We reported the wafer-scale highly-transparent and superhydrophilic sapphires with antireflective subwavelength structures (SWSs) which were fabricated by dry etching using thermally dewetted gold (Au) nanomasks. Their optical transmittance properties were experimentally and theoretically investigated. The density, size, and period of the thermally dewetted Au nanopatterns can be controlled by the Au film thickness. For the sapphire with both-side SWSs at 5 nm of Au film, the average total transmittance (Tavg) of ~96.5% at 350-800 nm was obtained, indicating a higher value than those of the flat sapphire (Tavg~85.6%) and the sapphire with one-side SWSs (Tavg~91%), and the less angle-dependent transmittance property was observed. The calculated transmittance results also showed a similar tendency to the measured data. The SWSs enhanced significantly the surface hydrophilicity of sapphires, exhibiting a water contact angle (θc) of < 5° for Au film of 5 nm compared to θc~37° of the flat sapphire.
© 2012 OSA
Sapphire has been widely used in optical and optoelectronic components including optical lenses, touch screen panels, safety glasses, display windows for mobile phones, and substrates for light-emitting diodes due to its good thermal/chemical stability and mechanical durability, such as chemical inertness and high resistance to environment-related causes of deterioration, high-temperature stability, and high mechanical strength, in harsh environments as well as the optical transparency in a wide range of wavelengths [1,2]. Particularly, the sapphire, which is one of the hardest materials, can only be scratched, besides by itself (i.e., Moh’s hardness scale of ~9), by a few substances including cubic boron nitride and diamond. However, the sapphire has a relatively lower transmission (T~85%) compared to other transparent substrates such as glasses, polymers, and quartzes (T~92%) [3,4]. This is caused by the high refractive index of ~1.76 in the visible wavelength, which leads to the surface reflectivity of ~7.5% at normal incidence on a flat single side of sapphire . The relatively poor transparency may limit the range of applications of the sapphire in practical optical elements or optoelectronic devices. Thus, efficient antireflection coatings (ARCs) are required to reduce the reflected glare and the performance degradation of devices caused by the optical loss due to the surface Fresnel reflection. Recently, antireflective subwavelength structures (SWSs) inspired from the corneas of moth eyes , which create a gradient refractive index profile between air and the semiconductor, have been considered as a promising alternative of conventional thin-film ARCs with many drawbacks of thermal-mismatch-induced lamination, material selection, poor adhesion, and layer thickness control as well as the limit of the low reflectance band in wavelengths and incident angles of light [6–8].
To fabricate the SWSs on sapphires, it is necessary to form the desirable large-area nanopatterns as an etch mask. The self-assembled metallic nanopatterns by the thermal dewetting process using metal thin films (e.g., Au, Ag, Ni, Pt etc.) have increasingly attracted attention because of their relatively simple, cost-effective, and size-tunable lithography method compared to the laser interference, nano-imprint, and e-beam lithography techniques [7,8]. Generally, conical SWSs exhibit a lower reflectivity than other SWSs with cylinder or truncated cone shape due to the linearly graded effective refractive index distribution between air and the semiconductor . For transparent substrates, meanwhile, the SWSs can enhance the hydrophilicity owing to the increase of surface roughness . The highly transparent substrate with a superhydrophilic surface is very useful in optical lenses, building or car windows, and optoelectronic devices because it has many functions such as anti-fog, quick dry, elimination of light scattering caused by the water droplets, and self-cleaning. There has been very little or no work on the fabrication of conical SWSs on wafer-level sapphires using thermally dewetted Au nanoparticles as an etch mask pattern. Thus, it is very meaningful to analyze the optical properties and wettability by applying the SWSs to sapphires. In this work, we reported the transmittance properties and wetting behavior of 2 inch sapphire wafers with the cone-shaped SWSs fabricated by a chlorine (Cl)-based inductively coupled plasma (ICP) etching using thermal dewetted Au nanoparticles as the etch mask patterns. A theoretical investigation using the rigorous coupled-wave analysis (RCWA) method was also performed.
2. Experimental and simulation modeling details
Figure 1 shows the schematic illustration of process steps for the fabrication of SWSs on sapphire substrates using the thermally dewetted Au nanopatterns. The top-view scanning electron microscope (SEM) images of the thermally dewetted Au nanoparticles for different Au film thicknesses of 5, 7, and 9 nm are also shown in Fig. 1. For the fabrication of SWSs on sapphires, the c-plane double-side polished 2 inch sapphire wafers were ultrasonically cleaned in acetone, methanol, and de-ionized (DI) water for 10 min, respectively, and then dried with a nitrogen gas. The Au thin films with different thicknesses of 5, 7, and 9 nm were deposited on the samples using 99.99% purity Au pellets by using a thermal evaporator. To form the desirable dot-like Au nanoparticles, the Au films were heat treated by rapid thermal annealing (RTA) at 600 °C for 3 min in a nitrogen environment. During the RTA process, the Au thin films are agglomerated into the nanosized particles to minimize the surface free energy, which results from the increased surface energy of Au by the heating . As shown in SEM images of Fig. 1, it can be observed that the dot-like Au nanoparticles were well formed on sapphire wafers for different thicknesses of Au films. To analyze the average density of the thermally dewetted Au nanoparticles, a public domain image processing program (ImageJ 1.42q, NIH) was utilized. As the thickness of Au films was increased from 5 to 9 nm, the average density of Au nanoparticles was decreased from 77 to 59%. The average diameter and period of Au nanoparticles were also increased from 185 ± 35 nm and 240 ± 45 nm for 5 nm of Au film to 265 ± 50 nm and 450 ± 85 nm for 9 nm, respectively. It is noted that the density, size, and period of thermally dewetted Au nanopatterns as the etch mask can be roughly controlled by adjusting the Au film thickness . Using the dot-like Au nanomask patterns, the cone-shaped SWSs were fabricated on sapphire substrates by an ICP dry etching in Cl-based plasma. During the etching, the Au nanopatterns were almost removed. Nevertheless, the remaining Au nanopatterns were eliminated by Au etchant solution.
The surface morphologies of the thermally dewetted Au nanopatterns and the etched profiles of the fabricated SWSs on sapphire substrates were observed by using a SEM. The total transmittance was measured by using a UV-vis-NIR spectrophotometer with an integrating sphere. For angle-dependent transmittance measurements, the spectroscopic ellipsometry was used at incident angles of 15-80° in specular mode for unpolarized light. The water contact angles were measured and averaged at three different positions on the surface of samples by using a contact angle measurement system. For the theoretical analysis based on the RCWA method, the optical transmittance calculations of the sapphires with SWSs were performed using a commercial software (DiffractMOD 3.1, Rsoft Design Group) . To design the theoretical models, the SWSs on sapphire substrates were roughly represented by a periodic geometry in the Cartesian coordinate system by a scalar-valued function of three variables, f(x, y, z), for simplicity. Further details can be found in our previous work . We assumed that the unpolarized incident light entered from air into the SWSs at incident angles of 0-80° and the thickness of sapphire substrate was set to 430 μm. The refractive index of sapphire used in this calculation was referred .
3. Results and discussion
Figure 2(a) shows the measured total transmittance spectra of the sapphires with one-side SWSs for different Au film thicknesses. For comparison, the total transmittance spectrum of flat sapphire is also shown in Fig. 2(a). The insets show the 30°-tilted and side-view SEM images of the corresponding structures. The dry etching process was performed with an additional 200 W ICP power at 3 mTorr for 10 min in Ar/Cl2/BCl3 (2:1:4) plasma. To achieve the cone-shaped SWSs with similar heights, the RF powers of 50, 70, and 90 W were used for the Au films of 5, 7, and 9 nm, respectively, exhibiting average heights of ~280 ± 40 nm. The conical SWSs can be obtained using the overall etching until the Au nanopatterns are completely removed . From the SEM images, the Au nanomask patterns were transferred directly into the underlying sapphire surface by the ICP etching, which led to the SWSs with a cone shape. The transmittance was strongly dependent on the period and density of SWSs. For sapphires with one-side SWSs, the transmittance spectra became lower and the low transmittance region of < 85% was shifted towards the longer wavelengths with increasing the thickness of the Au film. This is maybe attributed to the diffraction loss at shorter wavelengths than the period of SWSs due to the higher order diffracted waves as well as the low packing density of SWSs . However, the one-side sapphire SWSs for Au film of 5 nm exhibited the transmittance spectrum higher than 85% over a wide wavelength range of 350-800 nm, indicating the average transmittance value (Tavg) of ~91% which is higher than those of the flat sapphire (Tavg~85.6%) as well as other samples (Tavg~88.1% for 7 nm of Au film, Tavg~86.4% for 9 nm). This is because cone-shaped SWSs can effectively reduce the surface reflection via a linearly graded effective refractive index profile between air and the sapphire [6,7], which relatively enhances the transmission in transparent sapphires.
Figure 2(b) shows the contour plot of calculated total transmittance variations of the sapphire with one-side SWSs as a function of period of SWSs. The three-dimensional (3D) simulation model used in this calculation is also shown in Fig. 2(b). In RCWA simulations, the height and density of SWSs were set to 280 nm and 77%, respectively. At periods less than 250 nm (i.e., white dashed line), the transmittance values higher than 86% were obtained at wavelengths of 350-800 nm. The calculated total transmittance results reasonably provide a similar tendency to the measured data though there is a discrepancy between the experimentally measured and theoretically calculated results at some wavelengths due to the geometrical difference between the simulation model and the fabricated structure as well as the refractive index mismatch of the sapphire used in these experiments and calculations.
Figure 3 shows the photographs of the (a) water droplets on the surface of the flat sapphire and the sapphires with one-side SWSs for Au films of 5, 7, and 9 nm and (b) DI-water sprayed (left) flat sapphire and (right) sapphire with one-side SWSs for Au film of 5 nm. The SWSs can dramatically increase the hydrophilicity of the sapphire by inducing the surface roughness into the hydrophilic sapphire as proposed by the Wenzel’s equation though it is also related to the surface energy of materials . The water contact angles (θc) of the sapphires with SWSs were lower than that of the flat sapphire (θc~37°), exhibiting the θc values less than 15° for all samples. Especially, for the sapphire with SWSs at 5 nm of Au film, which had the values of θc< 5°, a water droplet completely spreads over the surface, confirming that a superhydrophilic surface was achieved. The superhydrophilic surface can effectively eliminate the light scattering caused by water droplets because it makes water droplets to spread out like a thin film on the surface. This thin water film will finally disappear due to the evaporation by the heat or the sliding down by the gravity. Also, it can wedge into the space between the dusts and the surface, and take the dusts away. As shown in Fig. 3(b), for the flat sapphire, the characters below it are blurred by the strong light scattering caused by the condensed water droplets. On the contrary, the sapphire wafer with SWSs for Au film of 5 nm remains highly transparent and the characters under it are clear. Therefore, the sapphire with SWSs can be used as purposes such as anti-fog, self-cleaning, and quick dry functions in optical components including eyeglasses, building or car windows, and safety lenses.
Figure 4 shows (a) the measured (solid lines) and calculated (dashed lines) total transmittance spectra of the flat sapphire and the sapphires with one- and both-side SWSs for Au film of 5 nm and (b) photographs of the corresponding samples exposed to a fluorescent lamp. The 3D simulation model of the sapphire with both-side SWSs used in this calculation is shown in the inset of Fig. 4(a). The SWSs considerably reduced the reflection loss at the both-side surfaces of sapphire, exhibiting a high Tavg value of ~96.5% over a wide wavelength region of 350-800 nm. This value is much higher than those of the flat sapphire (Tavg~85.6%) and sapphire with one-side SWSs (Tavg~91%). For all samples, it can be observed that the calculated total transmittance spectra were also similar with the measured results. These results can be confirmed in Fig. 4(b). For the flat sapphire, the characters under it are nearly not seen due to the strongly reflected white fluorescent light at the both-side surfaces. For the sapphire with one-side SWSs, similarly, it is also difficult to read the characters owing to the Fresnel reflection at the other flat-side surface. However, the sapphire with both-side SWSs exhibits much better legibility for the characters below it though there exist weak reflected fluorescent lights.
Figure 5(a) shows the measured specular transmittance of the flat sapphire and the sapphires with one- and both-side SWSs for Au film of 5 nm as a function of incident angle for unpolarized light at a wavelength of 525 nm. As the incident angle (θi) of light was increased, the transmittance of the samples was generally decreased. However, the transmittance of the sapphire with both-side SWSs was less dependent on the incident angle of light compared to the flat sapphire and the sapphire with one-side SWSs, exhibiting the average specular transmittance value of ~86.5% at incident angles of θi = 0-80° which is higher than those of other samples (i.e., ~77.8% for the flat sapphire and ~81.2% for the sapphire with one-side SWSs). For comparison, the contour plots of variations of calculated incident angle-dependent total transmittance spectra of (i) the flat sapphire and the sapphires with (ii) one- and (iii) both-side SWSs for unpolarized light are also shown in Fig. 5(b). In simulations, the sapphire with both-side SWSs also exhibits the less angle-dependent transmittance characteristics compared to other structures in wide ranges of wavelengths and incident angles.
The wafer-scale highly-transparent and superhydrophilic sapphires with antireflective SWSs were fabricated by the ICP dry etching using thermally dewetted Au nanopatterns as the etch mask. The density, size, and period of the thermally dewetted Au nanopatterns strongly rely on the thickness of Au film. By applying the SWSs to both side surfaces of 2 inch sapphire wafer, the higher transmittance spectrum as well as higher Tavg value of ~96.5% were achieved at wavelengths of 350-800 nm compared to the flat sapphire and the sapphires with one-side SWSs, exhibiting the less-angle dependent transmittance property up to a high θi = 80°. The theoretically calculated transmittance results showed a similar tendency to the experimentally measured data. The SWSs considerably enhanced the hydrophilicity of sapphires, indicating the water contact angle of θc< 5° for Au film of 5 nm (i.e., θc~37° for flat sapphire). These results can provide a better insight into the broadband and wide-angle highly-transparent sapphires of efficient antireflective SWSs with a superhydrophilicity for practical optical and optoelectronic applications.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0026393).
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