1-1 Background and Motivation
Naturally occurring structures is a rich source of products that meets specific functions which are imposed by natural selection. The surface relief structure of animals and plants is one of the most attractive topics. Many papers have revealed lots of micro and nano structures on the surface of creatures. Some kinds of these structures were observed complex. A lotus leaf, the most famous superhydrophobic example, was observed by W. Barthlott et al. in 1997 [1]. As shown in Fig. 1-1, the microscopic SEM image of a lotus leaf showed the two-scale roughness, dots on a dot array. The water contact angle of a lotus leaf is 161 o. Legs of a water strider, another famous superhydrophobic example, were observed by X. F. Gao et el. in 2004 [2]. As shown in Fig. 1-2, the leg of a water strider also shows two-scale roughness and the water contact angle was 167o. Nano structures on cicada wing, an anti-reflective surface relief example, were observed and replicated by G.Y. Xie et al. in 2008 [3].
As shown in Fig. 1-3, the nano structure of the cicada wing is also complex structure, rods on rod arrays. In Reference 3, the authors replicate the cicada wing to PMMA material and the PMMA replica showed excellent anti-reflective property compared to a flat PMMA sheet.
Fig. 1-1 An SEM image of a lotus leaf shows the two-scale roughness [1].
Fig. 1-2 Images of the non-wetting leg of a water strider. (a) A water droplet on a leg with a contact angle of 167.6o±4.4o; (b) and (c), SEM images of the leg show numerous oriented spindly microsetae (b) and the fine nano scale grooved structures on a seta (c). Scale bars: (b) 20 mm, (c) 200 nm [2].
Fig. 1-3 An SEM image of a cicada wing [3].
After reviewing some papers about biometric surface relief structures, it is believed that the micro or nano scale complex structures can have hydrophobic or anti-reflective property. The two properties related to wettability of a solid surface and optics. The two issues have been widely investigated recently and progressed very fast.
1-1-1 Wettability of a Solid Surface
The first topic, wettability of a solid surface, is an important property in practical applications. An intuitive expression about this property is water contact angle to any solid surface. The water contact angle to a surface below 90o is called hydrophilic. In contrary, the water contact angle to a surface beyond 90o is called hydrophobic.
Surfaces with water contact angle higher than 150o are super-hydrophobic or ultrahydrophobic. In fact, the super-hydrophobic surfaces are usually covered with micro- or nano- scale asperities. The water contact angles between rough surface and the original flat surface with the same material are different. The difference is
is the water contact angle on a flat surface, r is the roughness value of wet area, r
is the ratio of total surface area to the total base area, f is the ratio of the suspended water contact area to the total base area. Water drop on rough surfaces can either penetrate the asperities or suspend above the asperities. As shown in Fig. 1-4 (a), the first case that water drop penetrates the asperities is called Wenzel state and the
contact angle in this state,w, is expressed by Eq. (1.1). As shown in Fig. 1-4 (b), the second case that water drop suspends above the asperities is called Cassie-Baxter state
and the water contact angle in this state,c, is expressed by Eq. (1.2). The two states and their mathematic expressions are guidelines in determining water contact angle of any rough surface.
In pursuit of super-hydrophobic surface, higher contact angle and rough surface are needed. Water contact angles of rough surface in the two states can be higher than that of a flat surface. By taking advantage of the progress of chemistry and semi-conductor technology, many micro- and nano- scale rough surfaces can be prepared. Hydrophobic surfaces can be obtained by micro- or nano- structured hydrophilic surface. In this case, an extreme example was revealed by E. Hosono et al.
in 2005 [6]. A super-hydrophobic surface with water contact angle of 178o was fabricated on glass substrates. The glass substrate was originally hydrophilic.
Fig. 1-4 (a) Wenzel state [4], (b) Cassie-Baxter state [5].
Fig. 1-5 (a, b) Field emission SEM images of the cobalt hydroxide films observed from the top and side, respectively. (c) TEM images of the cobalt hydroxide films. (d) A simple model of the film with the fractal structure [6].
In other words, f of the top area is very small in Eq. (1.2) and cosc approaches -1.
When cosc approaches -1, c approaches 180o. The case was in Cassie-Baxter state.
Many researches focused on fabrication of superhydrophobic surface, but superhydrophobic surface doesn’t possess self-cleaning property absolutely. Some papers showed high-adhesion superhyFdrophobic surface was possible [7]. Actually, for self-cleaning property low sliding angle is more important than high contact angle.
A water drop is placed on a horizontal plane and slowly inclined. When the drop starts to slide, the angle between the tilt plane and horizontal plane is called sliding angle.
Lower sliding angle of a surface represents better self-clean property of a surface.
C. J. Lv et al. fabricated three kinds of square-shaped pillar-structured surfaces at micro scale with different period and related sliding angle to solid fraction and its contact angle of flat surface [8]. The solid fraction here means the area ratio of the structure top and the base. Reference 8 suggested lower solid fraction of squared-shaped pillar-structure can have lower sliding angle. The author of reference 8 also took note that this expression was established for pillar-structured surfaces with square-distributed square cross-sectional at micro scale and may not be applicable to other type roughness and scales. It still provided a guideline for designing roughness with low sliding angle.
1-1-2 Anti-Reflective Nano Structure
Nano scale surface relief structure can be designed to have anti-reflective property. Bernhard et al. discovered the outer surface of the facet lenses in moth-eyes consists of an array of cuticular protuberances. The structure is called corneal nipples.
This discovery initialized a large amount of researches about anti-reflective nano structure. The structure has graded refractive index from air to the interior part of moth eye. The graded refractive index reduces insertion loss when light propagates from outside into the moth eye. Later, many papers revealed all kinds of surface relief structures on insects, such as cicada wings [3] and butterfly eyes [9]. In additional to these surface structures of insects, many artificial nano structures have been fabricated.
With increasing solar cell applications and progress of semi-conductor technology, the artificial anti-reflective nano structures have been usually fabricated on silicon-based substrates.
A nano pillar array with tapered side wall and round top was one of the most popular artificial nano structures. As shown in Fig. 1-6, C.H. Sun et al. fabricated such moth-eye like structure on silicon substrate by silica colloidal lithography and reactive ion etching [10]. The average reflectance in visible range (wavelength between 400nm to 700nm) was about 1.2%. Rigorous coupled-wave analysis (RCWA) and thin film theory (TFT) simulated results were compared with the measurement
result. The spectra do not fit each other perfectly but the simulation results show the tendency of the measured reflectivity. H. M. Wu et al. fabricated similar structure on silicon and gallium nitride substrate by the same silica colloidal lithography and different reactive ion etching parameters [11]. The average reflectance in visible range was about 2.22%.
S. Wang et al. fabricated random nano pillars array in different feature size with tapered side wall on silicon by using metal film island mask and reactive ion etching [12]. The average reflectance in visible range was about 4%.
Y. Kanamori et al. fabricated periodic nano cone array by electron beam lithography and reactive ion etching [13]. The average reflectivity in visible range was 0.8%. H. L. Chen et al. fabricated periodic pillar array with cone top by colloidal lithography and reactive ion etching [14]. The lowest average reflectivity in visible range in this paper was 1.4%.
Fig. 1-6 Measured reflectance spectrum and simulated reflectance spectra by using RCWA and TFT models [11]. The SEM image of the structure is shown in the upper-left corner of the figure.
As shown in Fig. 1-7, C. H. Sun et al. fabricated periodic inverted pyramid array by colloidal lithography and KOH wet etching [15]. The reflectance in visible range was about 6%. The simulated result by using RCWA was compared to the measured reflection spectrum. The two spectra had a good agreement with each other in long wavelength but they had some deviation in short wavelength.
The best three structures, pillar with round top, pillar with cone top and cone array, are mentioned. RCWA or TFT simulation can be used to provide some information to determine how low the reflectivity of the structure can be. Some papers show that fill factor has important influence on reflectivity [11]. In other words, the base area of the anti-reflective structure should be as large as possible.
Fig. 1-7 (a) Inverted pyramid structure on a silicon substrate (b)Experimental (solid) and RCWA-simulated (dotted) specular optical reflectivity at normal incidence. Black:
bare (100) silicon wafer. Red: 360 nm size pyramids template from 320 nm silica spheres [15].
1-1-3 Hydrophobic and Anti-Reflective Nano
Structure
From discussion above, nano structure with small top area and proper shape can have high contact angle and low reflectivity. Recently, some papers reported on the fabrication of micro or nano structures which have the two properties simultaneously.
As shown in Fig. 1-8, Y. C. Chang et al. fabricated inverted pyramid array on silicon substrate by electron beam lithography and KOH wet etching in 2007 [16].
The period of the structure was 300 nm and the height was 212 nm. The aspect ratio of the structure was 0.7. The nano structure was coated with Teflon film and the wettability of the surface was changed from hydrophilic to hydrophobic. The average reflectance of the nano structure in visible range was 17%. The contact angle of the structure was 135.9o. The theoretical contact angle of the nano structure was 142.7 o and the case was in Wenzel state. The two contact angles are very close. The sliding angle was not reported.
As shown in Fig. 1-9, W. L. Min et al. fabricated periodic pillars array on silicon substrates by colloidal nano particles and reactive ion etching in 2008 [17].
The period of the structure was 98 nm and the height was 400 nm. The aspect ratio of the structure was 4.08.
Fig. 1-8 Inverted pyramid arrays on a silicon substrate [16].
Fig. 1-9 Silicon nanopillar array fabricated by nano particles and reactive ion etching [17].
The average reflectance of this nano structure in visible range was 1.58%. The structure was then fluorosilane modified. The measured contact angle of the modified nano structure was 158 o. The sliding angle was not reported here.
In 2009, Y. F. Li et al. fabricated biomimetic silicon hollow-tip arrays by using metal catalytic wet etching silicon followed by a short time reactive ion etching process [18]. The initial pattern was achieved by non close packed polystyrene particles. The tip arrays were vertical to the substrate with 686 nm in root diameter, 792 nm spacing, and 7.1μm in height. The SEM graph of the tip arrays are shown in Fig. 1-10. The aspect ratio of the needle array was higher than 12. From Fig. 1-11, the TEM image of the silicon tip, we can see straight pores inside, and the distribution of pores in the tip was random. The diameters of the pores were about 50–150 nm. The specular reflectance in the 250-1600 nm was lower than 1%. The measured water contact angle was 165o for the 7.1μm height hollow-tip arrays which was fluorosilane modified. The sliding angle was 2°. Actually, the artificial mirco or nano structures which possess both low reflectivity and hydrophobic property usually have high aspect ratios. The aspect ratios of these structures are usually higher than 4.
Many structures with moth-eye effect observed from butterfly usually have low aspect ratio. The aspect ratio of the structures on the surface of butterfly is usually lower than 2 [9].
Fig. 1-10 High aspect ratio hollow-tip array on silicon substrate [18].
Fig. 1-11 TEM image of the hollow-tip detached from the substrate [18].
This phenomenon may be explained by the requirement of mechanical strength.
Intuitively, micro or nano structure with low aspect ratio can bear higher mechanical loading. In the field of micro or nano engineering, nano structures with high aspect ratio have bad durability and make nano imprint process much harder.
The complex nano structure on cicada wing introduced by G.Y. Xie et al. [3] fit the two geometric requirements: small top area and large base area. The structure had low aspect ratio which was 2. The definition of aspect ratios for a dual structure is shown in Fig. 1-12. The authors replicate the nano structure from cicada wing to PMMA by gold evaporation and nano imprint. The replica of PMMA shows one fourth reflectance than that of a flat PMMA film. Other report also showed that cicada wing had anti wetting property [19]. The contact angle of cicada wing could be as high as 147o.
In this thesis, we introduce a process to fabricate subwavelength dual structure which was inspired by the structure of cicada wing. We also demonstrate that the dual structure can possess both anti-reflection and low sliding angle simultaneously.
Fig. 1-12 A general shape of dual structure. The aspect ratio of top part is defined by h1/w1. The aspect ratio of the total structure is defined by h2/w2.
1-2 Organization of the Thesis
In Chapter 2, grating equation, TFT and effective medium theory (EMT) will be introduced briefly. The three theories provided intuitive physical meaning of anti-reflective structures. RCWA, a much more accurate model for reflectance simulation of anti-reflective subwavelength structures will also be introduced. The simulated reflectance of complex subwavelength structure array will be compared to the simulated reflectance of bell-shaped structure and cone structure.
In Chapter 3, the whole fabrication process of complex structure will be presented. The modulation of structure shape by changing exposure dosage, RIE parameter, and ICP-RIE parameter will also be described.
In Chapter 4, the measured reflectance, contact angle, and sliding angle of our dual structure will be shown.
In Chapter 5, the conclusion of current work and results will be made. Possible future works will also be described.