and studied their wetting behavior. Dual rough structure shows higher static wetting angle and lower wetting angle hysteresis than that of mono structures. Dual rough structure could be effective structure for moving droplet application. Now I conclude our works. We fabricated various structures with mono and dual roughness through nano structuring of Si and studied their wetting behavior. Dual rough structure shows higher static wetting angle and lower wetting angle hysteresis than that of mono structures. Dual rough structure could be effective structure for moving droplet application.